[PATCH 10/18] alternates: provide helper for allocating alternate

[Jeff King]

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 <p...@peff.net>
---
 cache.h     |  6 ++++++
 sha1_file.c | 28 +++++++++++++++++++---------
 sha1_name.c |  7 +------
 3 files changed, 26 insertions(+), 15 deletions(-)

diff --git a/cache.h b/cache.h
index 9a91378..d36b2ad 100644
--- a/cache.h
+++ b/cache.h
@@ -1391,6 +1391,12 @@ 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.
diff --git a/sha1_file.c b/sha1_file.c
index 2e41b26..549cf1e 100644
--- a/sha1_file.c
+++ b/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));
diff --git a/sha1_name.c b/sha1_name.c
index faf873c..98152a6 100644
--- a/sha1_name.c
+++ b/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;
 
-- 
2.10.0.618.g82cc264