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>

commit-slab.h

@@ -15,7 +15,13 @@
  * - int *indegree_at(struct indegree *, struct commit *);
  *
  *   This function locates the data associated with the given commit in
- *   the indegree slab, and returns the pointer to it.
+ *   the indegree slab, and returns the pointer to it.  The location to
+ *   store the data is allocated as necessary.
+ *
+ * - int *indegree_peek(struct indegree *, struct commit *);
+ *
+ *   This function is similar to indegree_at(), but it will return NULL
+ *   until a call to indegree_at() was made for the commit.
  *
  * - void init_indegree(struct indegree *);
  *   void init_indegree_with_stride(struct indegree *, int);
@@ -80,8 +86,9 @@ static MAYBE_UNUSED void clear_ ##slabname(struct slabname *s)		\
 	s->slab = NULL;							\
 }									\
 									\
-static MAYBE_UNUSED elemtype *slabname## _at(struct slabname *s,	\
-				       const struct commit *c)		\
+static MAYBE_UNUSED elemtype *slabname## _at_peek(struct slabname *s,	\
+						  const struct commit *c, \
+						  int add_if_missing)   \
 {									\
 	int nth_slab, nth_slot;						\
 									\
@@ -90,6 +97,8 @@ static MAYBE_UNUSED elemtype *slabname## _at(struct slabname *s,	\
 									\
 	if (s->slab_count <= nth_slab) {				\
 		int i;							\
+		if (!add_if_missing)					\
+			return NULL;					\
 		s->slab = xrealloc(s->slab,				\
 				   (nth_slab + 1) * sizeof(*s->slab));	\
 		stat_ ##slabname## realloc++;				\
@@ -97,10 +106,25 @@ static MAYBE_UNUSED elemtype *slabname## _at(struct slabname *s,	\
 			s->slab[i] = NULL;				\
 		s->slab_count = nth_slab + 1;				\
 	}								\
-	if (!s->slab[nth_slab])						\
+	if (!s->slab[nth_slab]) {					\
+		if (!add_if_missing)					\
+			return NULL;					\
 		s->slab[nth_slab] = xcalloc(s->slab_size,		\
 					    sizeof(**s->slab) * s->stride);		\
-	return &s->slab[nth_slab][nth_slot * s->stride];				\
+	}								\
+	return &s->slab[nth_slab][nth_slot * s->stride];		\
+}									\
+									\
+static MAYBE_UNUSED elemtype *slabname## _at(struct slabname *s,	\
+					     const struct commit *c)	\
+{									\
+	return slabname##_at_peek(s, c, 1);				\
+}									\
+									\
+static MAYBE_UNUSED elemtype *slabname## _peek(struct slabname *s,	\
+					     const struct commit *c)	\
+{									\
+	return slabname##_at_peek(s, c, 0);				\
 }									\
 									\
 static int stat_ ##slabname## realloc

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;