Allow archiving via loadable modules.

Running a shell command for each file to be archived has a lot of
overhead and may not offer as much error checking as you want, or the
exact semantics that you want. So, offer the option to call a loadable
module for each file to be archived, rather than running a shell command.

Also, add a 'basic_archive' contrib module as an example implementation
that archives to a local directory.

Nathan Bossart, with a little bit of kibitzing by me.

Discussion: http://postgr.es/m/20220202224433.GA1036711@nathanxps13

26 files changed, 938 insertions, 64 deletions

diff --git a/contrib/Makefile b/contrib/Makefile
index 87bf87ab90..e3e221308b 100644
--- a/contrib/Makefile
+++ b/contrib/Makefile
@@ -9,6 +9,7 @@ SUBDIRS = \
 		amcheck		\
 		auth_delay	\
 		auto_explain	\
+		basic_archive	\
 		bloom		\
 		btree_gin	\
 		btree_gist	\
diff --git a/contrib/basic_archive/.gitignore b/contrib/basic_archive/.gitignore
new file mode 100644
index 0000000000..5dcb3ff972
--- /dev/null
+++ b/contrib/basic_archive/.gitignore
@@ -0,0 +1,4 @@
+# Generated subdirectories
+/log/
+/results/
+/tmp_check/
diff --git a/contrib/basic_archive/Makefile b/contrib/basic_archive/Makefile
new file mode 100644
index 0000000000..14d036e1c4
--- /dev/null
+++ b/contrib/basic_archive/Makefile
@@ -0,0 +1,20 @@
+# contrib/basic_archive/Makefile
+
+MODULES = basic_archive
+PGFILEDESC = "basic_archive - basic archive module"
+
+REGRESS = basic_archive
+REGRESS_OPTS = --temp-config $(top_srcdir)/contrib/basic_archive/basic_archive.conf
+
+NO_INSTALLCHECK = 1
+
+ifdef USE_PGXS
+PG_CONFIG = pg_config
+PGXS := $(shell $(PG_CONFIG) --pgxs)
+include $(PGXS)
+else
+subdir = contrib/basic_archive
+top_builddir = ../..
+include $(top_builddir)/src/Makefile.global
+include $(top_srcdir)/contrib/contrib-global.mk
+endif
diff --git a/contrib/basic_archive/basic_archive.c b/contrib/basic_archive/basic_archive.c
new file mode 100644
index 0000000000..16ddddccbb
--- /dev/null
+++ b/contrib/basic_archive/basic_archive.c
@@ -0,0 +1,371 @@
+/*-------------------------------------------------------------------------
+ *
+ * basic_archive.c
+ *
+ * This file demonstrates a basic archive library implementation that is
+ * roughly equivalent to the following shell command:
+ *
+ * 		test ! -f /path/to/dest && cp /path/to/src /path/to/dest
+ *
+ * One notable difference between this module and the shell command above
+ * is that this module first copies the file to a temporary destination,
+ * syncs it to disk, and then durably moves it to the final destination.
+ *
+ * Another notable difference is that if /path/to/dest already exists
+ * but has contents identical to /path/to/src, archiving will succeed,
+ * whereas the command shown above would fail. This prevents problems if
+ * a file is successfully archived and then the system crashes before
+ * a durable record of the success has been made.
+ *
+ * Copyright (c) 2022, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ *	  contrib/basic_archive/basic_archive.c
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include <sys/stat.h>
+#include <sys/time.h>
+#include <unistd.h>
+
+#include "common/int.h"
+#include "miscadmin.h"
+#include "postmaster/pgarch.h"
+#include "storage/copydir.h"
+#include "storage/fd.h"
+#include "utils/guc.h"
+#include "utils/memutils.h"
+
+PG_MODULE_MAGIC;
+
+void _PG_init(void);
+void _PG_archive_module_init(ArchiveModuleCallbacks *cb);
+
+static char *archive_directory = NULL;
+static MemoryContext basic_archive_context;
+
+static bool basic_archive_configured(void);
+static bool basic_archive_file(const char *file, const char *path);
+static void basic_archive_file_internal(const char *file, const char *path);
+static bool check_archive_directory(char **newval, void **extra, GucSource source);
+static bool compare_files(const char *file1, const char *file2);
+
+/*
+ * _PG_init
+ *
+ * Defines the module's GUC.
+ */
+void
+_PG_init(void)
+{
+	DefineCustomStringVariable("basic_archive.archive_directory",
+							   gettext_noop("Archive file destination directory."),
+							   NULL,
+							   &archive_directory,
+							   "",
+							   PGC_SIGHUP,
+							   0,
+							   check_archive_directory, NULL, NULL);
+
+	EmitWarningsOnPlaceholders("basic_archive");
+
+	basic_archive_context = AllocSetContextCreate(TopMemoryContext,
+												  "basic_archive",
+												  ALLOCSET_DEFAULT_SIZES);
+}
+
+/*
+ * _PG_archive_module_init
+ *
+ * Returns the module's archiving callbacks.
+ */
+void
+_PG_archive_module_init(ArchiveModuleCallbacks *cb)
+{
+	AssertVariableIsOfType(&_PG_archive_module_init, ArchiveModuleInit);
+
+	cb->check_configured_cb = basic_archive_configured;
+	cb->archive_file_cb = basic_archive_file;
+}
+
+/*
+ * check_archive_directory
+ *
+ * Checks that the provided archive directory exists.
+ */
+static bool
+check_archive_directory(char **newval, void **extra, GucSource source)
+{
+	struct stat st;
+
+	/*
+	 * The default value is an empty string, so we have to accept that value.
+	 * Our check_configured callback also checks for this and prevents archiving
+	 * from proceeding if it is still empty.
+	 */
+	if (*newval == NULL || *newval[0] == '\0')
+		return true;
+
+	/*
+	 * Make sure the file paths won't be too long.  The docs indicate that the
+	 * file names to be archived can be up to 64 characters long.
+	 */
+	if (strlen(*newval) + 64 + 2 >= MAXPGPATH)
+	{
+		GUC_check_errdetail("archive directory too long");
+		return false;
+	}
+
+	/*
+	 * Do a basic sanity check that the specified archive directory exists.  It
+	 * could be removed at some point in the future, so we still need to be
+	 * prepared for it not to exist in the actual archiving logic.
+	 */
+	if (stat(*newval, &st) != 0 || !S_ISDIR(st.st_mode))
+	{
+		GUC_check_errdetail("specified archive directory does not exist");
+		return false;
+	}
+
+	return true;
+}
+
+/*
+ * basic_archive_configured
+ *
+ * Checks that archive_directory is not blank.
+ */
+static bool
+basic_archive_configured(void)
+{
+	return archive_directory != NULL && archive_directory[0] != '\0';
+}
+
+/*
+ * basic_archive_file
+ *
+ * Archives one file.
+ */
+static bool
+basic_archive_file(const char *file, const char *path)
+{
+	sigjmp_buf	local_sigjmp_buf;
+	MemoryContext oldcontext;
+
+	/*
+	 * We run basic_archive_file_internal() in our own memory context so that we
+	 * can easily reset it during error recovery (thus avoiding memory leaks).
+	 */
+	oldcontext = MemoryContextSwitchTo(basic_archive_context);
+
+	/*
+	 * Since the archiver operates at the bottom of the exception stack, ERRORs
+	 * turn into FATALs and cause the archiver process to restart.  However,
+	 * using ereport(ERROR, ...) when there are problems is easy to code and
+	 * maintain.  Therefore, we create our own exception handler to catch ERRORs
+	 * and return false instead of restarting the archiver whenever there is a
