refs: add GIT_REFERENCE_BACKEND to specify reference backend

Git allows setting a different object directory via
'GIT_OBJECT_DIRECTORY', but provides no equivalent for references. In
the previous commit we extended the 'extensions.refStorage' config to
also support an URI input for reference backend with location.

Let's also add a new environment variable 'GIT_REFERENCE_BACKEND' that
takes in the same input as the config variable. Having an environment
variable allows us to modify the reference backend and location on the
fly for individual git commands.

Helped-by: Jean-Noël Avila <jn.avila@free.fr>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: KarthikNayak

Documentation/git.adoc

@@ -584,6 +584,11 @@ double-quotes and respecting backslash escapes. E.g., the value
 	repositories will be set to this value. The default is "files".
 	See `--ref-format` in linkgit:git-init[1].
 
+`GIT_REFERENCE_BACKEND`::
+    Specify which reference backend to be used along with its URI.
+    See `extensions.refStorage` option in linkgit:git-config[1] for more
+    description. Overrides the config variable when used.
+
 Git Commits
 ~~~~~~~~~~~
 `GIT_AUTHOR_NAME`::

environment.h

@@ -42,6 +42,7 @@
 #define GIT_OPTIONAL_LOCKS_ENVIRONMENT "GIT_OPTIONAL_LOCKS"
 #define GIT_TEXT_DOMAIN_DIR_ENVIRONMENT "GIT_TEXTDOMAINDIR"
 #define GIT_ATTR_SOURCE_ENVIRONMENT "GIT_ATTR_SOURCE"
+#define GIT_REFERENCE_BACKEND_ENVIRONMENT "GIT_REFERENCE_BACKEND"
 
 /*
  * Environment variable used to propagate the --no-advice global option to the

setup.c

@@ -1838,6 +1838,7 @@ const char *setup_git_directory_gently(int *nongit_ok)
 	static struct strbuf cwd = STRBUF_INIT;
 	struct strbuf dir = STRBUF_INIT, gitdir = STRBUF_INIT, report = STRBUF_INIT;
 	const char *prefix = NULL;
+	const char *ref_backend_uri;
 	struct repository_format repo_fmt = REPOSITORY_FORMAT_INIT;
 
 	/*
@@ -1995,6 +1996,25 @@ const char *setup_git_directory_gently(int *nongit_ok)
 		setenv(GIT_PREFIX_ENVIRONMENT, "", 1);
 	}
 
+	/*
+	 * The env variable should override the repository config
+	 * for 'extensions.refStorage'.
+	 */
+	ref_backend_uri = getenv(GIT_REFERENCE_BACKEND_ENVIRONMENT);
+	if (ref_backend_uri) {
+		char *backend, *location;
+		enum ref_storage_format format;
+
+		parse_reference_uri(ref_backend_uri, &backend, &location);
+		format = ref_storage_format_by_name(backend);
+		if (format == REF_STORAGE_FORMAT_UNKNOWN)
+			die(_("unknown ref storage format: '%s'"), backend);
+		repo_set_ref_storage_format(the_repository, format, location);
+
+		free(backend);
+		free(location);
+	}
+
 	setup_original_cwd();
 
 	strbuf_release(&dir);

t/t1423-ref-backend.sh

@@ -11,16 +11,25 @@ test_description='Test reference backend URIs'
 #   <backend> is the original ref storage of the repo.
 #   <uri> is the new URI to be set for the ref storage.
 #   <cmd> is the git subcommand to be run in the repository.
+#   <via> if 'config', set the backend via the 'extensions.refStorage' config.
+#         if 'env', set the backend via the 'GIT_REFERENCE_BACKEND' env.
 run_with_uri() {
 	repo=$1 &&
 	backend=$2 &&
 	uri=$3 &&
 	cmd=$4 &&
+	via=$5 &&
 
-	git -C "$repo" config set core.repositoryformatversion 1
-	git -C "$repo" config set extensions.refStorage "$uri" &&
-	git -C "$repo" $cmd &&
-	git -C "$repo" config set extensions.refStorage "$backend"
+	git -C "$repo" config set core.repositoryformatversion 1 &&
+	if test "$via" = "env"
+	then
+		test_env GIT_REFERENCE_BACKEND="$uri" git -C "$repo" $cmd
+	elif test "$via" = "config"
+	then
+		git -C "$repo" config set extensions.refStorage "$uri" &&
+		git -C "$repo" $cmd &&
+		git -C "$repo" config set extensions.refStorage "$backend"
+	fi
 }
 
 # Test a repository with a given reference storage by running and comparing
@@ -30,44 +39,57 @@ run_with_uri() {
 #   <repo> is the relative path to the repo to run the command in.
 #   <backend> is the original ref storage of the repo.
 #   <uri> is the new URI to be set for the ref storage.
+#   <via> if 'config', set the backend via the 'extensions.refStorage' config.
+#         if 'env', set the backend via the 'GIT_REFERENCE_BACKEND' env.
 #   <err_msg> (optional) if set, check if 'git-refs(1)' failed with the provided msg.
 test_refs_backend() {
 	repo=$1 &&
 	backend=$2 &&
 	uri=$3 &&
-	err_msg=$4 &&
+	via=$4 &&
+	err_msg=$5 &&
+
 
-	git -C "$repo" config set core.repositoryformatversion 1 &&
 	if test -n "$err_msg";
 	then
-		git -C "$repo" config set extensions.refStorage "$uri" &&
-		test_must_fail git -C "$repo" refs list 2>err &&
-		test_grep "$err_msg" err
+		if test "$via" = "env"
