Re: What's cooking in git.git (Nov 2016, #06; Mon, 28)

[Brandon Williams <bmwill@google.com>]

On 11/30, Jeff King wrote:
> On Wed, Nov 30, 2016 at 06:32:04PM -0500, Jeff King wrote:
> 
> > On Wed, Nov 30, 2016 at 03:28:23PM -0800, Brandon Williams wrote:
> > 
> > > So I couldn't find a race condition in the code.  I tracked the problem
> > > to grep_source_load_file which attempts to run lstat on the file so that
> > > it can read it into a buffer.  The lstat call fails with ENOENT (which
> > > conveniently is skipped by the if statement which calls error_errno).  So
> > > for some reason the file cannot be found and read into memory resulting
> > > in nothing being grep'ed for that particular file (since the buffer is
> > > NULL).
> > 
> > That's definitely weird. Is it possible that any of the underlying calls
> > from another thread are using chdir()? I think realpath() make do that
> > behind the scenes, and there may be others.
> > 
> > A full strace from a failing case would be interesting reading. In
> > theory we should be able to get that by running the stress script for
> > long enough. :)
> 
> Actually, it failed pretty much immediately. I guess the extra stracing
> changes the timing to make the problem _more_ likely.
> 
> And indeed, I see:
> 
> 20867 lstat("fi:le",  <unfinished ...>
> 20813 <... read resumed> "", 232)       = 0
> 20871 futex(0x558cdec8b164, FUTEX_WAIT_PRIVATE, 7, NULL <unfinished ...>
> 20813 close(7 <unfinished ...>
> 20870 <... futex resumed> )             = 0
> 20869 lstat(".gitmodules",  <unfinished ...>
> 20813 <... close resumed> )             = 0
> 20865 set_robust_list(0x7f1df92579e0, 24 <unfinished ...>
> 20813 lstat("su:b/../.git/modules/su:b/commondir", 0x7ffecc8b3ac0) = -1 ENOENT (No such file or directory)
> 20865 <... set_robust_list resumed> )   = 0
> 20868 set_robust_list(0x7f1df7a549e0, 24 <unfinished ...>
> 20813 access("su:b/../.git/modules/su:b/objects", X_OK) = 0
> 20813 access("su:b/../.git/modules/su:b/refs", X_OK) = 0
> 20813 stat("su:b/../.git/modules/su:b", {st_mode=S_IFDIR|0755, st_size=280, ...}) = 0
> 20813 getcwd("/var/ram/git-stress/root-4/trash directory.t7814-grep-recurse-submodules/parent", 129) = 80
> 20869 <... lstat resumed> {st_mode=S_IFREG|0644, st_size=47, ...}) = 0
> 20813 chdir("su:b/../.git/modules/su:b") = 0
> 20869 open(".gitmodules", O_RDONLY <unfinished ...>
> 20813 getcwd( <unfinished ...>
> 20867 <... lstat resumed> 0x7f1df8254cf0) = -1 ENOENT (No such file or directory)
> 
> where 20813 and 20867 are two threads of the main process. One is doing
> the lstat and the other calls chdir at the same moment.
> 
> -Peff

Yeah so it looks like the start_command function calls chdir.  Which
means any uses of the run-command interface are not thread safe....

For now the work around could be to just pass "-C <dir>" to the child
process instead of relying on run-command to chdir.

-- 
Brandon Williams