fast-import: do less work when given "from" matches current branch head

When building a fast-import stream, it's easy to forget the fact
that for non-merge commits happening on top of the current branch
head, there is no need for a "from" command. That is corroborated by
the fact that at least git-p4, hg-fast-export and felipec's
git-remote-hg all unconditionally use a "from" command.

Unfortunately, giving a "from" command always resets the branch
tree, forcing it to be re-read, and in many cases, the pack is also
closed and reopened through gfi_unpack_entry.  Both are unnecessary
overhead, and the latter is particularly slow at least on OSX.

Avoid resetting the tree when it's unmodified, and avoid calling
gfi_unpack_entry when the given mark points to the same commit as
the current branch head.

Signed-off-by: Mike Hommey <mh@glandium.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/fast-import.c b/fast-import.c
index 6378726993445694581ac36d6351f817fe488c72..ece009662379dd26a523cb082b50c11109c96b14 100644 (file)
--- a/fast-import.c
+++ b/fast-import.c
@@ -2588,14 +2588,12 @@ static int parse_from(struct branch *b)
 {
        const char *from;
        struct branch *s;
+       unsigned char sha1[20];
 
        if (!skip_prefix(command_buf.buf, "from ", &from))
                return 0;
 
-       if (b->branch_tree.tree) {
-               release_tree_content_recursive(b->branch_tree.tree);
-               b->branch_tree.tree = NULL;
-       }
+       hashcpy(sha1, b->branch_tree.versions[1].sha1);
 
        s = lookup_branch(from);
        if (b == s)
@@ -2610,14 +2608,16 @@ static int parse_from(struct branch *b)
                struct object_entry *oe = find_mark(idnum);
                if (oe->type != OBJ_COMMIT)
                        die("Mark :%" PRIuMAX " not a commit", idnum);
-               hashcpy(b->sha1, oe->idx.sha1);
-               if (oe->pack_id != MAX_PACK_ID) {
-                       unsigned long size;
-                       char *buf = gfi_unpack_entry(oe, &size);
-                       parse_from_commit(b, buf, size);
-                       free(buf);
-               } else
-                       parse_from_existing(b);
+               if (hashcmp(b->sha1, oe->idx.sha1)) {
+                       hashcpy(b->sha1, oe->idx.sha1);
+                       if (oe->pack_id != MAX_PACK_ID) {
+                               unsigned long size;
+                               char *buf = gfi_unpack_entry(oe, &size);
+                               parse_from_commit(b, buf, size);
+                               free(buf);
+                       } else
+                               parse_from_existing(b);
+               }
        } else if (!get_sha1(from, b->sha1)) {
                parse_from_existing(b);
                if (is_null_sha1(b->sha1))
@@ -2626,6 +2626,11 @@ static int parse_from(struct branch *b)
        else
                die("Invalid ref name or SHA1 expression: %s", from);
 
+       if (b->branch_tree.tree && hashcmp(sha1, b->branch_tree.versions[1].sha1)) {
+               release_tree_content_recursive(b->branch_tree.tree);
+               b->branch_tree.tree = NULL;
+       }
+
        read_next_command();
        return 1;
 }