refs: implement simple transactions for the packed-refs file
authorMichael Haggerty <mhagger@alum.mit.edu>
Thu, 20 Jun 2013 08:37:46 +0000 (10:37 +0200)
committerJunio C Hamano <gitster@pobox.com>
Thu, 20 Jun 2013 22:50:17 +0000 (15:50 -0700)
Handle simple transactions for the packed-refs file at the
packed_ref_cache level via new functions lock_packed_refs(),
commit_packed_refs(), and rollback_packed_refs().

Only allow the packed ref cache to be modified (via add_packed_ref())
while the packed refs file is locked.

Change clone to add the new references within a transaction.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
builtin/clone.c
refs.c
refs.h

index 66bff57..14b1323 100644 (file)
@@ -493,13 +493,16 @@ static void write_remote_refs(const struct ref *local_refs)
 {
        const struct ref *r;
 
+       lock_packed_refs(LOCK_DIE_ON_ERROR);
+
        for (r = local_refs; r; r = r->next) {
                if (!r->peer_ref)
                        continue;
                add_packed_ref(r->peer_ref->name, r->old_sha1);
        }
 
-       pack_refs(PACK_REFS_ALL);
+       if (commit_packed_refs())
+               die_errno("unable to overwrite old ref-pack file");
 }
 
 static void write_followtags(const struct ref *refs, const char *msg)
diff --git a/refs.c b/refs.c
index 373d95b..b345799 100644 (file)
--- a/refs.c
+++ b/refs.c
@@ -808,6 +808,13 @@ static int is_refname_available(const char *refname, const char *oldrefname,
 
 struct packed_ref_cache {
        struct ref_entry *root;
+
+       /*
+        * Iff the packed-refs file associated with this instance is
+        * currently locked for writing, this points at the associated
+        * lock (which is owned by somebody else).
+        */
+       struct lock_file *lock;
 };
 
 /*
@@ -826,9 +833,14 @@ static struct ref_cache {
        char name[1];
 } ref_cache, *submodule_ref_caches;
 
+/* Lock used for the main packed-refs file: */
+static struct lock_file packlock;
+
 static void clear_packed_ref_cache(struct ref_cache *refs)
 {
        if (refs->packed) {
+               if (refs->packed->lock)
+                       die("internal error: packed-ref cache cleared while locked");
                free_ref_entry(refs->packed->root);
                free(refs->packed);
                refs->packed = NULL;
@@ -1038,7 +1050,12 @@ static struct ref_dir *get_packed_refs(struct ref_cache *refs)
 
 void add_packed_ref(const char *refname, const unsigned char *sha1)
 {
-       add_ref(get_packed_refs(&ref_cache),
+       struct packed_ref_cache *packed_ref_cache =
+               get_packed_ref_cache(&ref_cache);
+
+       if (!packed_ref_cache->lock)
+               die("internal error: packed refs not locked");
+       add_ref(get_packed_ref_dir(packed_ref_cache),
                create_ref_entry(refname, sha1, REF_ISPACKED, 1));
 }
 
@@ -2035,6 +2052,52 @@ static int write_packed_entry_fn(struct ref_entry *entry, void *cb_data)
        return 0;
 }
 
+int lock_packed_refs(int flags)
+{
+       struct packed_ref_cache *packed_ref_cache;
+
+       /* Discard the old cache because it might be invalid: */
+       clear_packed_ref_cache(&ref_cache);
+       if (hold_lock_file_for_update(&packlock, git_path("packed-refs"), flags) < 0)
+               return -1;
+       /* Read the current packed-refs while holding the lock: */
+       packed_ref_cache = get_packed_ref_cache(&ref_cache);
+       packed_ref_cache->lock = &packlock;
+       return 0;
+}
+
+int commit_packed_refs(void)
+{
+       struct packed_ref_cache *packed_ref_cache =
+               get_packed_ref_cache(&ref_cache);
+       int error = 0;
+
+       if (!packed_ref_cache->lock)
+               die("internal error: packed-refs not locked");
+       write_or_die(packed_ref_cache->lock->fd,
+                    PACKED_REFS_HEADER, strlen(PACKED_REFS_HEADER));
+
+       do_for_each_entry_in_dir(get_packed_ref_dir(packed_ref_cache),
+                                0, write_packed_entry_fn,
+                                &packed_ref_cache->lock->fd);
+       if (commit_lock_file(packed_ref_cache->lock))
+               error = -1;
+       packed_ref_cache->lock = NULL;
+       return error;
+}
+
+void rollback_packed_refs(void)
+{
+       struct packed_ref_cache *packed_ref_cache =
+               get_packed_ref_cache(&ref_cache);
+
+       if (!packed_ref_cache->lock)
+               die("internal error: packed-refs not locked");
+       rollback_lock_file(packed_ref_cache->lock);
+       packed_ref_cache->lock = NULL;
+       clear_packed_ref_cache(&ref_cache);
+}
+
 struct ref_to_prune {
        struct ref_to_prune *next;
        unsigned char sha1[20];
@@ -2148,28 +2211,22 @@ static void prune_refs(struct ref_to_prune *r)
        }
 }
 
