Subject: bug#52835: [PATCH v6 2/3] Make system* and piped-process internally use spawn.

From: Josselin Poiret <[email-masked]>
Date: Thu, 22 Dec 2022 13:49:09 +0100

* libguile/posix.c (scm_system_star, scm_piped_process): Use do_spawn.
(start_child): Remove function.
---
 libguile/posix.c | 181 ++++++++++-------------------------------------
 1 file changed, 39 insertions(+), 142 deletions(-)

diff --git a/libguile/posix.c b/libguile/posix.c
index e92625483..f9c36d7ac 100644
--- a/libguile/posix.c
+++ b/libguile/posix.c
@@ -1308,125 +1308,6 @@ SCM_DEFINE (scm_fork, "primitive-fork", 0, 0, 0,
 #undef FUNC_NAME
 #endif /* HAVE_FORK */
 
-#ifdef HAVE_FORK
-/* 'renumber_file_descriptor' is a helper function for 'start_child'
-   below, and is specialized for that particular environment where it
-   doesn't make sense to report errors via exceptions.  It uses dup(2)
-   to duplicate the file descriptor FD, closes the original FD, and
-   returns the new descriptor.  If dup(2) fails, print an error message
-   to ERR and abort.  */
-static int
-renumber_file_descriptor (int fd, int err)
-{
-  int new_fd;
-
-  do
-    new_fd = dup (fd);
-  while (new_fd == -1 && errno == EINTR);
-
-  if (new_fd == -1)
-    {
-      /* At this point we are in the child process before exec.  We
-         cannot safely raise an exception in this environment.  */
-      const char *msg = strerror (errno);
-      fprintf (fdopen (err, "a"), "start_child: dup failed: %s\n", msg);
-      _exit (127);  /* Use exit status 127, as with other exec errors. */
-    }
-
-  close (fd);
-  return new_fd;
-}
-#endif /* HAVE_FORK */
-
-#ifdef HAVE_FORK
-#define HAVE_START_CHILD 1
-/* Since Guile uses threads, we have to be very careful to avoid calling
-   functions that are not async-signal-safe in the child.  That's why
-   this function is implemented in C.  */
-static pid_t
-start_child (const char *exec_file, char **exec_argv,
-	     int reading, int c2p[2], int writing, int p2c[2],
-             int in, int out, int err)
-{
-  int pid;
-  int max_fd = 1024;
-
-#if defined (HAVE_GETRLIMIT) && defined (RLIMIT_NOFILE)
-  {
-    struct rlimit lim = { 0, 0 };
-    if (getrlimit (RLIMIT_NOFILE, &lim) == 0)
-      max_fd = lim.rlim_cur;
-  }
-#endif
-
-  pid = fork ();
-
-  if (pid != 0)
-    /* The parent, with either and error (pid == -1), or the PID of the
-       child.  Return directly in either case.  */
-    return pid;
-
-  /* The child.  */
-  if (reading)
-    close (c2p[0]);
-  if (writing)
-    close (p2c[1]);
-
-  /* Close all file descriptors in ports inherited from the parent
-     except for in, out, and err.  Heavy-handed, but robust.  */
-  while (max_fd--)
-    if (max_fd != in && max_fd != out && max_fd != err)
-      close (max_fd);
-
-  /* Ignore errors on these open() calls.  */
-  if (in == -1)
-    in = open ("/dev/null", O_RDONLY);
-  if (out == -1)
-    out = open ("/dev/null", O_WRONLY);
-  if (err == -1)
-    err = open ("/dev/null", O_WRONLY);
-
-  if (in > 0)
-    {
-      if (out == 0)
-        out = renumber_file_descriptor (out, err);
-      if (err == 0)
-        err = renumber_file_descriptor (err, err);
-      do dup2 (in, 0); while (errno == EINTR);
-      close (in);
-    }
-  if (out > 1)
-    {
-      if (err == 1)
-        err = renumber_file_descriptor (err, err);
-      do dup2 (out, 1); while (errno == EINTR);
-      if (out > 2)
-        close (out);
-    }
-  if (err > 2)
-    {
-      do dup2 (err, 2); while (errno == EINTR);
-      close (err);
-    }
-
-  execvp (exec_file, exec_argv);
-
-  /* The exec failed!  There is nothing sensible to do.  */
-  {
-    const char *msg = strerror (errno);
-    fprintf (fdopen (2, "a"), "In execvp of %s: %s\n",
-             exec_file, msg);
-  }
-
-  /* Use exit status 127, like shells in this case, as per POSIX
-     <http://pubs.opengroup.org/onlinepubs/007904875/utilities/xcu_chap02.html#tag_02_09_01_01>.  */
-  _exit (127);
-
-  /* Not reached.  */
-  return -1;
-}
-#endif
-
 static int
 do_spawn (char *exec_file, char **exec_argv, char **exec_env, int in, int out, int err)
 {
@@ -1507,7 +1388,7 @@ SCM_DEFINE (scm_spawn_process, "spawn*", 5, 0, 0,
 }
 #undef FUNC_NAME
 
