[PATCH v3 1/8] git-p4: detect/prevent infinite loop in gitCommitByP4Change()

["Mazo, Andrey" <amazo@checkvideo.com>]

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>
---

Notes:
    I don't have a simple test-case for this yet,
    and I was able to perform a few complex initial `git p4 sync` runs
    without hitting this problem.
    
    I suspect, I had somehow messed up with branch definitions
    and --changesfile option at some point.

 git-p4.py | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/git-p4.py b/git-p4.py
index 5b79920f46..c0a3068b6f 100755
--- a/git-p4.py
+++ b/git-p4.py
@@ -3323,11 +3323,13 @@ def gitCommitByP4Change(self, ref, change):
                 return next
 
             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 ""
 
     def importNewBranch(self, branch, maxChange):
         # make fast-import flush all changes to disk and update the refs using the checkpoint
-- 
2.19.2