alternates: use a separate scratch space

The alternate_object_database struct uses a single buffer
both for storing the path to the alternate, and as a scratch
buffer for forming object names. This is efficient (since
otherwise we'd end up storing the path twice), but it makes
life hard for callers who just want to know the path to the
alternate. They have to remember to stop reading after
"alt->name - alt->base" bytes, and to subtract one for the
trailing '/'.

It would be much simpler if they could simply access a
NUL-terminated path string. We could encapsulate this in a
function which puts a NUL in the scratch buffer and returns
the string, but that opens up questions about the lifetime
of the result. The first time another caller uses the
alternate, the scratch buffer may get other data tacked onto
it.

Let's instead just store the root path separately from the
scratch buffer. There aren't enough alternates being stored
for the duplicated data to matter for performance, and this
keeps things simple and safe for the callers.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 3 insertions, 8 deletions

diff --git a/builtin/submodule--helper.c b/builtin/submodule--helper.c
index e3fdc0aa78..fd72c90442 100644
--- a/builtin/submodule--helper.c
+++ b/builtin/submodule--helper.c
@@ -492,20 +492,16 @@ static int add_possible_reference_from_superproject(
 {
 	struct submodule_alternate_setup *sas = sas_cb;
 
-	/* directory name, minus trailing slash */
-	size_t namelen = alt->name - alt->base - 1;
-	struct strbuf name = STRBUF_INIT;
-	strbuf_add(&name, alt->base, namelen);
-
 	/*
 	 * If the alternate object store is another repository, try the
 	 * standard layout with .git/modules/<name>/objects
 	 */
-	if (ends_with(name.buf, ".git/objects")) {
+	if (ends_with(alt->path, ".git/objects")) {
 		char *sm_alternate;
 		struct strbuf sb = STRBUF_INIT;
 		struct strbuf err = STRBUF_INIT;
-		strbuf_add(&sb, name.buf, name.len - strlen("objects"));
+		strbuf_add(&sb, alt->path, strlen(alt->path) - strlen("objects"));
+
 		/*
 		 * We need to end the new path with '/' to mark it as a dir,
 		 * otherwise a submodule name containing '/' will be broken
@@ -533,7 +529,6 @@ static int add_possible_reference_from_superproject(
 		strbuf_release(&sb);
 	}
 
-	strbuf_release(&name);
 	return 0;
 }