clone: clone from a repository with relative alternates

Cloning from a local repository blindly copies or hardlinks all the files
under objects/ hierarchy. This results in two issues:

 - If the repository cloned has an "objects/info/alternates" file, and the
   command line of clone specifies --reference, the ones specified on the
   command line get overwritten by the copy from the original repository.

 - An entry in a "objects/info/alternates" file can specify the object
   stores it borrows objects from as a path relative to the "objects/"
   directory. When cloning a repository with such an alternates file, if
   the new repository is not sitting next to the original repository, such
   relative paths needs to be adjusted so that they can be used in the new
   repository.

This updates add_to_alternates_file() to take the path to the alternate
object store, including the "/objects" part at the end (earlier, it was
taking the path to $GIT_DIR and was adding "/objects" itself), as it is
technically possible to specify in objects/info/alternates file the path
of a directory whose name does not end with "/objects".

Signed-off-by: Junio C Hamano <gitster@pobox.com>

3 files changed, 78 insertions, 25 deletions

diff --git a/builtin/clone.c b/builtin/clone.c
index ee5d6518c8..16b4fba71f 100644
--- a/builtin/clone.c
+++ b/builtin/clone.c
@@ -209,34 +209,34 @@ static void strip_trailing_slashes(char *dir)
 
 static int add_one_reference(struct string_list_item *item, void *cb_data)
 {
-	const char *ref_git;
-	char *ref_git_copy;
-
+	char *ref_git;
+	struct strbuf alternate = STRBUF_INIT;
 	struct remote *remote;
 	struct transport *transport;
 	const struct ref *extra;
 
-	ref_git = real_path(item->string);
-
-	if (is_directory(mkpath("%s/.git/objects", ref_git)))
-		ref_git = mkpath("%s/.git", ref_git);
-	else if (!is_directory(mkpath("%s/objects", ref_git)))
+	/* Beware: real_path() and mkpath() return static buffer */
+	ref_git = xstrdup(real_path(item->string));
+	if (is_directory(mkpath("%s/.git/objects", ref_git))) {
+		char *ref_git_git = xstrdup(mkpath("%s/.git", ref_git));
+		free(ref_git);
+		ref_git = ref_git_git;
+	} else if (!is_directory(mkpath("%s/objects", ref_git)))
 		die(_("reference repository '%s' is not a local directory."),
 		    item->string);
 
-	ref_git_copy = xstrdup(ref_git);
-
-	add_to_alternates_file(ref_git_copy);
+	strbuf_addf(&alternate, "%s/objects", ref_git);
+	add_to_alternates_file(alternate.buf);
+	strbuf_release(&alternate);
 
-	remote = remote_get(ref_git_copy);
-	transport = transport_get(remote, ref_git_copy);
+	remote = remote_get(ref_git);
+	transport = transport_get(remote, ref_git);
 	for (extra = transport_get_remote_refs(transport); extra;
 	     extra = extra->next)
 		add_extra_ref(extra->name, extra->old_sha1, 0);
 
 	transport_disconnect(transport);
-
-	free(ref_git_copy);
+	free(ref_git);
 	return 0;
 }
 
@@ -245,7 +245,42 @@ static void setup_reference(void)
 	for_each_string_list(&option_reference, add_one_reference, NULL);
 }
 
