[PATCH 09/14] pull: implement pulling into an unborn branch

[Paul Tan <pyokagan@gmail.com>]

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>
---
 builtin/pull.c  | 29 ++++++++++++++++++++++++++++-
 t/t5520-pull.sh |  4 ++--
 2 files changed, 30 insertions(+), 3 deletions(-)

diff --git a/builtin/pull.c b/builtin/pull.c
index 3b7029f..ba2ff01 100644
--- 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"
 
 /**
  * Given an option opt, where opt->value points to a char* and opt->defval is a
@@ -424,6 +425,27 @@ static int run_fetch(const char *repo, const char **refspecs)
 }
 
 /**
+ * "Pulls into void" by branching off merge_head.
+ */
+static int pull_into_void(unsigned char merge_head[GIT_SHA1_RAWSZ],
+		unsigned char curr_head[GIT_SHA1_RAWSZ])
+{
+	/*
+	 * Two-way merge: we claim the index is 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.
  */
 static int run_merge(void)
@@ -524,5 +546,10 @@ 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();
 }
diff --git a/t/t5520-pull.sh b/t/t5520-pull.sh
index 3645a59..2131749 100755
--- a/t/t5520-pull.sh
+++ b/t/t5520-pull.sh
@@ -26,7 +26,7 @@ test_expect_success 'pulling into void' '
 	test_cmp file cloned/file
 '
 
-test_expect_failure 'pulling into void using master:master' '
+test_expect_success 'pulling into void using master:master' '
 	git init cloned-uho &&
 	(
 		cd cloned-uho &&
@@ -76,7 +76,7 @@ test_expect_success 'pulling into void does not remove new staged files' '
 	)
 '
 
-test_expect_failure 'pulling into void must not create an octopus' '
+test_expect_success 'pulling into void must not create an octopus' '
 	git init cloned-octopus &&
 	(
 		cd cloned-octopus &&
-- 
2.1.4