memoize common git-path "constant" files

One of the most common uses of git_path() is to pass a
constant, like git_path("MERGE_MSG"). This has two
drawbacks:

  1. The return value is a static buffer, and the lifetime
     is dependent on other calls to git_path, etc.

  2. There's no compile-time checking of the pathname. This
     is OK for a one-off (after all, we have to spell it
     correctly at least once), but many of these constant
     strings appear throughout the code.

This patch introduces a series of functions to "memoize"
these strings, which are essentially globals for the
lifetime of the program. We compute the value once, take
ownership of the buffer, and return the cached value for
subsequent calls.  cache.h provides a helper macro for
defining these functions as one-liners, and defines a few
common ones for global use.

Using a macro is a little bit gross, but it does nicely
document the purpose of the functions. If we need to touch
them all later (e.g., because we learned how to change the
git_dir variable at runtime, and need to invalidate all of
the stored values), it will be much easier to have the
complete list.

Note that the shared-global functions have separate, manual
declarations. We could do something clever with the macros
(e.g., expand it to a declaration in some places, and a
declaration _and_ a definition in path.c). But there aren't
that many, and it's probably better to stay away from
too-magical macros.

Likewise, if we abandon the C preprocessor in favor of
generating these with a script, we could get much fancier.
E.g., normalizing "FOO/BAR-BAZ" into "git_path_foo_bar_baz".
But the small amount of saved typing is probably not worth
the resulting confusion to readers who want to grep for the
function's definition.

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

1 files changed, 5 insertions, 2 deletions

diff --git a/bisect.c b/bisect.c
index 03d5cd945..7a6498c8e 100644
--- a/bisect.c
+++ b/bisect.c
@@ -420,10 +420,13 @@ static int read_bisect_refs(void)
 	return for_each_ref_in("refs/bisect/", register_ref, NULL);
 }
 
+static GIT_PATH_FUNC(git_path_bisect_names, "BISECT_NAMES")
+static GIT_PATH_FUNC(git_path_bisect_expected_rev, "BISECT_EXPECTED_REV")
+
 static void read_bisect_paths(struct argv_array *array)
 {
 	struct strbuf str = STRBUF_INIT;
-	const char *filename = git_path("BISECT_NAMES");
+	const char *filename = git_path_bisect_names();
 	FILE *fp = fopen(filename, "r");
 
 	if (!fp)
@@ -644,7 +647,7 @@ static void exit_if_skipped_commits(struct commit_list *tried,
 
 static int is_expected_rev(const struct object_id *oid)
 {
-	const char *filename = git_path("BISECT_EXPECTED_REV");
+	const char *filename = git_path_bisect_expected_rev();
 	struct stat st;
 	struct strbuf str = STRBUF_INIT;
 	FILE *fp;