[PATCH v2 2/2] receive-pack: detect aliased updates which can occur with symrefs

[Jay Soffian <jaysoffian@gmail.com>]

When pushing to a remote repo the sending side filters out aliased
updates (e.g., foo:baz bar:baz). However, it is not possible for the
sender to know if two refs are aliased on the receiving side via
symrefs. Here is one such scenario:

  $ git init origin
  $ (cd origin && touch file && git add file && git commit -a -m intial)
  $ git clone --bare origin origin.git
  $ rm -rf origin

  $ git clone origin.git client

  $ git clone --mirror client backup.git &&
  $ (cd backup.git && git remote set-head origin --auto)

  $ (cd client &&
	git remote add --mirror backup ../backup.git &&
	echo change1 > file && git commit -a -m change1 &&
	git push origin &&
	git push backup
	)

The push to backup fails with:

  Counting objects: 5, done.
  Writing objects: 100% (3/3), 244 bytes, done.
  Total 3 (delta 0), reused 0 (delta 0)
  Unpacking objects: 100% (3/3), done.
  error: Ref refs/remotes/origin/master is at ef3... but expected 262...
  remote: error: failed to lock refs/remotes/origin/master
  To ../backup.git
     262cd57..ef307ff  master -> master
     262cd57..ef307ff  origin/HEAD -> origin/HEAD
   ! [remote rejected] origin/master -> origin/master (failed to lock)
  error: failed to push some refs to '../backup.git'

The reason is that refs/remotes/origin/HEAD is a symref to
refs/remotes/origin/master, but it is not possible for the sending side
to unambiguously know this.

This commit fixes the issue by having receive-pack ignore any update to
a symref whose target is being identically updated. If a symref and its
target are being updated inconsistently, then the update for both fails
with an error message ("refusing inconsistent update...") to help
diagnose the situation.

Signed-off-by: Jay Soffian <jaysoffian@gmail.com>
---
 builtin/receive-pack.c |   64 +++++++++++++++++++++++++++++++++++++++++++++++-
 t/t5516-fetch-push.sh  |   45 +++++++++++++++++++++++++++++++++
 2 files changed, 108 insertions(+), 1 deletions(-)

Changes from v1 (incorporating Junio's feedback):
- Reformatted commit message; minor rewording.
- Detect situation where there is an inconsistent aliased update and
  give a better diagnostic than "failed to lock"
- Add additional test case for inconsistent update situation

[
This one is correct, the interdiff from
<1271694343-31876-2-git-send-email-jaysoffian@gmail.com> is

  diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
  index 414446b..7b269d2 100644
  --- a/builtin/receive-pack.c
  +++ b/builtin/receive-pack.c
  @@ -512,6 +512,8 @@ static void check_aliased_update(struct command *cmd, struct string_list *list)
   	    !hashcmp(cmd->new_sha1, dst_cmd->new_sha1))
   		return;
   
  +	dst_cmd->skip_update = 1;
  +
   	strcpy(cmd_oldh, find_unique_abbrev(cmd->old_sha1, DEFAULT_ABBREV));
   	strcat(cmd_newh, find_unique_abbrev(cmd->new_sha1, DEFAULT_ABBREV));
   	strcpy(dst_oldh, find_unique_abbrev(dst_cmd->old_sha1, DEFAULT_ABBREV));
  )
]
  
diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index fffb6ea..7b269d2 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -9,6 +9,7 @@
 #include "object.h"
 #include "remote.h"
 #include "transport.h"
+#include "string-list.h"
 
 static const char receive_pack_usage[] = "git receive-pack <git-dir>";
 
@@ -129,6 +130,7 @@ static void write_head_info(void)
 struct command {
 	struct command *next;
 	const char *error_string;
+	unsigned int skip_update;
 	unsigned char old_sha1[20];
 	unsigned char new_sha1[20];
 	char ref_name[FLEX_ARRAY]; /* more */
@@ -486,6 +488,63 @@ static void run_update_post_hook(struct command *commands)
 	}
 }
 