+	 * failure.
+	 */
+	if (sigsetjmp(local_sigjmp_buf, 1) != 0)
+	{
+		/* Since not using PG_TRY, must reset error stack by hand */
+		error_context_stack = NULL;
+
+		/* Prevent interrupts while cleaning up */
+		HOLD_INTERRUPTS();
+
+		/* Report the error and clear ErrorContext for next time */
+		EmitErrorReport();
+		FlushErrorState();
+
+		/* Close any files left open by copy_file() or compare_files() */
+		AtEOSubXact_Files(false, InvalidSubTransactionId, InvalidSubTransactionId);
+
+		/* Reset our memory context and switch back to the original one */
+		MemoryContextSwitchTo(oldcontext);
+		MemoryContextReset(basic_archive_context);
+
+		/* Remove our exception handler */
+		PG_exception_stack = NULL;
+
+		/* Now we can allow interrupts again */
+		RESUME_INTERRUPTS();
+
+		/* Report failure so that the archiver retries this file */
+		return false;
+	}
+
+	/* Enable our exception handler */
+	PG_exception_stack = &local_sigjmp_buf;
+
+	/* Archive the file! */
+	basic_archive_file_internal(file, path);
+
+	/* Remove our exception handler */
+	PG_exception_stack = NULL;
+
+	/* Reset our memory context and switch back to the original one */
+	MemoryContextSwitchTo(oldcontext);
+	MemoryContextReset(basic_archive_context);
+
+	return true;
+}
+
+static void
+basic_archive_file_internal(const char *file, const char *path)
+{
+	char		destination[MAXPGPATH];
+	char		temp[MAXPGPATH + 256];
+	struct stat st;
+	struct timeval tv;
+	uint64		epoch;
+
+	ereport(DEBUG3,
+			(errmsg("archiving \"%s\" via basic_archive", file)));
+
+	snprintf(destination, MAXPGPATH, "%s/%s", archive_directory, file);
+
+	/*
+	 * First, check if the file has already been archived.  If it already exists
+	 * and has the same contents as the file we're trying to archive, we can
+	 * return success (after ensuring the file is persisted to disk). This
+	 * scenario is possible if the server crashed after archiving the file but
+	 * before renaming its .ready file to .done.
+	 *
+	 * If the archive file already exists but has different contents, something
+	 * might be wrong, so we just fail.
+	 */
+	if (stat(destination, &st) == 0)
+	{
+		if (compare_files(path, destination))
+		{
+			ereport(DEBUG3,
+					(errmsg("archive file \"%s\" already exists with identical contents",
+							destination)));
+
+			fsync_fname(destination, false);
+			fsync_fname(archive_directory, true);
+
+			return;
+		}
+
+		ereport(ERROR,
+				(errmsg("archive file \"%s\" already exists", destination)));
+	}
+	else if (errno != ENOENT)
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not stat file \"%s\": %m", destination)));
+
+	/*
+	 * Pick a sufficiently unique name for the temporary file so that a
+	 * collision is unlikely.  This helps avoid problems in case a temporary
+	 * file was left around after a crash or another server happens to be
+	 * archiving to the same directory.
+	 */
+	gettimeofday(&tv, NULL);
+	if (pg_mul_u64_overflow((uint64) 1000, (uint64) tv.tv_sec, &epoch) ||
+		pg_add_u64_overflow(epoch, (uint64) tv.tv_usec, &epoch))
+		elog(ERROR, "could not generate temporary file name for archiving");
+
+	snprintf(temp, sizeof(temp), "%s/%s.%s.%d." UINT64_FORMAT,
+			 archive_directory, "archtemp", file, MyProcPid, epoch);
+
+	/*
+	 * Copy the file to its temporary destination.  Note that this will fail if
+	 * temp already exists.
+	 */
+	copy_file(unconstify(char *, path), temp);
+
+	/*
+	 * Sync the temporary file to disk and move it to its final destination.
+	 * This will fail if destination already exists.
+	 */
+	(void) durable_rename_excl(temp, destination, ERROR);
+
+	ereport(DEBUG1,
+			(errmsg("archived \"%s\" via basic_archive", file)));
+}
+
+/*
+ * compare_files
+ *
+ * Returns whether the contents of the files are the same.
+ */
+static bool
+compare_files(const char *file1, const char *file2)
+{
+#define CMP_BUF_SIZE (4096)
+	char		buf1[CMP_BUF_SIZE];
+	char		buf2[CMP_BUF_SIZE];
+	int			fd1;
+	int			fd2;
+	bool		ret = true;
+
+	fd1 = OpenTransientFile(file1, O_RDONLY | PG_BINARY);
+	if (fd1 < 0)
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not open file \"%s\": %m", file1)));
+
+	fd2 = OpenTransientFile(file2, O_RDONLY | PG_BINARY);
+	if (fd2 < 0)
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not open file \"%s\": %m", file2)));
+
+	for (;;)
+	{
+		int		nbytes = 0;
+		int		buf1_len = 0;
+		int		buf2_len = 0;
+
+		while (buf1_len < CMP_BUF_SIZE)
+		{
+			nbytes = read(fd1, buf1 + buf1_len, CMP_BUF_SIZE - buf1_len);
+			if (nbytes < 0)
+				ereport(ERROR,
+						(errcode_for_file_access(),
+						 errmsg("could not read file \"%s\": %m", file1)));
+			else if (nbytes == 0)
+				break;
+
+			buf1_len += nbytes;
+		}
+
+		while (buf2_len < CMP_BUF_SIZE)
+		{
+			nbytes = read(fd2, buf2 + buf2_len, CMP_BUF_SIZE - buf2_len);
+			if (nbytes < 0)
+				ereport(ERROR,
+						(errcode_for_file_access(),
+						 errmsg("could not read file \"%s\": %m", file2)));
+			else if (nbytes == 0)
+				break;
+
+			buf2_len += nbytes;
+		}
+
+		if (buf1_len != buf2_len || memcmp(buf1, buf2, buf1_len) != 0)
+		{
+			ret = false;
+			break;
+		}
+		else if (buf1_len == 0)
+			break;
+	}
+
+	if (CloseTransientFile(fd1) != 0)
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not close file \"%s\": %m", file1)));
+
+	if (CloseTransientFile(fd2) != 0)
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not close file \"%s\": %m", file2)));
+
+	return ret;
+}
diff --git a/contrib/basic_archive/basic_archive.conf b/contrib/basic_archive/basic_archive.conf
new file mode 100644
index 0000000000..b26b2d4144
--- /dev/null
+++ b/contrib/basic_archive/basic_archive.conf
@@ -0,0 +1,3 @@
+archive_mode = 'on'
+archive_library = 'basic_archive'
+basic_archive.archive_directory = '.'
diff --git a/contrib/basic_archive/expected/basic_archive.out b/contrib/basic_archive/expected/basic_archive.out
new file mode 100644
index 0000000000..0015053e0f
--- /dev/null
+++ b/contrib/basic_archive/expected/basic_archive.out
@@ -0,0 +1,29 @@
+CREATE TABLE test (a INT);
+SELECT 1 FROM pg_switch_wal();
+ ?column? 
+----------
+        1
+(1 row)
+
+DO $$
+DECLARE
+	archived bool;
+	loops int := 0;
+BEGIN
+	LOOP
+		archived := count(*) > 0 FROM pg_ls_dir('.', false, false) a
+			WHERE a ~ '^[0-9A-F]{24}$';
+		IF archived OR loops > 120 * 10 THEN EXIT; END IF;
+		PERFORM pg_sleep(0.1);
+		loops := loops + 1;
+	END LOOP;
+END
+$$;
+SELECT count(*) > 0 FROM pg_ls_dir('.', false, false) a
+	WHERE a ~ '^[0-9A-F]{24}$';
+ ?column? 
+----------
+ t
+(1 row)
+
+DROP TABLE test;
diff --git a/contrib/basic_archive/sql/basic_archive.sql b/contrib/basic_archive/sql/basic_archive.sql
new file mode 100644
index 0000000000..14e236d57a
--- /dev/null
+++ b/contrib/basic_archive/sql/basic_archive.sql
@@ -0,0 +1,22 @@
+CREATE TABLE test (a INT);
+SELECT 1 FROM pg_switch_wal();
+
+DO $$
+DECLARE
+	archived bool;
+	loops int := 0;
+BEGIN
+	LOOP
