Re: master 2c79a8f 2/2: Use posix_spawn if possible.

[Aaron Jensen <aaronjensen@gmail.com>]

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On Tue, Nov 9, 2021 at 9:46 AM Philipp <p.stephani2@gmail.com> wrote:
>
> I guess either way is fine.  I'd slightly lean towards enabling it whenever it's available (i.e. remove the condition on DARWIN_OS):

I'm good with that, I only added it because I believe in an earlier
message in the thread that was called for. Patch for that attached.
It'd be best and totally fine with me if you wanted to commit this
with your name attached and not mine as I only did the conflict
resolution and am not familiar with the code itself.

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From f20deb3924b00dea21361ee7178c5892e8da01f9 Mon Sep 17 00:00:00 2001
From: Aaron Jensen <aaronjensen@gmail.com>
Date: Mon, 8 Nov 2021 05:38:11 -0500
Subject: [PATCH] Use posix_spawn if possible

posix_spawn is less error-prone than vfork + execve, and can make
better use of system-specific enhancements like 'clone' on Linux.  Use
it if we don't need to configure a pseudoterminal.

* configure.ac (HAVE_SPAWN_H, HAVE_POSIX_SPAWN)
(HAVE_POSIX_SPAWN_FILE_ACTIONS_ADDCHDIR)
(HAVE_POSIX_SPAWN_FILE_ACTIONS_ADDCHDIR_NP)
(HAVE_POSIX_SPAWNATTR_SETFLAGS, HAVE_DECL_POSIX_SPAWN_SETSID): New
configuration variables.
* src/callproc.c (USABLE_POSIX_SPAWN): New configuration macro.
(emacs_posix_spawn_init_actions)
(emacs_posix_spawn_init_attributes, emacs_posix_spawn_init): New
helper functions.
(emacs_spawn): Use posix_spawn if possible.

Originally authored by: Philipp Stephani <phst@google.com>
---
 configure.ac   |  17 +++++
 src/callproc.c | 192 ++++++++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 208 insertions(+), 1 deletion(-)

diff --git a/configure.ac b/configure.ac
index 33e7037afe..c231c2ceae 100644
--- a/configure.ac
+++ b/configure.ac
@@ -4771,6 +4771,23 @@ AC_DEFUN
 dnl the current CFLAGS etc.
 AC_CHECK_FUNCS(snprintf)
 
+dnl posix_spawn.  The chdir and setsid functionality is relatively
+dnl recent, so we check for it specifically.
+AC_CHECK_HEADERS([spawn.h])
+AC_SUBST([HAVE_SPAWN_H])
+AC_CHECK_FUNCS([posix_spawn \
+                posix_spawn_file_actions_addchdir \
+                posix_spawn_file_actions_addchdir_np \
+                posix_spawnattr_setflags])
+AC_SUBST([HAVE_POSIX_SPAWN])
+AC_SUBST([HAVE_POSIX_SPAWN_FILE_ACTIONS_ADDCHDIR])
+AC_SUBST([HAVE_POSIX_SPAWN_FILE_ACTIONS_ADDCHDIR_NP])
+AC_SUBST([HAVE_POSIX_SPAWNATTR_SETFLAGS])
+AC_CHECK_DECLS([POSIX_SPAWN_SETSID], [], [], [[
+               #include <spawn.h>
+               ]])
+AC_SUBST([HAVE_DECL_POSIX_SPAWN_SETSID])
+
 dnl Check for glib.  This differs from other library checks in that
 dnl Emacs need not link to glib unless some other library is already
 dnl linking to glib.  Although glib provides no facilities that Emacs
diff --git a/src/callproc.c b/src/callproc.c
index fa43f97384..cb69b8ddd8 100644
--- a/src/callproc.c
+++ b/src/callproc.c
@@ -28,6 +28,20 @@ Copyright (C) 1985-1988, 1993-1995, 1999-2021 Free Software Foundation,
 #include <sys/file.h>
 #include <fcntl.h>
 