-static void copy_or_link_directory(struct strbuf *src, struct strbuf *dest)
+static void copy_alternates(struct strbuf *src, struct strbuf *dst,
+			    const char *src_repo)
+{
+	/*
+	 * Read from the source objects/info/alternates file
+	 * and copy the entries to corresponding file in the
+	 * destination repository with add_to_alternates_file().
+	 * Both src and dst have "$path/objects/info/alternates".
+	 *
+	 * Instead of copying bit-for-bit from the original,
+	 * we need to append to existing one so that the already
+	 * created entry via "clone -s" is not lost, and also
+	 * to turn entries with paths relative to the original
+	 * absolute, so that they can be used in the new repository.
+	 */
+	FILE *in = fopen(src->buf, "r");
+	struct strbuf line = STRBUF_INIT;
+
+	while (strbuf_getline(&line, in, '\n') != EOF) {
+		char *abs_path, abs_buf[PATH_MAX];
+		if (!line.len || line.buf[0] == '#')
+			continue;
+		if (is_absolute_path(line.buf)) {
+			add_to_alternates_file(line.buf);
+			continue;
+		}
+		abs_path = mkpath("%s/objects/%s", src_repo, line.buf);
+		normalize_path_copy(abs_buf, abs_path);
+		add_to_alternates_file(abs_buf);
+	}
+	strbuf_release(&line);
+	fclose(in);
+}
+
+static void copy_or_link_directory(struct strbuf *src, struct strbuf *dest,
+				   const char *src_repo, int src_baselen)
 {
 	struct dirent *de;
 	struct stat buf;
@@ -281,7 +316,14 @@ static void copy_or_link_directory(struct strbuf *src, struct strbuf *dest)
 		}
 		if (S_ISDIR(buf.st_mode)) {
 			if (de->d_name[0] != '.')
-				copy_or_link_directory(src, dest);
+				copy_or_link_directory(src, dest,
+						       src_repo, src_baselen);
+			continue;
+		}
+
+		/* Files that cannot be copied bit-for-bit... */
+		if (!strcmp(src->buf + src_baselen, "/info/alternates")) {
+			copy_alternates(src, dest, src_repo);
 			continue;
 		}
 
@@ -304,17 +346,20 @@ static const struct ref *clone_local(const char *src_repo,
 				     const char *dest_repo)
 {
 	const struct ref *ret;
-	struct strbuf src = STRBUF_INIT;
-	struct strbuf dest = STRBUF_INIT;
 	struct remote *remote;
 	struct transport *transport;
 
-	if (option_shared)
-		add_to_alternates_file(src_repo);
-	else {
+	if (option_shared) {
+		struct strbuf alt = STRBUF_INIT;
+		strbuf_addf(&alt, "%s/objects", src_repo);
+		add_to_alternates_file(alt.buf);
+		strbuf_release(&alt);
+	} else {
+		struct strbuf src = STRBUF_INIT;
+		struct strbuf dest = STRBUF_INIT;
 		strbuf_addf(&src, "%s/objects", src_repo);
 		strbuf_addf(&dest, "%s/objects", dest_repo);
-		copy_or_link_directory(&src, &dest);
+		copy_or_link_directory(&src, &dest, src_repo, src.len);
 		strbuf_release(&src);
 		strbuf_release(&dest);
 	}
diff --git a/sha1_file.c b/sha1_file.c
index 064a330408..f7c3408de6 100644
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -380,7 +380,7 @@ void add_to_alternates_file(const char *reference)
 {
 	struct lock_file *lock = xcalloc(1, sizeof(struct lock_file));
 	int fd = hold_lock_file_for_append(lock, git_path("objects/info/alternates"), LOCK_DIE_ON_ERROR);
-	char *alt = mkpath("%s/objects\n", reference);
+	char *alt = mkpath("%s\n", reference);
 	write_or_die(fd, alt, strlen(alt));
 	if (commit_lock_file(lock))
 		die("could not close alternates file");
diff --git a/t/t5601-clone.sh b/t/t5601-clone.sh
index 0163ad1e21..d87214cfbf 100755
--- a/t/t5601-clone.sh
+++ b/t/t5601-clone.sh
@@ -207,7 +207,7 @@ test_expect_success 'clone separate gitdir where target already exists' '
 	test_must_fail git clone --separate-git-dir realgitdir src dst
 '
 
-test_expect_failure 'clone --reference from original' '
+test_expect_success 'clone --reference from original' '
 	git clone --shared --bare src src-1 &&
 	git clone --bare src src-2 &&
 	git clone --reference=src-2 --bare src-1 target-8 &&
@@ -222,4 +222,12 @@ test_expect_success 'clone with more than one --reference' '
 	grep /src-4/ target-9/.git/objects/info/alternates
 '
 
+test_expect_success 'clone from original with relative alternate' '
+	mkdir nest &&
+	git clone --bare src nest/src-5 &&
+	echo ../../../src/.git/objects >nest/src-5/objects/info/alternates &&
+	git clone --bare nest/src-5 target-10 &&
+	grep /src/\\.git/objects target-10/objects/info/alternates
+'
+
 test_done