+		archived := count(*) > 0 FROM pg_ls_dir('.', false, false) a
+			WHERE a ~ '^[0-9A-F]{24}$';
+		IF archived OR loops > 120 * 10 THEN EXIT; END IF;
+		PERFORM pg_sleep(0.1);
+		loops := loops + 1;
+	END LOOP;
+END
+$$;
+
+SELECT count(*) > 0 FROM pg_ls_dir('.', false, false) a
+	WHERE a ~ '^[0-9A-F]{24}$';
+
+DROP TABLE test;
diff --git a/doc/src/sgml/archive-modules.sgml b/doc/src/sgml/archive-modules.sgml
new file mode 100644
index 0000000000..f1189ddcd5
--- /dev/null
+++ b/doc/src/sgml/archive-modules.sgml
@@ -0,0 +1,136 @@
+<!-- doc/src/sgml/archive-modules.sgml -->
+
+<chapter id="archive-modules">
+ <title>Archive Modules</title>
+ <indexterm zone="archive-modules">
+  <primary>Archive Modules</primary>
+ </indexterm>
+
+ <para>
+  PostgreSQL provides infrastructure to create custom modules for continuous
+  archiving (see <xref linkend="continuous-archiving"/>).  While archiving via
+  a shell command (i.e., <xref linkend="guc-archive-command"/>) is much
+  simpler, a custom archive module will often be considerably more robust and
+  performant.
+ </para>
+
+ <para>
+  When a custom <xref linkend="guc-archive-library"/> is configured, PostgreSQL
+  will submit completed WAL files to the module, and the server will avoid
+  recyling or removing these WAL files until the module indicates that the files
+  were successfully archived.  It is ultimately up to the module to decide what
+  to do with each WAL file, but many recommendations are listed at
+  <xref linkend="backup-archiving-wal"/>.
+ </para>
+
+ <para>
+  Archiving modules must at least consist of an initialization function (see
+  <xref linkend="archive-module-init"/>) and the required callbacks (see
+  <xref linkend="archive-module-callbacks"/>).  However, archive modules are
+  also permitted to do much more (e.g., declare GUCs and register background
+  workers).
+ </para>
+
+ <para>
+  The <filename>contrib/basic_archive</filename> module contains a working
+  example, which demonstrates some useful techniques.
+ </para>
+
+ <sect1 id="archive-module-init">
+  <title>Initialization Functions</title>
+  <indexterm zone="archive-module-init">
+   <primary>_PG_archive_module_init</primary>
+  </indexterm>
+  <para>
+   An archive library is loaded by dynamically loading a shared library with the
+   <xref linkend="guc-archive-library"/>'s name as the library base name.  The
+   normal library search path is used to locate the library.  To provide the
+   required archive module callbacks and to indicate that the library is
+   actually an archive module, it needs to provide a function named
+   <function>_PG_archive_module_init</function>.  This function is passed a
+   struct that needs to be filled with the callback function pointers for
+   individual actions.
+
+<programlisting>
+typedef struct ArchiveModuleCallbacks
+{
+    ArchiveCheckConfiguredCB check_configured_cb;
+    ArchiveFileCB archive_file_cb;
+    ArchiveShutdownCB shutdown_cb;
+} ArchiveModuleCallbacks;
+typedef void (*ArchiveModuleInit) (struct ArchiveModuleCallbacks *cb);
+</programlisting>
+
+   Only the <function>archive_file_cb</function> callback is required.  The
+   others are optional.
+  </para>
+ </sect1>
+
+ <sect1 id="archive-module-callbacks">
+  <title>Archive Module Callbacks</title>
+  <para>
+   The archive callbacks define the actual archiving behavior of the module.
+   The server will call them as required to process each individual WAL file.
+  </para>
+
+  <sect2 id="archive-module-check">
+   <title>Check Callback</title>
+   <para>
+    The <function>check_configured_cb</function> callback is called to determine
+    whether the module is fully configured and ready to accept WAL files (e.g.,
+    its configuration parameters are set to valid values).  If no
+    <function>check_configured_cb</function> is defined, the server always
+    assumes the module is configured.
+
+<programlisting>
+typedef bool (*ArchiveCheckConfiguredCB) (void);
+</programlisting>
+
+    If <literal>true</literal> is returned, the server will proceed with
+    archiving the file by calling the <function>archive_file_cb</function>
+    callback.  If <literal>false</literal> is returned, archiving will not
+    proceed, and the archiver will emit the following message to the server log:
+<screen>
+WARNING:  archive_mode enabled, yet archiving is not configured
+</screen>
+    In the latter case, the server will periodically call this function, and
+    archiving will proceed only when it returns <literal>true</literal>.
+   </para>
+  </sect2>
+
+  <sect2 id="archive-module-archive">
+   <title>Archive Callback</title>
+   <para>
+    The <function>archive_file_cb</function> callback is called to archive a
+    single WAL file.
+
+<programlisting>
+typedef bool (*ArchiveFileCB) (const char *file, const char *path);
+</programlisting>
+
+    If <literal>true</literal> is returned, the server proceeds as if the file
+    was successfully archived, which may include recycling or removing the
+    original WAL file.  If <literal>false</literal> is returned, the server will
+    keep the original WAL file and retry archiving later.
+    <literal>file</literal> will contain just the file name of the WAL file to
+    archive, while <literal>path</literal> contains the full path of the WAL
+    file (including the file name).
+   </para>
+  </sect2>
+
+  <sect2 id="archive-module-shutdown">
+   <title>Shutdown Callback</title>
+   <para>
+    The <function>shutdown_cb</function> callback is called when the archiver
+    process exits (e.g., after an error) or the value of
+    <xref linkend="guc-archive-library"/> changes.  If no
+    <function>shutdown_cb</function> is defined, no special action is taken in
+    these situations.
+
+<programlisting>
+typedef void (*ArchiveShutdownCB) (void);
+</programlisting>
+   </para>
+  </sect2>
+ </sect1>
+</chapter>
diff --git a/doc/src/sgml/backup.sgml b/doc/src/sgml/backup.sgml
index cba32b6eb3..0d69851bb1 100644
--- a/doc/src/sgml/backup.sgml
+++ b/doc/src/sgml/backup.sgml
@@ -593,20 +593,23 @@ tar -cf backup.tar /usr/local/pgsql/data
     provide the database administrator with flexibility,
     <productname>PostgreSQL</productname> tries not to make any assumptions about how
     the archiving will be done.  Instead, <productname>PostgreSQL</productname> lets
-    the administrator specify a shell command to be executed to copy a
-    completed segment file to wherever it needs to go.  The command could be
-    as simple as a <literal>cp</literal>, or it could invoke a complex shell
-    script &mdash; it's all up to you.
+    the administrator specify an archive library to be executed to copy a
+    completed segment file to wherever it needs to go.  This could be as simple
+    as a shell command that uses <literal>cp</literal>, or it could invoke a
+    complex C function &mdash; it's all up to you.
    </para>
 