+/* In order to be able to use `posix_spawn', it needs to support some
+   variant of `chdir' as well as `setsid'.  */
+#if defined HAVE_SPAWN_H && defined HAVE_POSIX_SPAWN        \
+  && defined HAVE_POSIX_SPAWNATTR_SETFLAGS                  \
+  && (defined HAVE_POSIX_SPAWN_FILE_ACTIONS_ADDCHDIR        \
+      || defined HAVE_POSIX_SPAWN_FILE_ACTIONS_ADDCHDIR_NP) \
+  && defined HAVE_DECL_POSIX_SPAWN_SETSID                   \
+  && HAVE_DECL_POSIX_SPAWN_SETSID == 1
+# include <spawn.h>
+# define USABLE_POSIX_SPAWN 1
+#else
+# define USABLE_POSIX_SPAWN 0
+#endif
+
 #include "lisp.h"
 
 #ifdef SETUP_SLAVE_PTY
@@ -1247,6 +1261,130 @@ child_setup (int in, int out, int err, char **new_argv, char **env,
 #endif  /* not WINDOWSNT */
 }
 
+#if USABLE_POSIX_SPAWN
+
+/* Set up ACTIONS and ATTRIBUTES for `posix_spawn'.  Return an error
+   number.  */
+
+static int
+emacs_posix_spawn_init_actions (posix_spawn_file_actions_t *actions,
+                                int std_in, int std_out, int std_err,
+                                const char *cwd)
+{
+  int error = posix_spawn_file_actions_init (actions);
+  if (error != 0)
+    return error;
+
+  error = posix_spawn_file_actions_adddup2 (actions, std_in,
+                                            STDIN_FILENO);
+  if (error != 0)
+    goto out;
+
+  error = posix_spawn_file_actions_adddup2 (actions, std_out,
+                                            STDOUT_FILENO);
+  if (error != 0)
+    goto out;
+
+  error = posix_spawn_file_actions_adddup2 (actions,
+                                            std_err < 0 ? std_out
+                                                        : std_err,
+                                            STDERR_FILENO);
+  if (error != 0)
+    goto out;
+
+  error =
+#ifdef HAVE_POSIX_SPAWN_FILE_ACTIONS_ADDCHDIR
+    posix_spawn_file_actions_addchdir
+#else
+    posix_spawn_file_actions_addchdir_np
+#endif
+    (actions, cwd);
+  if (error != 0)
+    goto out;
+
+ out:
+  if (error != 0)
+    posix_spawn_file_actions_destroy (actions);
+  return error;
+}
+
+static int
+emacs_posix_spawn_init_attributes (posix_spawnattr_t *attributes)
+{
+  int error = posix_spawnattr_init (attributes);
+  if (error != 0)
+    return error;
+
+  error = posix_spawnattr_setflags (attributes,
+                                    POSIX_SPAWN_SETSID
+                                      | POSIX_SPAWN_SETSIGDEF
+                                      | POSIX_SPAWN_SETSIGMASK);
+  if (error != 0)
+    goto out;
+
+  sigset_t sigdefault;
+  sigemptyset (&sigdefault);
+
+#ifdef DARWIN_OS
+  /* Work around a macOS bug, where SIGCHLD is apparently
+     delivered to a vforked child instead of to its parent.  See:
+     https://lists.gnu.org/r/emacs-devel/2017-05/msg00342.html
+  */
+  sigaddset (&sigdefault, SIGCHLD);
+#endif
+
+  sigaddset (&sigdefault, SIGINT);
+  sigaddset (&sigdefault, SIGQUIT);
+#ifdef SIGPROF
+  sigaddset (&sigdefault, SIGPROF);
+#endif
+
+  /* Emacs ignores SIGPIPE, but the child should not.  */
+  sigaddset (&sigdefault, SIGPIPE);
+  /* Likewise for SIGPROF.  */
+#ifdef SIGPROF
+  sigaddset (&sigdefault, SIGPROF);
+#endif
+
+  error = posix_spawnattr_setsigdefault (attributes, &sigdefault);
+  if (error != 0)
+    goto out;
+
+  /* Stop blocking SIGCHLD in the child.  */
+  sigset_t oldset;
+  error = pthread_sigmask (SIG_SETMASK, NULL, &oldset);
+  if (error != 0)
+    goto out;
+  error = posix_spawnattr_setsigmask (attributes, &oldset);
+  if (error != 0)
+    goto out;
+
+ out:
+  if (error != 0)
+    posix_spawnattr_destroy (attributes);
+
+  return error;
+}
+
+static int
+emacs_posix_spawn_init (posix_spawn_file_actions_t *actions,
+                        posix_spawnattr_t *attributes, int std_in,
+                        int std_out, int std_err, const char *cwd)
+{
+  int error = emacs_posix_spawn_init_actions (actions, std_in,
+                                              std_out, std_err, cwd);
+  if (error != 0)
+    return error;
+
+  error = emacs_posix_spawn_init_attributes (attributes);
+  if (error != 0)
+    return error;
+
+  return 0;
+}
+
+#endif
+
 /* Start a new asynchronous subprocess.  If successful, return zero
    and store the process identifier of the new process in *NEWPID.
    Use STDIN, STDOUT, and STDERR as standard streams for the new
@@ -1266,10 +1404,58 @@ emacs_spawn (pid_t *newpid, int std_in, int std_out, int std_err,
              char **argv, char **envp, const char *cwd,
              const char *pty, const sigset_t *oldset)
 {
+#if USABLE_POSIX_SPAWN
+  /* Prefer the simpler `posix_spawn' if available.  `posix_spawn'
+     doesn't yet support setting up pseudoterminals, so we fall back
+     to `vfork' if we're supposed to use a pseudoterminal.  */
+
+  bool use_posix_spawn = pty == NULL;
+
+  posix_spawn_file_actions_t actions;
+  posix_spawnattr_t attributes;
+
+  if (use_posix_spawn)
+    {
+      /* Initialize optional attributes before blocking. */
+      int error
+        = emacs_posix_spawn_init (&actions, &attributes, std_in,
+                                  std_out, std_err, cwd);
+      if (error != 0)
+	return error;
+    }
+#endif
+
   int pid;
+  int vfork_error;
 
