[PATCH v2 12/19] 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 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/builtin/pull.c b/builtin/pull.c
index 703fc1d..8db0db2 100644
--- a/builtin/pull.c
+++ b/builtin/pull.c
@@ -411,6 +411,7 @@ 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];
 
 	if (!getenv("_GIT_USE_BUILTIN_PULL")) {
 		const char *path = mkpath("%s/git-pull", git_exec_path());
@@ -434,12 +435,41 @@ int cmd_pull(int argc, const char **argv, const char *prefix)
 	if (!opt_ff.len)
 		config_get_ff(&opt_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)
-- 
2.1.4