    <para>
     To enable WAL archiving, set the <xref linkend="guc-wal-level"/>
     configuration parameter to <literal>replica</literal> or higher,
     <xref linkend="guc-archive-mode"/> to <literal>on</literal>,
-    and specify the shell command to use in the <xref
-    linkend="guc-archive-command"/> configuration parameter.  In practice
+    and specify the library to use in the <xref
+    linkend="guc-archive-library"/> configuration parameter.  In practice
     these settings will always be placed in the
     <filename>postgresql.conf</filename> file.
+    One simple way to archive is to set <varname>archive_library</varname> to
+    an empty string and to specify a shell command in
+    <xref linkend="guc-archive-command"/>.
     In <varname>archive_command</varname>,
     <literal>%p</literal> is replaced by the path name of the file to
     archive, while <literal>%f</literal> is replaced by only the file name.
@@ -631,7 +634,17 @@ test ! -f /mnt/server/archivedir/00000001000000A900000065 &amp;&amp; cp pg_wal/0
    </para>
 
    <para>
-    The archive command will be executed under the ownership of the same
+    Another way to archive is to use a custom archive module as the
+    <varname>archive_library</varname>.  Since such modules are written in
+    <literal>C</literal>, creating your own may require considerably more effort
+    than writing a shell command.  However, archive modules can be more
+    performant than archiving via shell, and they will have access to many
+    useful server resources.  For more information about archive modules, see
+    <xref linkend="archive-modules"/>.
+   </para>
+
+   <para>
+    The archive library will be executed under the ownership of the same
     user that the <productname>PostgreSQL</productname> server is running as.  Since
     the series of WAL files being archived contains effectively everything
     in your database, you will want to be sure that the archived data is
@@ -640,25 +653,31 @@ test ! -f /mnt/server/archivedir/00000001000000A900000065 &amp;&amp; cp pg_wal/0
    </para>
 
    <para>
-    It is important that the archive command return zero exit status if and
-    only if it succeeds.  Upon getting a zero result,
+    It is important that the archive function return <literal>true</literal> if
+    and only if it succeeds.  If <literal>true</literal> is returned,
     <productname>PostgreSQL</productname> will assume that the file has been
-    successfully archived, and will remove or recycle it.  However, a nonzero
-    status tells <productname>PostgreSQL</productname> that the file was not archived;
-    it will try again periodically until it succeeds.
+    successfully archived, and will remove or recycle it.  However, a return
+    value of <literal>false</literal> tells
+    <productname>PostgreSQL</productname> that the file was not archived; it
+    will try again periodically until it succeeds.  If you are archiving via a
+    shell command, the appropriate return values can be achieved by returning
+    <literal>0</literal> if the command succeeds and a nonzero value if it
+    fails.
    </para>
 
    <para>
-    When the archive command is terminated by a signal (other than
-    <systemitem>SIGTERM</systemitem> that is used as part of a server
-    shutdown) or an error by the shell with an exit status greater than
-    125 (such as command not found), the archiver process aborts and gets
-    restarted by the postmaster. In such cases, the failure is
-    not reported in <xref linkend="pg-stat-archiver-view"/>.
+    If the archive function emits an <literal>ERROR</literal> or
+    <literal>FATAL</literal>, the archiver process aborts and gets restarted by
+    the postmaster.  If you are archiving via shell command, FATAL is emitted if
+    the command is terminated by a signal (other than
+    <systemitem>SIGTERM</systemitem> that is used as part of a server shutdown)
+    or an error by the shell with an exit status greater than 125 (such as
+    command not found).  In such cases, the failure is not reported in
+    <xref linkend="pg-stat-archiver-view"/>.
    </para>
 
    <para>
-    The archive command should generally be designed to refuse to overwrite
+    The archive library should generally be designed to refuse to overwrite
     any pre-existing archive file.  This is an important safety feature to
     preserve the integrity of your archive in case of administrator error
     (such as sending the output of two different servers to the same archive
@@ -666,9 +685,9 @@ test ! -f /mnt/server/archivedir/00000001000000A900000065 &amp;&amp; cp pg_wal/0
    </para>
 
    <para>
-    It is advisable to test your proposed archive command to ensure that it
+    It is advisable to test your proposed archive library to ensure that it
     indeed does not overwrite an existing file, <emphasis>and that it returns
-    nonzero status in this case</emphasis>.
+    <literal>false</literal> in this case</emphasis>.
     The example command above for Unix ensures this by including a separate
     <command>test</command> step.  On some Unix platforms, <command>cp</command> has
     switches such as <option>-i</option> that can be used to do the same thing
@@ -680,7 +699,7 @@ test ! -f /mnt/server/archivedir/00000001000000A900000065 &amp;&amp; cp pg_wal/0
 
    <para>
     While designing your archiving setup, consider what will happen if
-    the archive command fails repeatedly because some aspect requires
+    the archive library fails repeatedly because some aspect requires
     operator intervention or the archive runs out of space. For example, this
     could occur if you write to tape without an autochanger; when the tape
     fills, nothing further can be archived until the tape is swapped.
@@ -695,7 +714,7 @@ test ! -f /mnt/server/archivedir/00000001000000A900000065 &amp;&amp; cp pg_wal/0
    </para>
 
    <para>
-    The speed of the archiving command is unimportant as long as it can keep up
+    The speed of the archive library is unimportant as long as it can keep up
     with the average rate at which your server generates WAL data.  Normal
     operation continues even if the archiving process falls a little behind.
     If archiving falls significantly behind, this will increase the amount of
@@ -707,11 +726,11 @@ test ! -f /mnt/server/archivedir/00000001000000A900000065 &amp;&amp; cp pg_wal/0
    </para>
 
    <para>
-    In writing your archive command, you should assume that the file names to
+    In writing your archive library, you should assume that the file names to
     be archived can be up to 64 characters long and can contain any
     combination of ASCII letters, digits, and dots.  It is not necessary to
-    preserve the original relative path (<literal>%p</literal>) but it is necessary to
-    preserve the file name (<literal>%f</literal>).
+    preserve the original relative path but it is necessary to preserve the file
+    name.
    </para>
 
