alternates: provide helper for allocating alternate

Allocating a struct alternate_object_database is tricky, as
we must over-allocate the buffer to provide scratch space,
and then put in particular '/' and NUL markers.

Let's encapsulate this in a function so that the complexity
doesn't leak into callers (and so that we can modify it
later).

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

cache.h

@@ -1392,6 +1392,12 @@ extern char *compute_alternate_path(const char *path, struct strbuf *err);
 typedef int alt_odb_fn(struct alternate_object_database *, void *);
 extern int foreach_alt_odb(alt_odb_fn, void*);
 
+/*
+ * Allocate a "struct alternate_object_database" but do _not_ actually
+ * add it to the list of alternates.
+ */
+struct alternate_object_database *alloc_alt_odb(const char *dir);
+
 /*
  * Add the directory to the on-disk alternates file; the new entry will also
  * take effect in the current process.

sha1_file.c

@@ -283,7 +283,6 @@ static int link_alt_odb_entry(const char *entry, const char *relative_base,
 	int depth, const char *normalized_objdir)
 {
 	struct alternate_object_database *ent;
-	size_t entlen;
 	struct strbuf pathbuf = STRBUF_INIT;
 
 	if (!is_absolute_path(entry) && relative_base) {
@@ -311,14 +310,7 @@ static int link_alt_odb_entry(const char *entry, const char *relative_base,
 		return -1;
 	}
 
-	entlen = st_add(pathbuf.len, 43); /* '/' + 2 hex + '/' + 38 hex + NUL */
-	ent = xmalloc(st_add(sizeof(*ent), entlen));
-	memcpy(ent->base, pathbuf.buf, pathbuf.len);
-
-	ent->name = ent->base + pathbuf.len + 1;
-	ent->base[pathbuf.len] = '/';
-	ent->base[pathbuf.len + 3] = '/';
-	ent->base[entlen-1] = 0;
+	ent = alloc_alt_odb(pathbuf.buf);
 
 	/* add the alternate entry */
 	*alt_odb_tail = ent;
@@ -395,6 +387,24 @@ void read_info_alternates(const char * relative_base, int depth)
 	munmap(map, mapsz);
 }
 
+struct alternate_object_database *alloc_alt_odb(const char *dir)
+{
+	struct alternate_object_database *ent;
+	size_t dirlen = strlen(dir);
+	size_t entlen;
+
+	entlen = st_add(dirlen, 43); /* '/' + 2 hex + '/' + 38 hex + NUL */
+	ent = xmalloc(st_add(sizeof(*ent), entlen));
+	memcpy(ent->base, dir, dirlen);
+
+	ent->name = ent->base + dirlen + 1;
+	ent->base[dirlen] = '/';
+	ent->base[dirlen + 3] = '/';
+	ent->base[entlen-1] = 0;
+
+	return ent;
+}
+
 void add_to_alternates_file(const char *reference)
 {
 	struct lock_file *lock = xcalloc(1, sizeof(struct lock_file));

sha1_name.c

@@ -86,12 +86,7 @@ static void find_short_object_filename(int len, const char *hex_pfx, struct disa
 		 * alt->name/alt->base while iterating over the
 		 * object databases including our own.
 		 */
-		const char *objdir = get_object_directory();
-		size_t objdir_len = strlen(objdir);
-		fakeent = xmalloc(st_add3(sizeof(*fakeent), objdir_len, 43));
-		memcpy(fakeent->base, objdir, objdir_len);
-		fakeent->name = fakeent->base + objdir_len + 1;
-		fakeent->name[-1] = '/';
+		fakeent = alloc_alt_odb(get_object_directory());
 	}
 	fakeent->next = alt_odb_list;