compression: add build-time option to select default

Compression and decompression are controlled by the same build flag,
so if one wants to use, say, LZ4 to compress, ZSTD has to be disabled,
which means one loses the ability to read zstd-compressed journals.

Add a default-compression meson option, that allows to select any of
the available compression algorithms as the default.

5 files changed, 37 insertions, 13 deletions

diff --git a/meson.build b/meson.build
index d60be1b050..723bc8dac1 100644
--- a/meson.build
+++ b/meson.build
@@ -1452,6 +1452,30 @@ conf.set10('HAVE_ZSTD', have_zstd)
 
 conf.set10('HAVE_COMPRESSION', have_xz or have_lz4 or have_zstd)
 
+compression = get_option('default-compression')
+if compression == 'auto'
+        if have_zstd
+                compression = 'zstd'
+        elif have_lz4
+                compression = 'lz4'
+        elif have_xz
+                compression = 'xz'
+        else
+                compression = 'none'
+        endif
+elif compression == 'zstd' and not have_zstd
+        error('default-compression=zstd requires zstd')
+elif compression == 'lz4' and not have_lz4
+        error('default-compression=lz4 requires lz4')
+elif compression == 'xz' and not have_xz
+        error('default-compression=xz requires xz')
+endif
+conf.set('OBJECT_COMPRESSED_NONE', 0)
+conf.set('OBJECT_COMPRESSED_XZ', 1)
+conf.set('OBJECT_COMPRESSED_LZ4', 2)
+conf.set('OBJECT_COMPRESSED_ZSTD', 4)
+conf.set('DEFAULT_COMPRESSION', 'OBJECT_COMPRESSED_@0@'.format(compression.to_upper()))
+
 want_xkbcommon = get_option('xkbcommon')
 if want_xkbcommon != 'false' and not skip_deps
         libxkbcommon = dependency('xkbcommon',
diff --git a/meson_options.txt b/meson_options.txt
index a315ca47c5..2a030ac28e 100644
--- a/meson_options.txt
+++ b/meson_options.txt
@@ -411,6 +411,8 @@ option('lz4', type : 'combo', choices : ['auto', 'true', 'false'],
        description : 'lz4 compression support')
 option('zstd', type : 'combo', choices : ['auto', 'true', 'false'],
        description : 'zstd compression support')
+option('default-compression', type : 'combo', choices : ['auto', 'zstd', 'lz4', 'xz'], value: 'auto',
+       description : 'default compression algorithm')
 option('xkbcommon', type : 'combo', choices : ['auto', 'true', 'false'],
        description : 'xkbcommon keymap support')
 option('pcre2', type : 'combo', choices : ['auto', 'true', 'false'],
diff --git a/src/libsystemd/sd-journal/compress.h b/src/libsystemd/sd-journal/compress.h
index 005e60e2e3..40ab43c4cb 100644
--- a/src/libsystemd/sd-journal/compress.h
+++ b/src/libsystemd/sd-journal/compress.h
@@ -18,15 +18,15 @@ int compress_blob_zstd(const void *src, uint64_t src_size,
 static inline int compress_blob(const void *src, uint64_t src_size,
                                 void *dst, size_t dst_alloc_size, size_t *dst_size) {
         int r;
-#if HAVE_ZSTD
+#if DEFAULT_COMPRESSION == OBJECT_COMPRESSED_ZSTD
         r = compress_blob_zstd(src, src_size, dst, dst_alloc_size, dst_size);
         if (r == 0)
                 return OBJECT_COMPRESSED_ZSTD;
-#elif HAVE_LZ4
+#elif DEFAULT_COMPRESSION == OBJECT_COMPRESSED_LZ4
         r = compress_blob_lz4(src, src_size, dst, dst_alloc_size, dst_size);
         if (r == 0)
                 return OBJECT_COMPRESSED_LZ4;
-#elif HAVE_XZ
+#elif DEFAULT_COMPRESSION == OBJECT_COMPRESSED_XZ
         r = compress_blob_xz(src, src_size, dst, dst_alloc_size, dst_size);
         if (r == 0)
                 return OBJECT_COMPRESSED_XZ;
@@ -72,13 +72,13 @@ int decompress_stream_xz(int fdf, int fdt, uint64_t max_size);
 int decompress_stream_lz4(int fdf, int fdt, uint64_t max_size);
 int decompress_stream_zstd(int fdf, int fdt, uint64_t max_size);
 
-#if HAVE_ZSTD
+#if DEFAULT_COMPRESSION == OBJECT_COMPRESSED_ZSTD
 #  define compress_stream compress_stream_zstd
 #  define COMPRESSED_EXT ".zst"
-#elif HAVE_LZ4
+#elif DEFAULT_COMPRESSION == OBJECT_COMPRESSED_LZ4
 #  define compress_stream compress_stream_lz4
 #  define COMPRESSED_EXT ".lz4"
-#elif HAVE_XZ
+#elif DEFAULT_COMPRESSION == OBJECT_COMPRESSED_XZ
 #  define compress_stream compress_stream_xz
 #  define COMPRESSED_EXT ".xz"
 #else
diff --git a/src/libsystemd/sd-journal/journal-def.h b/src/libsystemd/sd-journal/journal-def.h
index d64c70cbe6..3259b0a515 100644
--- a/src/libsystemd/sd-journal/journal-def.h
+++ b/src/libsystemd/sd-journal/journal-def.h
@@ -42,11 +42,9 @@ typedef enum ObjectType {
         _OBJECT_TYPE_MAX
 } ObjectType;
 
-/* Object flags */
+/* Object flags
+ * The per-compression enums are defined in meson.build so that config.h is self-contained */
 enum {
-        OBJECT_COMPRESSED_XZ   = 1 << 0,
-        OBJECT_COMPRESSED_LZ4  = 1 << 1,
-        OBJECT_COMPRESSED_ZSTD = 1 << 2,
         OBJECT_COMPRESSION_MASK = (OBJECT_COMPRESSED_XZ | OBJECT_COMPRESSED_LZ4 | OBJECT_COMPRESSED_ZSTD),
         _OBJECT_COMPRESSED_MAX = OBJECT_COMPRESSION_MASK,
 };
diff --git a/src/libsystemd/sd-journal/journal-file.c b/src/libsystemd/sd-journal/journal-file.c
index e72f041bdc..a6b2a0f326 100644
--- a/src/libsystemd/sd-journal/journal-file.c
+++ b/src/libsystemd/sd-journal/journal-file.c
@@ -3359,11 +3359,11 @@ int journal_file_open(
                 .open_flags = open_flags,
                 .writable = (open_flags & O_ACCMODE) != O_RDONLY,
 
-#if HAVE_ZSTD
+#if DEFAULT_COMPRESSION == OBJECT_COMPRESSED_ZSTD
                 .compress_zstd = FLAGS_SET(file_flags, JOURNAL_COMPRESS),
-#elif HAVE_LZ4
+#elif DEFAULT_COMPRESSION == OBJECT_COMPRESSED_LZ4
                 .compress_lz4 = FLAGS_SET(file_flags, JOURNAL_COMPRESS),
-#elif HAVE_XZ
+#elif DEFAULT_COMPRESSION == OBJECT_COMPRESSED_XZ
                 .compress_xz = FLAGS_SET(file_flags, JOURNAL_COMPRESS),
 #endif
                 .compress_threshold_bytes = compress_threshold_bytes == UINT64_MAX ?