    <para>
@@ -728,7 +747,7 @@ test ! -f /mnt/server/archivedir/00000001000000A900000065 &amp;&amp; cp pg_wal/0
    </para>
 
    <para>
-    The archive command is only invoked on completed WAL segments.  Hence,
+    The archive function is only invoked on completed WAL segments.  Hence,
     if your server generates only little WAL traffic (or has slack periods
     where it does so), there could be a long delay between the completion
     of a transaction and its safe recording in archive storage.  To put
@@ -757,8 +776,9 @@ test ! -f /mnt/server/archivedir/00000001000000A900000065 &amp;&amp; cp pg_wal/0
     turned on during execution of one of these statements, WAL would not
     contain enough information for archive recovery.  (Crash recovery is
     unaffected.)  For this reason, <varname>wal_level</varname> can only be changed at
-    server start.  However, <varname>archive_command</varname> can be changed with a
-    configuration file reload.  If you wish to temporarily stop archiving,
+    server start.  However, <varname>archive_library</varname> can be changed with a
+    configuration file reload.  If you are archiving via shell and wish to
+    temporarily stop archiving,
     one way to do it is to set <varname>archive_command</varname> to the empty
     string (<literal>''</literal>).
     This will cause WAL files to accumulate in <filename>pg_wal/</filename> until a
@@ -938,11 +958,11 @@ SELECT * FROM pg_stop_backup(false, true);
      On a standby, <varname>archive_mode</varname> must be <literal>always</literal> in order
      for <function>pg_stop_backup</function> to wait.
      Archiving of these files happens automatically since you have
-     already configured <varname>archive_command</varname>. In most cases this
+     already configured <varname>archive_library</varname>. In most cases this
      happens quickly, but you are advised to monitor your archive
      system to ensure there are no delays.
      If the archive process has fallen behind
-     because of failures of the archive command, it will keep retrying
+     because of failures of the archive library, it will keep retrying
      until the archive succeeds and the backup is complete.
      If you wish to place a time limit on the execution of
      <function>pg_stop_backup</function>, set an appropriate
@@ -1500,9 +1520,10 @@ restore_command = 'cp /mnt/server/archivedir/%f %p'
       To prepare for low level standalone hot backups, make sure
       <varname>wal_level</varname> is set to
       <literal>replica</literal> or higher, <varname>archive_mode</varname> to
-      <literal>on</literal>, and set up an <varname>archive_command</varname> that performs
+      <literal>on</literal>, and set up an <varname>archive_library</varname> that performs
       archiving only when a <emphasis>switch file</emphasis> exists.  For example:
 <programlisting>
+archive_library = ''  # use shell command
 archive_command = 'test ! -f /var/lib/pgsql/backup_in_progress || (test ! -f /var/lib/pgsql/archive/%f &amp;&amp; cp %p /var/lib/pgsql/archive/%f)'
 </programlisting>
       This command will perform archiving when
diff --git a/doc/src/sgml/basic-archive.sgml b/doc/src/sgml/basic-archive.sgml
new file mode 100644
index 0000000000..0b650f17a8
--- /dev/null
+++ b/doc/src/sgml/basic-archive.sgml
@@ -0,0 +1,81 @@
+<!-- doc/src/sgml/basic-archive.sgml -->
+
+<sect1 id="basic-archive" xreflabel="basic_archive">
+ <title>basic_archive</title>
+
+ <indexterm zone="basic-archive">
+  <primary>basic_archive</primary>
+ </indexterm>
+
+ <para>
+  <filename>basic_archive</filename> is an example of an archive module.  This
+  module copies completed WAL segment files to the specified directory.  This
+  may not be especially useful, but it can serve as a starting point for
+  developing your own archive module.  For more information about archive
+  modules, see <xref linkend="archive-modules"/>.
+ </para>
+
+ <para>
+  In order to function, this module must be loaded via
+  <xref linkend="guc-archive-library"/>, and <xref linkend="guc-archive-mode"/>
+  must be enabled.
+ </para>
+
+ <sect2>
+  <title>Configuration Parameters</title>
+
+  <variablelist>
+   <varlistentry>
+    <term>
+     <varname>basic_archive.archive_directory</varname> (<type>string</type>)
+     <indexterm>
+      <primary><varname>basic_archive.archive_directory</varname> configuration parameter</primary>
+     </indexterm>
+    </term>
+    <listitem>
+     <para>
+      The directory where the server should copy WAL segment files.  This
+      directory must already exist.  The default is an empty string, which
+      effectively halts WAL archiving, but if <xref linkend="guc-archive-mode"/>
+      is enabled, the server will accumulate WAL segment files in the
+      expectation that a value will soon be provided.
+     </para>
+    </listitem>
+   </varlistentry>
+  </variablelist>
+
+  <para>
+   These parameters must be set in <filename>postgresql.conf</filename>.
+   Typical usage might be:
+  </para>
+
+<programlisting>
+# postgresql.conf
+archive_mode = 'on'
+archive_library = 'basic_archive'
+basic_archive.archive_directory = '/path/to/archive/directory'
+</programlisting>
+ </sect2>
+
+ <sect2>
+  <title>Notes</title>
+
+  <para>
+   Server crashes may leave temporary files with the prefix
+   <filename>archtemp</filename> in the archive directory.  It is recommended to
+   delete such files before restarting the server after a crash.  It is safe to
+   remove such files while the server is running as long as they are unrelated
+   to any archiving still in progress, but users should use extra caution when
+   doing so.
+  </para>
+ </sect2>
+
+ <sect2>
+  <title>Author</title>
+
+  <para>
+   Nathan Bossart
+  </para>
+ </sect2>
+
+</sect1>
diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 692d8a2a17..fc63172efd 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -3479,7 +3479,7 @@ include_dir 'conf.d'
         Maximum size to let the WAL grow during automatic
         checkpoints. This is a soft limit; WAL size can exceed
         <varname>max_wal_size</varname> under special circumstances, such as
-        heavy load, a failing <varname>archive_command</varname>, or a high
+        heavy load, a failing <varname>archive_library</varname>, or a high
         <varname>wal_keep_size</varname> setting.
         If this value is specified without units, it is taken as megabytes.
         The default is 1 GB.
@@ -3528,7 +3528,7 @@ include_dir 'conf.d'
        <para>
         When <varname>archive_mode</varname> is enabled, completed WAL segments
         are sent to archive storage by setting
-        <xref linkend="guc-archive-command"/>. In addition to <literal>off</literal>,
+        <xref linkend="guc-archive-library"/>. In addition to <literal>off</literal>,
         to disable, there are two modes: <literal>on</literal>, and
         <literal>always</literal>. During normal operation, there is no
         difference between the two modes, but when set to <literal>always</literal>
@@ -3538,9 +3538,6 @@ include_dir 'conf.d'
         <xref linkend="continuous-archiving-in-standby"/> for details.
        </para>
        <para>
-        <varname>archive_mode</varname> and <varname>archive_command</varname> are
-        separate variables so that <varname>archive_command</varname> can be
-        changed without leaving archiving mode.
         This parameter can only be set at server start.
         <varname>archive_mode</varname> cannot be enabled when
         <varname>wal_level</varname> is set to <literal>minimal</literal>.
@@ -3548,6 +3545,28 @@ include_dir 'conf.d'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-archive-library" xreflabel="archive_library">
+      <term><varname>archive_library</varname> (<type>string</type>)
+      <indexterm>
+       <primary><varname>archive_library</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        The library to use for archiving completed WAL file segments.  If set to
+        an empty string (the default), archiving via shell is enabled, and
+        <xref linkend="guc-archive-command"/> is used.  Otherwise, the specified
+        shared library is used for archiving.  For more information, see
+        <xref linkend="backup-archiving-wal"/> and
+        <xref linkend="archive-modules"/>.
+       </para>
+       <para>
+        This parameter can only be set in the
+        <filename>postgresql.conf</filename> file or on the server command line.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-archive-command" xreflabel="archive_command">
       <term><varname>archive_command</varname> (<type>string</type>)
       <indexterm>
@@ -3570,9 +3589,11 @@ include_dir 'conf.d'
        <para>
         This parameter can only be set in the <filename>postgresql.conf</filename>
         file or on the server command line.  It is ignored unless
-        <varname>archive_mode</varname> was enabled at server start.
+        <varname>archive_mode</varname> was enabled at server start and
+        <varname>archive_library</varname> specifies to archive via shell command.
         If <varname>archive_command</varname> is an empty string (the default) while
-        <varname>archive_mode</varname> is enabled, WAL archiving is temporarily
+        <varname>archive_mode</varname> is enabled and <varname>archive_library</varname>
+        specifies archiving via shell, WAL archiving is temporarily
         disabled, but the server continues to accumulate WAL segment files in
         the expectation that a command will soon be provided.  Setting
         <varname>archive_command</varname> to a command that does nothing but
@@ -3592,7 +3613,7 @@ include_dir 'conf.d'
       </term>
       <listitem>
        <para>
-        The <xref linkend="guc-archive-command"/> is only invoked for
+        The <xref linkend="guc-archive-library"/> is only invoked for
         completed WAL segments. Hence, if your server generates little WAL
         traffic (or has slack periods where it does so), there could be a
         long delay between the completion of a transaction and its safe
diff --git a/doc/src/sgml/contrib.sgml b/doc/src/sgml/contrib.sgml
index d3ca4b6932..be9711c6f2 100644
--- a/doc/src/sgml/contrib.sgml
+++ b/doc/src/sgml/contrib.sgml
@@ -99,6 +99,7 @@ CREATE EXTENSION <replaceable>module_name</replaceable>;
  &amcheck;
  &auth-delay;
  &auto-explain;
+ &basic-archive;
  &bloom;
  &btree-gin;
  &btree-gist;
diff --git a/doc/src/sgml/filelist.sgml b/doc/src/sgml/filelist.sgml
index 89454e99b9..328cd1f378 100644
--- a/doc/src/sgml/filelist.sgml
+++ b/doc/src/sgml/filelist.sgml
@@ -99,6 +99,7 @@
 <!ENTITY custom-scan SYSTEM "custom-scan.sgml">
 <!ENTITY logicaldecoding SYSTEM "logicaldecoding.sgml">
 <!ENTITY replication-origins SYSTEM "replication-origins.sgml">
+<!ENTITY archive-modules SYSTEM "archive-modules.sgml">
 <!ENTITY protocol   SYSTEM "protocol.sgml">
 <!ENTITY sources    SYSTEM "sources.sgml">
 <!ENTITY storage    SYSTEM "storage.sgml">
@@ -112,6 +113,7 @@
 <!ENTITY amcheck         SYSTEM "amcheck.sgml">
 <!ENTITY auth-delay      SYSTEM "auth-delay.sgml">
 <!ENTITY auto-explain    SYSTEM "auto-explain.sgml">
+<!ENTITY basic-archive   SYSTEM "basic-archive.sgml">
 <!ENTITY bloom           SYSTEM "bloom.sgml">
 <!ENTITY btree-gin       SYSTEM "btree-gin.sgml">
 <!ENTITY btree-gist      SYSTEM "btree-gist.sgml">
diff --git a/doc/src/sgml/high-availability.sgml b/doc/src/sgml/high-availability.sgml
index a265409f02..437712762a 100644
--- a/doc/src/sgml/high-availability.sgml
+++ b/doc/src/sgml/high-availability.sgml
@@ -935,7 +935,7 @@ primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass'
     In lieu of using replication slots, it is possible to prevent the removal
     of old WAL segments using <xref linkend="guc-wal-keep-size"/>, or by
     storing the segments in an archive using
-    <xref linkend="guc-archive-command"/>.
+    <xref linkend="guc-archive-library"/>.
     However, these methods often result in retaining more WAL segments than
     required, whereas replication slots retain only the number of segments
     known to be needed.  On the other hand, replication slots can retain so
@@ -1386,10 +1386,10 @@ synchronous_standby_names = 'ANY 2 (s1, s2, s3)'
      to <literal>always</literal>, and the standby will call the archive
      command for every WAL segment it receives, whether it's by restoring
      from the archive or by streaming replication. The shared archive can
-     be handled similarly, but the <varname>archive_command</varname> must
+     be handled similarly, but the <varname>archive_library</varname> must
      test if the file being archived exists already, and if the existing file
      has identical contents. This requires more care in the
-     <varname>archive_command</varname>, as it must
+     <varname>archive_library</varname>, as it must
      be careful to not overwrite an existing file with different contents,
      but return success if the exactly same file is archived twice. And
      all that must be done free of race conditions, if two servers attempt
diff --git a/doc/src/sgml/postgres.sgml b/doc/src/sgml/postgres.sgml
index dba9cf413f..3db6d2160b 100644
--- a/doc/src/sgml/postgres.sgml
+++ b/doc/src/sgml/postgres.sgml
@@ -233,6 +233,7 @@ break is not needed in a wider output rendering.
   &bgworker;
   &logicaldecoding;
   &replication-origins;
+  &archive-modules;
 
