Add spawnfd for use where fork / forkfd can't be used

In certain environments, using fork() is not recommended due to the need
for an MMU. This commit adds support for those environments, by using
posix_spawn. Limitations of this environment are:
 - we cannot reliably detect failure to exec (e.g. non-existing executable)
 - we cannot do setsid(); we do setpgrp(0, 0) instead
 - we cannot thread-safely chdir() to the requested dir

Because of the former limitation, the QProcess unit tests that rely on
failure-to-start error conditions are either skipped or marked as
expected failures. There's a non-reliable solution that is implemented
in a another commit.

This change also makes it easier to transition the QNX builds to using
fork(), which is supported from QNX Neutrino 6.6 and onwards.

Change-Id: I5cb46abf2ef8783941525d35cc991f00d2bf2d58
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/3rdparty/forkfd/forkfd.c

@@ -590,3 +590,66 @@ err_free:
     freeInfo(header, info);
     return -1;
 }
+
+#ifdef _POSIX_SPAWN
+int spawnfd(int flags, pid_t *ppid, const char *path, const posix_spawn_file_actions_t *file_actions,
+            posix_spawnattr_t *attrp, char *const argv[], char *const envp[])
+{
+    Header *header;
+    ProcessInfo *info;
+    siginfo_t si;
+    pid_t pid;
+    int death_pipe[2];
+    int ret = -1;
+    /* we can only do work if we have a way to start the child in stopped mode;
+     * otherwise, we have a major race condition. */
+
+    (void) pthread_once(&forkfd_initialization, forkfd_initialize);
+
+    info = allocateInfo(&header);
+    if (info == NULL) {
+        errno = ENOMEM;
+        goto out;
+    }
+
+    /* create the pipe before we spawn */
+    if (create_pipe(death_pipe, flags) == -1)
+        goto err_free; /* failed to create the pipes, pass errno */
+
+    /* start the process */
+    if (flags & FFD_SPAWN_SEARCH_PATH) {
+        /* use posix_spawnp */
+        if (posix_spawnp(&pid, path, file_actions, attrp, argv, envp) != 0)
+            goto err_close;
+    } else {
+        if (posix_spawn(&pid, path, file_actions, attrp, argv, envp) != 0)
+            goto err_close;
+    }
+
+    if (ppid)
+        *ppid = pid;
+
+    /* Store the child's PID in the info structure.
+     */
+    info->deathPipe = death_pipe[1];
+    ffd_atomic_store(&info->pid, pid, FFD_ATOMIC_RELEASE);
+
+    /* check if the child has already exited */
+    if (tryReaping(pid, &si))
+        notifyAndFreeInfo(header, info, &si);
+
+    ret = death_pipe[0];
+    return ret;
+
+err_close:
+    EINTR_LOOP(ret, close(death_pipe[0]));
+    EINTR_LOOP(ret, close(death_pipe[1]));
+
+err_free:
+    /* free the info pointer */
+    freeInfo(header, info);
+
+out:
+    return -1;
+}
+#endif // _POSIX_SPAWN

src/3rdparty/forkfd/forkfd.h

@@ -39,6 +39,11 @@
 #define FORKFD_H
 
 #include <fcntl.h>
+#include <unistd.h> // to get the POSIX flags
+
+#ifdef _POSIX_SPAWN
+#  include <spawn.h>
+#endif
 
 #ifdef __cplusplus
 extern "C" {
@@ -51,6 +56,14 @@ extern "C" {
 
 int forkfd(int flags, pid_t *ppid);
 
+#ifdef _POSIX_SPAWN
+/* only for spawnfd: */
+#  define FFD_SPAWN_SEARCH_PATH   O_RDWR
+
+int spawnfd(int flags, pid_t *ppid, const char *path, const posix_spawn_file_actions_t *file_actions,
+            posix_spawnattr_t *attrp, char *const argv[], char *const envp[]);
+#endif
+
 #ifdef __cplusplus
 }
 #endif