git-p4: detect/prevent infinite loop in gitCommitByP4Change()

Under certain circumstances, gitCommitByP4Change() can enter an infinite
loop resulting in `git p4 sync` hanging forever.

The problem is that
`git rev-list --bisect <latest> ^<earliest>` can return `<latest>`,
which would result in reinspecting <latest> and potentially an infinite loop.

This can happen when importing just a subset of P4 repository
and/or with explicit "--changesfile" option.

A real-life example:
"""
looking in ref refs/remotes/p4/mybranch for change 26894 using bisect...
Reading pipe: git rev-parse refs/remotes/p4/mybranch
trying: earliest  latest 4daff81c520a82678e1ef347f2b5e97258101ae1
Reading pipe: git rev-list --bisect 4daff81c520a82678e1ef347f2b5e97258101ae1
Reading pipe: git cat-file commit 147f5d3292af2e1cc4a56a7b96db845144c68486
current change 25339
trying: earliest ^147f5d3292af2e1cc4a56a7b96db845144c68486 latest 4daff81c520a82678e1ef347f2b5e97258101ae1
Reading pipe: git rev-list --bisect 4daff81c520a82678e1ef347f2b5e97258101ae1 ^147f5d3292af2e1cc4a56a7b96db845144c68486
Reading pipe: git cat-file commit 51db83df9d588010d0bd995641c85aa0408a5bb9
current change 25420
trying: earliest ^51db83df9d588010d0bd995641c85aa0408a5bb9 latest 4daff81c520a82678e1ef347f2b5e97258101ae1
Reading pipe: git rev-list --bisect 4daff81c520a82678e1ef347f2b5e97258101ae1 ^51db83df9d588010d0bd995641c85aa0408a5bb9
Reading pipe: git cat-file commit e8f83909ceb570f5a7e48c2853f3c5d8207cea52
current change 25448
trying: earliest ^e8f83909ceb570f5a7e48c2853f3c5d8207cea52 latest 4daff81c520a82678e1ef347f2b5e97258101ae1
Reading pipe: git rev-list --bisect 4daff81c520a82678e1ef347f2b5e97258101ae1 ^e8f83909ceb570f5a7e48c2853f3c5d8207cea52
Reading pipe: git cat-file commit 09a48eb7acd594dce52e06681be9c366e1844d66
current change 25521
trying: earliest ^09a48eb7acd594dce52e06681be9c366e1844d66 latest 4daff81c520a82678e1ef347f2b5e97258101ae1
Reading pipe: git rev-list --bisect 4daff81c520a82678e1ef347f2b5e97258101ae1 ^09a48eb7acd594dce52e06681be9c366e1844d66
Reading pipe: git cat-file commit 4daff81c520a82678e1ef347f2b5e97258101ae1
current change 26907
trying: earliest ^09a48eb7acd594dce52e06681be9c366e1844d66 latest 4daff81c520a82678e1ef347f2b5e97258101ae1
Reading pipe: git rev-list --bisect 4daff81c520a82678e1ef347f2b5e97258101ae1 ^09a48eb7acd594dce52e06681be9c366e1844d66
Reading pipe: git cat-file commit 4daff81c520a82678e1ef347f2b5e97258101ae1
current change 26907
trying: earliest ^09a48eb7acd594dce52e06681be9c366e1844d66 latest 4daff81c520a82678e1ef347f2b5e97258101ae1
Reading pipe: git rev-list --bisect 4daff81c520a82678e1ef347f2b5e97258101ae1 ^09a48eb7acd594dce52e06681be9c366e1844d66
Reading pipe: git cat-file commit 4daff81c520a82678e1ef347f2b5e97258101ae1
current change 26907
...
"""

The fix is two-fold:
* detect an infinite loop and die right away
instead of looping forever;
* make sure, `git rev-list --bisect` can't return "latestCommit" again
by excluding it from the rev-list range explicitly.

Signed-off-by: Andrey Mazo <amazo@checkvideo.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/git-p4.py b/git-p4.py
index 5b79920f46a972437ff05966d042a84d44f050d9..c0a3068b6f3994a0389f5458f7b35e06ee589519 100755 (executable)
--- a/git-p4.py
+++ b/git-p4.py
@@ -3325,7 +3325,9 @@ def gitCommitByP4Change(self, ref, change):
             if currentChange < change:
                 earliestCommit = "^%s" % next
             else:
-                latestCommit = "%s" % next
+                if next == latestCommit:
+                    die("Infinite loop while looking in ref %s for change %s. Check your branch mappings" % (ref, change))
+                latestCommit = "%s^@" % next
 
         return ""