  </part>
 
diff --git a/doc/src/sgml/ref/pg_basebackup.sgml b/doc/src/sgml/ref/pg_basebackup.sgml
index 1546f10c0d..e7ae29ec3d 100644
--- a/doc/src/sgml/ref/pg_basebackup.sgml
+++ b/doc/src/sgml/ref/pg_basebackup.sgml
@@ -102,8 +102,8 @@ PostgreSQL documentation
      <para>
       All WAL records required for the backup must contain sufficient full-page writes,
       which requires you to enable <varname>full_page_writes</varname> on the primary and
-      not to use a tool like <application>pg_compresslog</application> as
-      <varname>archive_command</varname> to remove full-page writes from WAL files.
+      not to use a tool in your <varname>archive_library</varname> to remove
+      full-page writes from WAL files.
      </para>
     </listitem>
    </itemizedlist>
diff --git a/doc/src/sgml/ref/pg_receivewal.sgml b/doc/src/sgml/ref/pg_receivewal.sgml
index b2e41ea814..b846213fb7 100644
--- a/doc/src/sgml/ref/pg_receivewal.sgml
+++ b/doc/src/sgml/ref/pg_receivewal.sgml
@@ -40,7 +40,7 @@ PostgreSQL documentation
   <para>
    <application>pg_receivewal</application> streams the write-ahead
    log in real time as it's being generated on the server, and does not wait
-   for segments to complete like <xref linkend="guc-archive-command"/> does.
+   for segments to complete like <xref linkend="guc-archive-library"/> does.
    For this reason, it is not necessary to set
    <xref linkend="guc-archive-timeout"/> when using
     <application>pg_receivewal</application>.
@@ -487,11 +487,11 @@ PostgreSQL documentation
 
