commit-slab: introduce slabname##_peek() function

There is no API to ask "Does this commit have associated data in
slab?".  If an application wants to (1) parse just a few commits at
the beginning of a process, (2) store data for only these commits,
and then (3) start processing many commits, taking into account the
data stored (for a few of them) in the slab, the application would
use slabname##_at() to allocate a space to store data in (2), but
there is no API other than slabname##_at() to use in step (3).  This
allocates and wastes new space for these commits the caller is only
interested in checking if they have data stored in step (2).

Introduce slabname##_peek(), which is similar to slabname##_at() but
returns NULL when there is no data already associated to it in such
a use case.

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

1 files changed, 20 insertions, 8 deletions

diff --git a/commit.c b/commit.c
index 65179f9..5fb9496 100644
--- a/commit.c
+++ b/commit.c
@@ -244,7 +244,12 @@ void set_commit_buffer(struct commit *commit, void *buffer, unsigned long size)
 
 const void *get_cached_commit_buffer(const struct commit *commit, unsigned long *sizep)
 {
-	struct commit_buffer *v = buffer_slab_at(&buffer_slab, commit);
+	struct commit_buffer *v = buffer_slab_peek(&buffer_slab, commit);
+	if (!v) {
+		if (sizep)
+			*sizep = 0;
+		return NULL;
+	}
 	if (sizep)
 		*sizep = v->size;
 	return v->buffer;
@@ -271,24 +276,31 @@ const void *get_commit_buffer(const struct commit *commit, unsigned long *sizep)
 
 void unuse_commit_buffer(const struct commit *commit, const void *buffer)
 {
-	struct commit_buffer *v = buffer_slab_at(&buffer_slab, commit);
-	if (v->buffer != buffer)
+	struct commit_buffer *v = buffer_slab_peek(&buffer_slab, commit);
+	if (!(v && v->buffer == buffer))
 		free((void *)buffer);
 }
 
 void free_commit_buffer(struct commit *commit)
 {
-	struct commit_buffer *v = buffer_slab_at(&buffer_slab, commit);
-	free(v->buffer);
-	v->buffer = NULL;
-	v->size = 0;
+	struct commit_buffer *v = buffer_slab_peek(&buffer_slab, commit);
+	if (v) {
+		free(v->buffer);
+		v->buffer = NULL;
+		v->size = 0;
+	}
 }
 
 const void *detach_commit_buffer(struct commit *commit, unsigned long *sizep)
 {
-	struct commit_buffer *v = buffer_slab_at(&buffer_slab, commit);
+	struct commit_buffer *v = buffer_slab_peek(&buffer_slab, commit);
 	void *ret;
 
+	if (!v) {
+		if (sizep)
+			*sizep = 0;
+		return NULL;
+	}
 	ret = v->buffer;
 	if (sizep)
 		*sizep = v->size;