+		then
+			test_env GIT_REFERENCE_BACKEND="$uri" test_must_fail git -C "$repo" refs list 2>err
+		elif test "$via" = "config"
+		then
+			git -C "$repo" config set extensions.refStorage "$uri" &&
+			test_must_fail git -C "$repo" refs list 2>err &&
+			test_grep "$err_msg" err
+		fi
 	else
 		git -C "$repo" refs list >expect &&
-		run_with_uri "$repo" "$backend" "$uri" "refs list" >actual &&
+		run_with_uri "$repo" "$backend" "$uri" "refs list" "$via">actual &&
 		test_cmp expect actual
 	fi
 }
 
-test_expect_success 'URI is invalid' '
+methods="config"
+for method in $methods
+do
+
+test_expect_success "$method: URI is invalid" '
 	test_when_finished "rm -rf repo" &&
 	git init repo &&
-	test_refs_backend repo files "reftable@/home/reftable" \
+	test_refs_backend repo files "reftable@/home/reftable" "$method" \
 		"invalid value for ${SQ}extensions.refstorage${SQ}"
 '
 
-test_expect_success 'URI ends with colon' '
+test_expect_success "$method: URI ends with colon" '
 	test_when_finished "rm -rf repo" &&
 	git init repo &&
-	test_refs_backend repo files "reftable:" \
+	test_refs_backend repo files "reftable:" "$method" \
 		"invalid value for ${SQ}extensions.refstorage${SQ}"
 '
 
-test_expect_success 'unknown reference backend' '
+test_expect_success "$method: unknown reference backend" '
 	test_when_finished "rm -rf repo" &&
 	git init repo &&
-	test_refs_backend repo files "db://.git" \
+	test_refs_backend repo files "db://.git" "$method" \
 		"invalid value for ${SQ}extensions.refstorage${SQ}"
 '
 
@@ -86,7 +108,7 @@ do
 	for dir in "$(pwd)/repo/.git" "./"
 	do
 
-		test_expect_success "$read from $to_format backend, $dir dir" '
+		test_expect_success "$method: $read from $to_format backend, $dir dir" '
 			test_when_finished "rm -rf repo" &&
 			git init --ref-format=$from_format repo &&
 			(
@@ -101,7 +123,7 @@ do
 			)
 		'
 
-		test_expect_success "$write to $to_format backend, $dir dir" '
+		test_expect_success "$method: $write to $to_format backend, $dir dir" '
 			test_when_finished "rm -rf repo" &&
 			git init --ref-format=$from_format repo &&
 			(
@@ -113,20 +135,22 @@ do
 				git refs migrate --dry-run --ref-format=$to_format >out &&
 				BACKEND_PATH="$dir/$(sed "s/.* ${SQ}.git\/\(.*\)${SQ}/\1/" out)" &&
 
-				test_refs_backend . $from_format "$to_format://$BACKEND_PATH" &&
+				test_refs_backend . $from_format "$to_format://$BACKEND_PATH" "$method" &&
 
 				git refs list >expect &&
-				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" "tag -d 1" &&
+				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" \
+					"tag -d 1" "$method" &&
 				git refs list >actual &&
 				test_cmp expect actual &&
 
 				git refs list | grep -v "refs/tags/1" >expect &&
-				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" "refs list" >actual &&
+				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" \
+					"refs list" "$method" >actual &&
 				test_cmp expect actual
 			)
 		'
 
-		test_expect_success "with worktree and $to_format backend, $dir dir" '
+		test_expect_success "$method: with worktree and $to_format backend, $dir dir" '
 			test_when_finished "rm -rf repo wt" &&
 			git init --ref-format=$from_format repo &&
 			(
@@ -138,22 +162,26 @@ do
 				git refs migrate --dry-run --ref-format=$to_format >out &&
 				BACKEND_PATH="$dir/$(sed "s/.* ${SQ}.git\/\(.*\)${SQ}/\1/" out)" &&
 
-				git config set core.repositoryformatversion 1 &&
-				git config set extensions.refStorage "$to_format://$BACKEND_PATH" &&
+				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" \
+					"worktree add ../wt 2" "$method" &&
 
-				git worktree add ../wt 2
-			) &&
+				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" \
+					"for-each-ref --include-root-refs" "$method" >actual &&
+				run_with_uri ../wt "$from_format" "$to_format://$BACKEND_PATH" \
+					"for-each-ref --include-root-refs" "$method" >expect &&
+				! test_cmp expect actual &&
 
-			git -C repo for-each-ref --include-root-refs >expect &&
-			git -C wt for-each-ref --include-root-refs >expect &&
-			! test_cmp expect actual &&
-
-			git -C wt rev-parse 2 >expect &&
-			git -C wt rev-parse HEAD >actual &&
-			test_cmp expect actual
+				run_with_uri . "$from_format" "$to_format://$BACKEND_PATH" \
+					"rev-parse 2" "$method" >actual &&
+				run_with_uri ../wt "$from_format" "$to_format://$BACKEND_PATH" \
+					"rev-parse HEAD" "$method" >expect &&
+				test_cmp expect actual
+			)
 		'
 	done # closes dir
 done # closes to_format
-done # closes from_format
+done # closes to_format
+
+done # closes method
 
 test_done