   <para>
    When using <application>pg_receivewal</application> instead of
-   <xref linkend="guc-archive-command"/> as the main WAL backup method, it is
+   <xref linkend="guc-archive-library"/> as the main WAL backup method, it is
    strongly recommended to use replication slots.  Otherwise, the server is
    free to recycle or remove write-ahead log files before they are backed up,
    because it does not have any information, either
-   from <xref linkend="guc-archive-command"/> or the replication slots, about
+   from <xref linkend="guc-archive-library"/> or the replication slots, about
    how far the WAL stream has been archived.  Note, however, that a
    replication slot will fill up the server's disk space if the receiver does
    not keep up with fetching the WAL data.
diff --git a/doc/src/sgml/wal.sgml b/doc/src/sgml/wal.sgml
index 24e1c89503..2bb27a8468 100644
--- a/doc/src/sgml/wal.sgml
+++ b/doc/src/sgml/wal.sgml
@@ -636,7 +636,7 @@
    WAL files plus one additional WAL file are
    kept at all times. Also, if WAL archiving is used, old segments cannot be
    removed or recycled until they are archived. If WAL archiving cannot keep up
-   with the pace that WAL is generated, or if <varname>archive_command</varname>
+   with the pace that WAL is generated, or if <varname>archive_library</varname>
    fails repeatedly, old WAL files will accumulate in <filename>pg_wal</filename>
    until the situation is resolved. A slow or failed standby server that
    uses a replication slot will have the same effect (see
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index dfe2a0bcce..958220c495 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -8831,7 +8831,7 @@ ShutdownXLOG(int code, Datum arg)
 		 * process one more time at the end of shutdown). The checkpoint
 		 * record will go to the next XLOG file and won't be archived (yet).
 		 */
-		if (XLogArchivingActive() && XLogArchiveCommandSet())
+		if (XLogArchivingActive())
 			RequestXLogSwitch(false);
 
 		CreateCheckPoint(CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_IMMEDIATE);
diff --git a/src/backend/postmaster/pgarch.c b/src/backend/postmaster/pgarch.c
index 6e3fcedc97..d916ed39a8 100644
--- a/src/backend/postmaster/pgarch.c
+++ b/src/backend/postmaster/pgarch.c
@@ -89,6 +89,8 @@ typedef struct PgArchData
 	slock_t		arch_lck;
 } PgArchData;
 
+char *XLogArchiveLibrary = "";
+
 
 /* ----------
  * Local data
@@ -96,6 +98,8 @@ typedef struct PgArchData
  */
 static time_t last_sigterm_time = 0;
 static PgArchData *PgArch = NULL;
+static ArchiveModuleCallbacks ArchiveContext;
+
 
 /*
  * Stuff for tracking multiple files to archive from each scan of
@@ -140,6 +144,8 @@ static void pgarch_archiveDone(char *xlog);
 static void pgarch_die(int code, Datum arg);
 static void HandlePgArchInterrupts(void);
 static int ready_file_comparator(Datum a, Datum b, void *arg);
+static void LoadArchiveLibrary(void);
+static void call_archive_module_shutdown_callback(int code, Datum arg);
 
 /* Report shared memory space needed by PgArchShmemInit */
 Size
@@ -244,7 +250,16 @@ PgArchiverMain(void)
 	arch_files->arch_heap = binaryheap_allocate(NUM_FILES_PER_DIRECTORY_SCAN,
 												ready_file_comparator, NULL);
 
-	pgarch_MainLoop();
+	/* Load the archive_library. */
+	LoadArchiveLibrary();
+
+	PG_ENSURE_ERROR_CLEANUP(call_archive_module_shutdown_callback, 0);
+	{
+		pgarch_MainLoop();
+	}
+	PG_END_ENSURE_ERROR_CLEANUP(call_archive_module_shutdown_callback, 0);
+
+	call_archive_module_shutdown_callback(0, 0);
 
 	proc_exit(0);
 }
@@ -407,11 +422,12 @@ pgarch_ArchiverCopyLoop(void)
 			 */
 			HandlePgArchInterrupts();
 
-			/* can't do anything if no command ... */
-			if (!XLogArchiveCommandSet())
+			/* can't do anything if not configured ... */
+			if (ArchiveContext.check_configured_cb != NULL &&
+				!ArchiveContext.check_configured_cb())
 			{
 				ereport(WARNING,
-						(errmsg("archive_mode enabled, yet archive_command is not set")));
+						(errmsg("archive_mode enabled, yet archiving is not configured")));
 				return;
 			}
 
@@ -492,7 +508,7 @@ pgarch_ArchiverCopyLoop(void)
 /*
  * pgarch_archiveXlog
  *
- * Invokes system(3) to copy one archive file to wherever it should go
+ * Invokes archive_file_cb to copy one archive file to wherever it should go
  *
  * Returns true if successful
  */
@@ -509,7 +525,7 @@ pgarch_archiveXlog(char *xlog)
 	snprintf(activitymsg, sizeof(activitymsg), "archiving %s", xlog);
 	set_ps_display(activitymsg);
 
-	ret = shell_archive_file(xlog, pathname);
+	ret = ArchiveContext.archive_file_cb(xlog, pathname);
 	if (ret)
 		snprintf(activitymsg, sizeof(activitymsg), "last was %s", xlog);
 	else
@@ -759,13 +775,89 @@ HandlePgArchInterrupts(void)
 	if (ProcSignalBarrierPending)
 		ProcessProcSignalBarrier();
 
+	/* Perform logging of memory contexts of this process */
+	if (LogMemoryContextPending)
+		ProcessLogMemoryContextInterrupt();
+
 	if (ConfigReloadPending)
 	{
+		char	   *archiveLib = pstrdup(XLogArchiveLibrary);
+		bool		archiveLibChanged;
+
 		ConfigReloadPending = false;
 		ProcessConfigFile(PGC_SIGHUP);
+
+		archiveLibChanged = strcmp(XLogArchiveLibrary, archiveLib) != 0;
+		pfree(archiveLib);
+
+		if (archiveLibChanged)
+		{
+			/*
+			 * Call the currently loaded archive module's shutdown callback, if
+			 * one is defined.
+			 */
+			call_archive_module_shutdown_callback(0, 0);
+
+			/*
+			 * Ideally, we would simply unload the previous archive module and
+			 * load the new one, but there is presently no mechanism for
+			 * unloading a library (see the comment above
+			 * internal_unload_library()).  To deal with this, we simply restart
+			 * the archiver.  The new archive module will be loaded when the new
+			 * archiver process starts up.
+			 */
+			ereport(LOG,
+					(errmsg("restarting archiver process because value of "
+							"\"archive_library\" was changed")));
+
+			proc_exit(0);
+		}
 	}
+}
 