-#ifdef HAVE_START_CHILD
+#ifdef HAVE_FORK
 static SCM
 scm_piped_process (SCM prog, SCM args, SCM from, SCM to)
 #define FUNC_NAME "piped-process"
@@ -1519,6 +1400,7 @@ scm_piped_process (SCM prog, SCM args, SCM from, SCM to)
   int pid;
   char *exec_file;
   char **exec_argv;
+  char **exec_env = environ;
 
   exec_file = scm_to_locale_string (prog);
   exec_argv = scm_i_allocate_string_pointers (scm_cons (prog, args));
@@ -1551,27 +1433,44 @@ scm_piped_process (SCM prog, SCM args, SCM from, SCM to)
       in = SCM_FPORT_FDES (port);
   }
 
-  pid = start_child (exec_file, exec_argv, reading, c2p, writing, p2c,
-                     in, out, err);
+  pid = do_spawn (exec_file, exec_argv, exec_env, in, out, err);
+  int errno_save = errno;
 
   if (pid == -1)
     {
-      int errno_save = errno;
-      free (exec_file);
-      if (reading)
-        {
-          close (c2p[0]);
-          close (c2p[1]);
-        }
-      if (writing)
-        {
-          close (p2c[0]);
-          close (p2c[1]);
-        }
-      errno = errno_save;
-      SCM_SYSERROR;
+      /* TODO This is a compatibility shim until the next major release */
+      switch (errno) {
+      /* If the error seemingly comes from fork */
+      case EAGAIN:
+      case ENOMEM:
+      case ENOSYS:
+        free (exec_file);
+
+        if (reading)
+          {
+            close (c2p[0]);
+          }
+        if (writing)
+          {
+            close (p2c[1]);
+          }
+        errno = errno_save;
+        SCM_SYSERROR;
+        break;
+      /* Else create a dummy process that exits with value 127 */
+      default:
+        dprintf (err, "In execvp of %s: %s\n", exec_file,
+                 strerror (errno_save));
+        pid = fork ();
+        if (pid == -1)
+          SCM_SYSERROR;
+        if (pid == 0)
+          _exit (127);
+      }
     }
 
+  free (exec_file);
+
   if (reading)
     close (c2p[1]);
   if (writing)
@@ -1651,7 +1550,7 @@ SCM_DEFINE (scm_system_star, "system*", 0, 0, 1,
   return scm_from_int (status);
 }
 #undef FUNC_NAME
-#endif /* HAVE_START_CHILD */
+#endif /* HAVE_FORK */
 
 #ifdef HAVE_UNAME
 SCM_DEFINE (scm_uname, "uname", 0, 0, 0,
@@ -2497,13 +2396,13 @@ SCM_DEFINE (scm_gethostname, "gethostname", 0, 0, 0,
 #endif /* HAVE_GETHOSTNAME */
 
 
-#ifdef HAVE_START_CHILD
+#ifdef HAVE_FORK
 static void
 scm_init_popen (void)
 {
   scm_c_define_gsubr ("piped-process", 2, 2, 0, scm_piped_process);
 }
-#endif /* HAVE_START_CHILD */
+#endif /* HAVE_FORK */
 
 void
 scm_init_posix ()
@@ -2621,12 +2520,10 @@ scm_init_posix ()
 
 #ifdef HAVE_FORK
   scm_add_feature ("fork");
-#endif	/* HAVE_FORK */
-#ifdef HAVE_START_CHILD
   scm_add_feature ("popen");
   scm_c_register_extension ("libguile-" SCM_EFFECTIVE_VERSION,
                             "scm_init_popen",
 			    (scm_t_extension_init_func) scm_init_popen,
 			    NULL);
-#endif /* HAVE_START_CHILD */
+#endif	/* HAVE_FORK */
 }
-- 
2.38.1