+static void check_aliased_update(struct command *cmd, struct string_list *list)
+{
+	struct string_list_item *item;
+	struct command *dst_cmd;
+	unsigned char sha1[20];
+	char cmd_oldh[41], cmd_newh[41], dst_oldh[41], dst_newh[41];
+	int flag;
+
+	const char *dst_name = resolve_ref(cmd->ref_name, sha1, 0, &flag);
+
+	if (!(flag & REF_ISSYMREF))
+		return;
+
+	if ((item = string_list_lookup(dst_name, list)) == NULL)
+		return;
+
+	cmd->skip_update = 1;
+
+	dst_cmd = (struct command *) item->util;
+
+	if (!hashcmp(cmd->old_sha1, dst_cmd->old_sha1) &&
+	    !hashcmp(cmd->new_sha1, dst_cmd->new_sha1))
+		return;
+
+	dst_cmd->skip_update = 1;
+
+	strcpy(cmd_oldh, find_unique_abbrev(cmd->old_sha1, DEFAULT_ABBREV));
+	strcat(cmd_newh, find_unique_abbrev(cmd->new_sha1, DEFAULT_ABBREV));
+	strcpy(dst_oldh, find_unique_abbrev(dst_cmd->old_sha1, DEFAULT_ABBREV));
+	strcat(dst_newh, find_unique_abbrev(dst_cmd->new_sha1, DEFAULT_ABBREV));
+	rp_error("refusing inconsistent update between symref '%s' (%s..%s) and"
+		 " its target '%s' (%s..%s)",
+		 cmd->ref_name, cmd_oldh, cmd_newh,
+		 dst_cmd->ref_name, dst_oldh, dst_newh);
+
+	cmd->error_string = dst_cmd->error_string =
+		"inconsistent aliased update";
+}
+
+static void check_aliased_updates(struct command *commands)
+{
+	struct command *cmd;
+	struct string_list ref_list = { NULL, 0, 0, 0 };
+
+	for (cmd = commands; cmd; cmd = cmd->next) {
+		struct string_list_item *item =
+			string_list_append(cmd->ref_name, &ref_list);
+		item->util = (void *)cmd;
+	}
+	sort_string_list(&ref_list);
+
+	for (cmd = commands; cmd; cmd = cmd->next)
+		check_aliased_update(cmd, &ref_list);
+
+	string_list_clear(&ref_list, 0);
+}
+
 static void execute_commands(struct command *commands, const char *unpacker_error)
 {
 	struct command *cmd;
@@ -503,9 +562,11 @@ static void execute_commands(struct command *commands, const char *unpacker_erro
 		return;
 	}
 
+	check_aliased_updates(commands);
+
 	head_name = resolve_ref("HEAD", sha1, 0, NULL);
 
-	for (cmd = commands; cmd; cmd = cmd->next)
+	for (cmd = commands; cmd && !cmd->skip_update; cmd = cmd->next)
 		cmd->error_string = update(cmd);
 }
 
@@ -545,6 +606,7 @@ static struct command *read_head_info(void)
 		hashcpy(cmd->old_sha1, old_sha1);
 		hashcpy(cmd->new_sha1, new_sha1);
 		memcpy(cmd->ref_name, line + 82, len - 81);
+		cmd->skip_update = 0;
 		cmd->error_string = NULL;
 		cmd->next = NULL;
 		*p = cmd;
diff --git a/t/t5516-fetch-push.sh b/t/t5516-fetch-push.sh
index 2de98e6..bc3f24f 100755
--- a/t/t5516-fetch-push.sh
+++ b/t/t5516-fetch-push.sh
@@ -660,6 +660,51 @@ test_expect_success 'push with branches containing #' '
 	git checkout master
 '
 
+test_expect_success 'push into aliased refs (consistent)' '
+	mk_test heads/master &&
+	mk_child child1 &&
+	mk_child child2 &&
+	(cd child1 &&
+		git branch foo &&
+		git symbolic-ref refs/heads/bar refs/heads/foo
+		git config receive.denyCurrentBranch false
+	) &&
+	(cd child2 &&
+		: >path2 &&
+		git add path2 &&
+		test_tick &&
+		git commit -a -m child2 &&
+		git branch foo &&
+		git branch bar &&
+		git push ../child1 foo bar
+	)
+'
+
+test_expect_success 'push into aliased refs (inconsistent)' '
+	mk_test heads/master &&
+	mk_child child1 &&
+	mk_child child2 &&
+	(cd child1 &&
+		git branch foo &&
+		git symbolic-ref refs/heads/bar refs/heads/foo
+		git config receive.denyCurrentBranch false
+	) &&
+	(cd child2 &&
+		: >path2 &&
+		git add path2 &&
+		test_tick &&
+		git commit -a -m child2 &&
+		git branch foo &&
+		: >path3 &&
+		git add path3 &&
+		test_tick &&
+		git commit -a -m child2 &&
+		git branch bar &&
+		test_must_fail git push ../child1 foo bar 2>stderr &&
+		grep "refusing inconsistent update" stderr
+	)
+'
+
 test_expect_success 'push --porcelain' '
 	mk_empty &&
 	echo >.git/foo  "To testrepo" &&
-- 
1.7.0.3.436.g2b878