[PATCH 16/17] commit: convert commit->buffer to a slab

[Jeff King <peff@peff.net>]

This will make it easier to manage the buffer cache
independently of the "struct commit" objects. It also
shrinks "struct commit" by one pointer, which may be
helpful.

Unfortunately it does not reduce the max memory size of
something like "rev-list", because rev-list uses
get_cached_commit_buffer() to decide not to show each
commit's output (and due to the design of slab_at, accessing
the slab requires us to extend it, allocating exactly the
same number of buffer pointers we dropped from the commit
structs).

Signed-off-by: Jeff King <peff@peff.net>
---
 commit.c | 20 +++++++++++++-------
 commit.h |  1 -
 2 files changed, 13 insertions(+), 8 deletions(-)

diff --git a/commit.c b/commit.c
index 1903dde..e289c78 100644
--- a/commit.c
+++ b/commit.c
@@ -245,14 +245,17 @@ int unregister_shallow(const unsigned char *sha1)
 	return 0;
 }
 
+define_commit_slab(buffer_slab, void *);
+static struct buffer_slab buffer_slab = COMMIT_SLAB_INIT(1, buffer_slab);
+
 void set_commit_buffer(struct commit *commit, void *buffer)
 {
-	commit->buffer = buffer;
+	*buffer_slab_at(&buffer_slab, commit) = buffer;
 }
 
 const void *get_cached_commit_buffer(const struct commit *commit)
 {
-	return commit->buffer;
+	return *buffer_slab_at(&buffer_slab, commit);
 }
 
 const void *get_commit_buffer(const struct commit *commit)
@@ -274,20 +277,23 @@ const void *get_commit_buffer(const struct commit *commit)
 
 void unuse_commit_buffer(const struct commit *commit, const void *buffer)
 {
-	if (commit->buffer != buffer)
+	void *cached = *buffer_slab_at(&buffer_slab, commit);
+	if (cached != buffer)
 		free((void *)buffer);
 }
 
 void free_commit_buffer(struct commit *commit)
 {
-	free(commit->buffer);
-	commit->buffer = NULL;
+	void **b = buffer_slab_at(&buffer_slab, commit);
+	free(*b);
+	*b = NULL;
 }
 
 const void *detach_commit_buffer(struct commit *commit)
 {
-	void *ret = commit->buffer;
-	commit->buffer = NULL;
+	void **b = buffer_slab_at(&buffer_slab, commit);
+	void *ret = *b;
+	*b = NULL;
 	return ret;
 }
 
diff --git a/commit.h b/commit.h
index 5ce5ce7..e1c2569 100644
--- a/commit.h
+++ b/commit.h
@@ -20,7 +20,6 @@ struct commit {
 	unsigned long date;
 	struct commit_list *parents;
 	struct tree *tree;
-	char *buffer;
 };
 
 extern int save_commit_buffer;
-- 
2.0.0.729.g520999f