   eassert (input_blocked_p ());
 
+#if USABLE_POSIX_SPAWN
+  if (use_posix_spawn)
+    {
+      vfork_error = posix_spawn (&pid, argv[0], &actions, &attributes,
+                                 argv, envp);
+      if (vfork_error != 0)
+	pid = -1;
+
+      int error = posix_spawn_file_actions_destroy (&actions);
+      if (error != 0)
+	{
+	  errno = error;
+	  emacs_perror ("posix_spawn_file_actions_destroy");
+	}
+
+      error = posix_spawnattr_destroy (&attributes);
+      if (error != 0)
+	{
+	  errno = error;
+	  emacs_perror ("posix_spawnattr_destroy");
+	}
+
+      goto fork_done;
+    }
+#endif
+
 #ifndef WINDOWSNT
   /* vfork, and prevent local vars from being clobbered by the vfork.  */
   pid_t *volatile newpid_volatile = newpid;
@@ -1413,7 +1599,11 @@ emacs_spawn (pid_t *newpid, int std_in, int std_out, int std_err,
 
   /* Back in the parent process.  */
 
-  int vfork_error = pid < 0 ? errno : 0;
+  vfork_error = pid < 0 ? errno : 0;
+
+#if USABLE_POSIX_SPAWN
+ fork_done:
+#endif
 
   if (pid < 0)
     {
-- 
2.33.1