pull: implement pulling into an unborn branch

b4dc085 (pull: merge into unborn by fast-forwarding from empty
tree, 2013-06-20) established git-pull's current behavior of pulling
into an unborn branch by fast-forwarding the work tree from an empty
tree to the merge head, then setting HEAD to the merge head.

Re-implement this behavior by introducing pull_into_void() which will be
called instead of run_merge() if HEAD is invalid.

Helped-by: Stephen Robin <stephen.robin@gmail.com>
Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/builtin/pull.c b/builtin/pull.c
index 110e71948148f51d29288daf040667f4db12f5d1..492bb0e28ccfaa5503e16cbe3262d9319e3cfb66 100644 (file)
--- a/builtin/pull.c
+++ b/builtin/pull.c
@@ -13,6 +13,7 @@
 #include "sha1-array.h"
 #include "remote.h"
 #include "dir.h"
+#include "refs.h"
 
 static const char * const pull_usage[] = {
        N_("git pull [options] [<repository> [<refspec>...]]"),
@@ -367,6 +368,27 @@ static int run_fetch(const char *repo, const char **refspecs)
        return ret;
 }
 
+/**
+ * "Pulls into void" by branching off merge_head.
+ */
+static int pull_into_void(const unsigned char *merge_head,
+               const unsigned char *curr_head)
+{
+       /*
+        * Two-way merge: we treat the index as based on an empty tree,
+        * and try to fast-forward to HEAD. This ensures we will not lose
+        * index/worktree changes that the user already made on the unborn
+        * branch.
+        */
+       if (checkout_fast_forward(EMPTY_TREE_SHA1_BIN, merge_head, 0))
+               return 1;
+
+       if (update_ref("initial pull", "HEAD", merge_head, curr_head, 0, UPDATE_REFS_DIE_ON_ERR))
+               return 1;
+
+       return 0;
+}
+
 /**
  * Runs git-merge, returning its exit status.
  */
@@ -476,5 +498,10 @@ int cmd_pull(int argc, const char **argv, const char *prefix)
        if (!merge_heads.nr)
                die_no_merge_candidates(repo, refspecs);
 
-       return run_merge();
+       if (is_null_sha1(orig_head)) {
+               if (merge_heads.nr > 1)
+                       die(_("Cannot merge multiple branches into empty head."));
+               return pull_into_void(*merge_heads.sha1, curr_head);
+       } else
+               return run_merge();
 }