-static struct lock_file packlock;
-
 int pack_refs(unsigned int flags)
 {
        struct pack_refs_cb_data cbdata;
-       int fd;
 
        memset(&cbdata, 0, sizeof(cbdata));
        cbdata.flags = flags;
 
-       fd = hold_lock_file_for_update(&packlock, git_path("packed-refs"),
-                                      LOCK_DIE_ON_ERROR);
+       lock_packed_refs(LOCK_DIE_ON_ERROR);
        cbdata.packed_refs = get_packed_refs(&ref_cache);
 
        do_for_each_entry_in_dir(get_loose_refs(&ref_cache), 0,
                                 pack_if_possible_fn, &cbdata);
 
-       write_or_die(fd, PACKED_REFS_HEADER, strlen(PACKED_REFS_HEADER));
-       do_for_each_entry_in_dir(cbdata.packed_refs, 0, write_packed_entry_fn, &fd);
-
-       if (commit_lock_file(&packlock) < 0)
+       if (commit_packed_refs())
                die_errno("unable to overwrite old ref-pack file");
+
        prune_refs(cbdata.ref_to_prune);
        return 0;
 }
@@ -2233,7 +2290,6 @@ static int curate_packed_ref_fn(struct ref_entry *entry, void *cb_data)
 
 static int repack_without_ref(const char *refname)
 {
-       int fd;
        struct ref_dir *packed;
        struct string_list refs_to_delete = STRING_LIST_INIT_DUP;
        struct string_list_item *ref_to_delete;
@@ -2241,12 +2297,10 @@ static int repack_without_ref(const char *refname)
        if (!get_packed_ref(refname))
                return 0; /* refname does not exist in packed refs */
 
-       fd = hold_lock_file_for_update(&packlock, git_path("packed-refs"), 0);
-       if (fd < 0) {
+       if (lock_packed_refs(0)) {
                unable_to_lock_error(git_path("packed-refs"), errno);
                return error("cannot delete '%s' from packed refs", refname);
        }
-       clear_packed_ref_cache(&ref_cache);
        packed = get_packed_refs(&ref_cache);
 
        /* Remove refname from the cache: */
@@ -2255,7 +2309,7 @@ static int repack_without_ref(const char *refname)
                 * The packed entry disappeared while we were
                 * acquiring the lock.
                 */
-               rollback_lock_file(&packlock);
+               rollback_packed_refs();
                return 0;
        }
 
@@ -2267,9 +2321,7 @@ static int repack_without_ref(const char *refname)
        }
 
        /* Write what remains: */
-       write_or_die(fd, PACKED_REFS_HEADER, strlen(PACKED_REFS_HEADER));
-       do_for_each_entry_in_dir(packed, 0, write_packed_entry_fn, &fd);
-       return commit_lock_file(&packlock);
+       return commit_packed_refs();
 }
 
 int delete_ref(const char *refname, const unsigned char *sha1, int delopt)
diff --git a/refs.h b/refs.h
index 246bf60..9e5db3a 100644 (file)
--- a/refs.h
+++ b/refs.h
@@ -77,11 +77,33 @@ extern int for_each_rawref(each_ref_fn, void *);
 extern void warn_dangling_symref(FILE *fp, const char *msg_fmt, const char *refname);
 
 /*
- * Add a reference to the in-memory packed reference cache.  To actually
- * write the reference to the packed-refs file, call pack_refs().
+ * Lock the packed-refs file for writing.  Flags is passed to
+ * hold_lock_file_for_update().  Return 0 on success.
+ */
+extern int lock_packed_refs(int flags);
+
+/*
+ * Add a reference to the in-memory packed reference cache.  This may
+ * only be called while the packed-refs file is locked (see
+ * lock_packed_refs()).  To actually write the packed-refs file, call
+ * commit_packed_refs().
  */
 extern void add_packed_ref(const char *refname, const unsigned char *sha1);
 
+/*
+ * Write the current version of the packed refs cache from memory to
+ * disk.  The packed-refs file must already be locked for writing (see
+ * lock_packed_refs()).  Return zero on success.
+ */
+extern int commit_packed_refs(void);
+
+/*
+ * Rollback the lockfile for the packed-refs file, and discard the
+ * in-memory packed reference cache.  (The packed-refs file will be
+ * read anew if it is needed again after this function is called.)
+ */
+extern void rollback_packed_refs(void);
+
 /*
  * Flags for controlling behaviour of pack_refs()
  * PACK_REFS_PRUNE: Prune loose refs after packing