-	/* Perform logging of memory contexts of this process */
-	if (LogMemoryContextPending)
-		ProcessLogMemoryContextInterrupt();
+/*
+ * LoadArchiveLibrary
+ *
+ * Loads the archiving callbacks into our local ArchiveContext.
+ */
+static void
+LoadArchiveLibrary(void)
+{
+	ArchiveModuleInit archive_init;
+
+	memset(&ArchiveContext, 0, sizeof(ArchiveModuleCallbacks));
+
+	/*
+	 * If shell archiving is enabled, use our special initialization
+	 * function.  Otherwise, load the library and call its
+	 * _PG_archive_module_init().
+	 */
+	if (XLogArchiveLibrary[0] == '\0')
+		archive_init = shell_archive_init;
+	else
+		archive_init = (ArchiveModuleInit)
+			load_external_function(XLogArchiveLibrary,
+								   "_PG_archive_module_init", false, NULL);
+
+	if (archive_init == NULL)
+		ereport(ERROR,
+				(errmsg("archive modules have to declare the _PG_archive_module_init symbol")));
+
+	(*archive_init) (&ArchiveContext);
+
+	if (ArchiveContext.archive_file_cb == NULL)
+		ereport(ERROR,
+				(errmsg("archive modules must register an archive callback")));
+}
+
+/*
+ * call_archive_module_shutdown_callback
+ *
+ * Calls the loaded archive module's shutdown callback, if one is defined.
+ */
+static void
+call_archive_module_shutdown_callback(int code, Datum arg)
+{
+	if (ArchiveContext.shutdown_cb != NULL)
+		ArchiveContext.shutdown_cb();
 }
diff --git a/src/backend/postmaster/shell_archive.c b/src/backend/postmaster/shell_archive.c
index b54e701da4..19e240c205 100644
--- a/src/backend/postmaster/shell_archive.c
+++ b/src/backend/postmaster/shell_archive.c
@@ -2,6 +2,10 @@
  *
  * shell_archive.c
  *
+ * This archiving function uses a user-specified shell command (the
+ * archive_command GUC) to copy write-ahead log files.  It is used as the
+ * default, but other modules may define their own custom archiving logic.
+ *
  * Copyright (c) 2022, PostgreSQL Global Development Group
  *
  * IDENTIFICATION
@@ -17,7 +21,25 @@
 #include "pgstat.h"
 #include "postmaster/pgarch.h"
 
-bool
+static bool shell_archive_configured(void);
+static bool shell_archive_file(const char *file, const char *path);
+
+void
+shell_archive_init(ArchiveModuleCallbacks *cb)
+{
+	AssertVariableIsOfType(&shell_archive_init, ArchiveModuleInit);
+
+	cb->check_configured_cb = shell_archive_configured;
+	cb->archive_file_cb = shell_archive_file;
+}
+
+static bool
+shell_archive_configured(void)
+{
+	return XLogArchiveCommand[0] != '\0';
+}
+
+static bool
 shell_archive_file(const char *file, const char *path)
 {
 	char		xlogarchcmd[MAXPGPATH];
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 0f2570d626..0868e5a24f 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -38,6 +38,7 @@
 #include "pgstat.h"
 #include "postmaster/autovacuum.h"
 #include "postmaster/interrupt.h"
+#include "postmaster/pgarch.h"
 #include "postmaster/postmaster.h"
 #include "storage/fd.h"
 #include "storage/ipc.h"
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index b3fd42e0f1..f505413a7f 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -3881,7 +3881,7 @@ static struct config_string ConfigureNamesString[] =
 	{
 		{"archive_command", PGC_SIGHUP, WAL_ARCHIVING,
 			gettext_noop("Sets the shell command that will be called to archive a WAL file."),
-			NULL
+			gettext_noop("This is used only if \"archive_library\" is not set.")
 		},
 		&XLogArchiveCommand,
 		"",
@@ -3889,6 +3889,16 @@ static struct config_string ConfigureNamesString[] =
 	},
 
 	{
+		{"archive_library", PGC_SIGHUP, WAL_ARCHIVING,
+			gettext_noop("Sets the library that will be called to archive a WAL file."),
+			gettext_noop("An empty string indicates that \"archive_command\" should be used.")
+		},
+		&XLogArchiveLibrary,
+		"",
+		NULL, NULL, NULL
+	},
+
+	{
 		{"restore_command", PGC_SIGHUP, WAL_ARCHIVE_RECOVERY,
 			gettext_noop("Sets the shell command that will be called to retrieve an archived WAL file."),
 			NULL
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index 817d5f5324..56d0bee6d9 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -245,6 +245,9 @@
 
 #archive_mode = off		# enables archiving; off, on, or always
 				# (change requires restart)
+#archive_library = ''		# library to use to archive a logfile segment
+				# (empty string indicates archive_command should
+				# be used)
 #archive_command = ''		# command to use to archive a logfile segment
 				# placeholders: %p = path of file to archive
 				#               %f = file name only
diff --git a/src/include/access/xlog.h b/src/include/access/xlog.h
index 5f934dd65a..a4b1c1286f 100644
--- a/src/include/access/xlog.h
+++ b/src/include/access/xlog.h
@@ -154,7 +154,6 @@ extern PGDLLIMPORT int wal_level;
 /* Is WAL archiving enabled always (even during recovery)? */
 #define XLogArchivingAlways() \
 	(AssertMacro(XLogArchiveMode == ARCHIVE_MODE_OFF || wal_level >= WAL_LEVEL_REPLICA), XLogArchiveMode == ARCHIVE_MODE_ALWAYS)
-#define XLogArchiveCommandSet() (XLogArchiveCommand[0] != '\0')
 
 /*
  * Is WAL-logging necessary for archival or log-shipping, or can we skip
diff --git a/src/include/postmaster/pgarch.h b/src/include/postmaster/pgarch.h
index 991a6d0616..9bc7593a2d 100644
--- a/src/include/postmaster/pgarch.h
+++ b/src/include/postmaster/pgarch.h
@@ -33,7 +33,41 @@ extern void PgArchiverMain(void) pg_attribute_noreturn();
 extern void PgArchWakeup(void);
 extern void PgArchForceDirScan(void);
 
-/* in shell_archive.c */
-extern bool shell_archive_file(const char *file, const char *path);
+/*
+ * The value of the archive_library GUC.
+ */
+extern char *XLogArchiveLibrary;
+
+/*
+ * Archive module callbacks
+ *
+ * These callback functions should be defined by archive libraries and returned
+ * via _PG_archive_module_init().  ArchiveFileCB is the only required callback.
+ * For more information about the purpose of each callback, refer to the
+ * archive modules documentation.
+ */
+typedef bool (*ArchiveCheckConfiguredCB) (void);
+typedef bool (*ArchiveFileCB) (const char *file, const char *path);
+typedef void (*ArchiveShutdownCB) (void);
+
+typedef struct ArchiveModuleCallbacks
+{
+	ArchiveCheckConfiguredCB check_configured_cb;
+	ArchiveFileCB archive_file_cb;
+	ArchiveShutdownCB shutdown_cb;
+} ArchiveModuleCallbacks;
+
+/*
+ * Type of the shared library symbol _PG_archive_module_init that is looked
+ * up when loading an archive library.
+ */
+typedef void (*ArchiveModuleInit) (ArchiveModuleCallbacks *cb);
+
+/*
+ * Since the logic for archiving via a shell command is in the core server
+ * and does not need to be loaded via a shared library, it has a special
+ * initialization function.
+ */
+extern void shell_archive_init(ArchiveModuleCallbacks *cb);
 
 #endif							/* _PGARCH_H */