On Jul 21 23:26, Eric Blake wrote:
> I finally figured out why autoconf is still failing its flock-related tests,
> and why perl was reliably failing even though my simple attempts in C were
> always passing. It turns out that if you do:
>
> open
> flock(LOCK_EX)
> if (!fork)
> execlp("sleep","sleep","10",NULL);
> sleep(10);
>
> then ProcessExplorer shows that the Event in the global namespace of
> flock-dev-
> ino\20-2-* exists in both parent and child, and with a notification level of
> false, blocking any outside influence until both the parent and forkee exit.
> But if you do:
>
> open
> fcntl (fd, F_SETFD, FD_CLOEXEC | fcntl (fd, F_GETFD))
> flock(LOCK_EX)
> if (!fork)
> execlp("sleep","sleep","10",NULL);
> sleep(10);
>
> then only the parent holds a handle to the Event, but with a notification
> level
> of true, allowing any outside party to do whatever they want.
Do you have a working C testcase to demonstrate this?
> I'm still trying to figure out why the close-on-exec cleanup appears to be
> spuriously triggering the flock Event to unlock. But my understanding is
> that
> F_FLOCK locks should survive over exec, so the close-on-exec cleanup should
> only trigger lock release on F_POSIX locks.
I have a hunch it's a thinko in fhandler_base::del_my_locks. In case of
close_on_exec, the underlying file handle is already invalid.
Here's a question: If you strace this, do you get a debug message
from get_obj_handle_count with a status code from NtQueryObject?
Does the below patch fix the problem? Does the reasoning sound...
reasonable?
Index: flock.cc
===================================================================
RCS file: /cvs/src/src/winsup/cygwin/flock.cc,v
retrieving revision 1.23
diff -u -p -r1.23 flock.cc
--- flock.cc 14 Jul 2009 17:37:42 -0000 1.23
+++ flock.cc 22 Jul 2009 10:07:17 -0000
@@ -350,8 +350,18 @@ fhandler_base::del_my_locks (bool after_
inode_t *node = inode_t::get (get_dev (), get_ino (), false);
if (node)
{
+ /* In the close-on-exec case, our io handle is already invalid.
+ We can't use it to test for the object reference count.
+ However, that shouldn't be necessary for the following reason.
+ After exec, there are no threads in the current process
+ waiting for the lock. So, either we're the only process
+ accessing the file table entry and there are no threads
+ which require signalling, or we have a parent process still
+ accessing the file object and signalling the lock event would
+ be premature. */
bool no_locks_left =
- node->del_my_locks (after_fork ? 0 : get_unique_id (), get_handle ());
+ node->del_my_locks (after_fork ? 0 : get_unique_id (),
+ close_on_exec () ? NULL : get_handle ());
if (no_locks_left)
{
LIST_REMOVE (node, i_next);
Corinna
--
Corinna Vinschen Please, send mails regarding Cygwin to
Cygwin Project Co-Leader cygwin AT cygwin DOT com
Red Hat
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