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Cygwin: posix_spawn: add Cygwin-specific code fixing process synchronisation

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Newlib's posix_spawn has been taken from FreeBSD.  The code relies on
BSD-specific behaviour of vfork, namely the fact that vfork blocks
the parent until the child exits or calls execve as well as the fact
that the child shares parent memory in non-COW mode.

This behaviour can't be emulated by Cygwin.  Cygwin's vfork is
equivalent to fork.  This is POSIX-compliant, but it's lacking BSD's
vfork ingrained synchronization of the parent to wait for the child
calling execve, or the chance to just write a variable and the parent
will see the result.

So this requires a Cygwin-specific solution.  The core function of
posix_spawn, called do_posix_spawn is now implemented twice, once using
the BSD method, and once for Cygwin using Windows synchronization under
the hood waiting for the child to call execve and signalling errors
upstream.  The Windows specifics are hidden inside Cygwin, so newlib
only calls internal Cygwin functions.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>
This commit is contained in:
Corinna Vinschen
2020-08-01 21:35:58 +02:00
parent c222c1b294
commit 3fbfcd11fb
4 changed files with 206 additions and 8 deletions
Download Patch File Download Diff File Expand all files Collapse all files

104
winsup/cygwin/spawn.cc
View File

@ -252,6 +252,8 @@ struct system_call_handle
child_info_spawn NO_COPY ch_spawn;
extern "C" void __posix_spawn_sem_release (void *sem, int error);
int
child_info_spawn::worker (const char *prog_arg, const char *const *argv,
const char *const envp[], int mode,
@ -897,6 +899,8 @@ child_info_spawn::worker (const char *prog_arg, const char *const *argv,
&& WaitForSingleObject (pi.hProcess, 0) == WAIT_TIMEOUT)
wait_for_myself ();
}
if (sem)
__posix_spawn_sem_release (sem, 0);
myself.exit (EXITCODE_NOSET);
break;
case _P_WAIT:
@ -1295,3 +1299,103 @@ err:
__seterrno ();
return -1;
}
/* The following __posix_spawn_* functions are called from newlib's posix_spawn
implementation. The original code in newlib has been taken from FreeBSD,
and the core code relies on specific, non-portable behaviour of vfork(2).
Our replacement implementation uses a semaphore to synchronize parent and
child process. Note: __posix_spawn_fork in fork.cc is part of the set. */
/* Create an inheritable semaphore. Set it to 0 (== non-signalled), so the
parent can wait on the semaphore immediately. */
extern "C" int
__posix_spawn_sem_create (void **semp)
{
HANDLE sem;
OBJECT_ATTRIBUTES attr;
NTSTATUS status;
if (!semp)
return EINVAL;
InitializeObjectAttributes (&attr, NULL, OBJ_INHERIT, NULL, NULL);
status = NtCreateSemaphore (&sem, SEMAPHORE_ALL_ACCESS, &attr, 0, INT_MAX);
if (!NT_SUCCESS (status))
return geterrno_from_nt_status (status);
*semp = sem;
return 0;
}
/* Signal the semaphore. "error" should be 0 if all went fine and the
exec'd child process is up and running, a useful POSIX error code otherwise.
After releasing the semaphore, the value of the semaphore reflects
the error code + 1. Thus, after WFMO in__posix_spawn_sem_wait_and_close,
querying the value of the semaphore returns either 0 if all went well,
or a value > 0 equivalent to the POSIX error code. */
extern "C" void
__posix_spawn_sem_release (void *sem, int error)
{
ReleaseSemaphore (sem, error + 1, NULL);
}
/* Helper to check the semaphore value. */
static inline int
__posix_spawn_sem_query (void *sem)
{
SEMAPHORE_BASIC_INFORMATION sbi;
NtQuerySemaphore (sem, SemaphoreBasicInformation, &sbi, sizeof sbi, NULL);
return sbi.CurrentCount;
}
/* Called from parent to wait for fork/exec completion. We're waiting for
the semaphore as well as the child's process handle, so even if the
child crashes without signalling the semaphore, we won't wait infinitely. */
extern "C" int
__posix_spawn_sem_wait_and_close (void *sem, void *proc)
{
int ret = 0;
HANDLE w4[2] = { sem, proc };
switch (WaitForMultipleObjects (2, w4, FALSE, INFINITE))
{
case WAIT_OBJECT_0:
ret = __posix_spawn_sem_query (sem);
break;
case WAIT_OBJECT_0 + 1:
/* If we return here due to the child process dying, the semaphore is
very likely not signalled. Check this here and return a valid error
code. */
ret = __posix_spawn_sem_query (sem);
if (ret == 0)
ret = ECHILD;
break;
default:
ret = geterrno_from_win_error ();
break;
}
CloseHandle (sem);
return ret;
}
/* Replacement for execve/execvpe, called from forked child in newlib's
posix_spawn. The relevant difference is the additional semaphore
so the worker method (which is not supposed to return on success)
can signal the semaphore after sync'ing with the exec'd child. */
extern "C" int
__posix_spawn_execvpe (const char *path, char * const *argv, char *const *envp,
HANDLE sem, int use_env_path)
{
path_conv buf;
static char *const empty_env[] = { NULL };
if (!envp)
envp = empty_env;
ch_spawn.set_sem (sem);
ch_spawn.worker (use_env_path ? (find_exec (path, buf, "PATH", FE_NNF) ?: "")
: path,
argv, envp,
_P_OVERLAY | (use_env_path ? _P_PATH_TYPE_EXEC : 0));
__posix_spawn_sem_release (sem, errno);
return -1;
}