[PATCH 08/14] pull: fast-forward working tree if head is updated

[Paul Tan <pyokagan@gmail.com>]

Since b10ac50 (Fix pulling into the same branch., 2005-08-25), git-pull,
upon detecting that git-fetch updated the current head, will
fast-forward the working tree to the updated head commit.

Re-implement this behavior.

Signed-off-by: Paul Tan <pyokagan@gmail.com>
---
 builtin/pull.c  | 30 ++++++++++++++++++++++++++++++
 t/t5520-pull.sh |  4 ++--
 2 files changed, 32 insertions(+), 2 deletions(-)

diff --git a/builtin/pull.c b/builtin/pull.c
index 9b06dfa..3b7029f 100644
--- a/builtin/pull.c
+++ b/builtin/pull.c
@@ -468,6 +468,7 @@ int cmd_pull(int argc, const char **argv, const char *prefix)
 {
 	const char *repo, **refspecs;
 	struct sha1_array merge_heads = SHA1_ARRAY_INIT;
+	unsigned char orig_head[GIT_SHA1_RAWSZ], curr_head[GIT_SHA1_RAWSZ];
 
 	argc = parse_options(argc, argv, prefix, pull_options, pull_usage, 0);
 	parse_repo_refspecs(argc, argv, &repo, &refspecs);
@@ -483,12 +484,41 @@ int cmd_pull(int argc, const char **argv, const char *prefix)
 	if (!opt_ff)
 		opt_ff = xstrdup_or_null(config_get_ff());
 
+	if (get_sha1("HEAD", orig_head))
+		hashclr(orig_head);
+
 	if (run_fetch(repo, refspecs))
 		return 1;
 
 	if (opt_dry_run)
 		return 0;
 
+	if (get_sha1("HEAD", curr_head))
+		hashclr(curr_head);
+
+	if (!is_null_sha1(orig_head) && !is_null_sha1(curr_head) &&
+			hashcmp(orig_head, curr_head)) {
+		/*
+		 * The fetch involved updating the current branch.
+		 *
+		 * The working tree and the index file are still based on
+		 * orig_head commit, but we are merging into curr_head.
+		 * Update the working tree to match curr_head.
+		 */
+
+		warning(_("fetch updated the current branch head.\n"
+			"fast-forwarding your working tree from\n"
+			"commit %s."), sha1_to_hex(orig_head));
+
+		if (checkout_fast_forward(orig_head, curr_head, 0))
+			die(_("Cannot fast-forward your working tree.\n"
+				"After making sure that you saved anything precious from\n"
+				"$ git diff %s\n"
+				"output, run\n"
+				"$ git reset --hard\n"
+				"to recover."), sha1_to_hex(orig_head));
+	}
+
 	get_merge_heads(&merge_heads);
 
 	if (!merge_heads.nr)
diff --git a/t/t5520-pull.sh b/t/t5520-pull.sh
index 4c52175..3645a59 100755
--- a/t/t5520-pull.sh
+++ b/t/t5520-pull.sh
@@ -191,7 +191,7 @@ test_expect_success 'fail if the index has unresolved entries' '
 	test_cmp expected file
 '
 
-test_expect_failure 'fast-forwards working tree if branch head is updated' '
+test_expect_success 'fast-forwards working tree if branch head is updated' '
 	git checkout -b third second^ &&
 	test_when_finished "git checkout -f copy && git branch -D third" &&
 	test "$(cat file)" = file &&
@@ -201,7 +201,7 @@ test_expect_failure 'fast-forwards working tree if branch head is updated' '
 	test "$(git rev-parse third)" = "$(git rev-parse second)"
 '
 
-test_expect_failure 'fast-forward fails with conflicting work tree' '
+test_expect_success 'fast-forward fails with conflicting work tree' '
 	git checkout -b third second^ &&
 	test_when_finished "git checkout -f copy && git branch -D third" &&
 	test "$(cat file)" = file &&
-- 
2.1.4