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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

34
winsup/cygwin/fork.cc
View File

@ -31,7 +31,7 @@ details. */
/* FIXME: Once things stabilize, bump up to a few minutes. */
#define FORK_WAIT_TIMEOUT (300 * 1000) /* 300 seconds */
static int dofork (bool *with_forkables);
static int dofork (void **proc, bool *with_forkables);
class frok
{
frok (bool *forkables)
@ -47,7 +47,7 @@ class frok
int __stdcall parent (volatile char * volatile here);
int __stdcall child (volatile char * volatile here);
bool error (const char *fmt, ...);
friend int dofork (bool *with_forkables);
friend int dofork (void **proc, bool *with_forkables);
};
static void
@ -583,17 +583,36 @@ extern "C" int
fork ()
{
bool with_forkables = false; /* do not force hardlinks on first try */
int res = dofork (&with_forkables);
int res = dofork (NULL, &with_forkables);
if (res >= 0)
return res;
if (with_forkables)
return res; /* no need for second try when already enabled */
with_forkables = true; /* enable hardlinks for second try */
return dofork (&with_forkables);
return dofork (NULL, &with_forkables);
}
/* __posix_spawn_fork is 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 needs the forked child's HANDLE for
synchronization, so __posix_spawn_fork returns it in proc. */
extern "C" int
__posix_spawn_fork (void **proc)
{
bool with_forkables = false; /* do not force hardlinks on first try */
int res = dofork (proc, &with_forkables);
if (res >= 0)
return res;
if (with_forkables)
return res; /* no need for second try when already enabled */
with_forkables = true; /* enable hardlinks for second try */
return dofork (proc, &with_forkables);
}
static int
dofork (bool *with_forkables)
dofork (void **proc, bool *with_forkables)
{
frok grouped (with_forkables);
@ -671,6 +690,11 @@ dofork (bool *with_forkables)
set_errno (grouped.this_errno);
}
else if (proc)
{
/* Return child process handle to posix_fork. */
*proc = grouped.hchild;
}
syscall_printf ("%R = fork()", res);
return res;
}