posix: osiSpawnDetachedProcess() find out if exec() errors.

Presently, success just means fork()

modules/libcom/src/osi/os/posix/osdProcess.c

@@ -24,7 +24,9 @@
 #include <unistd.h>
 #include <sched.h>
 #include <sys/types.h>
+#include <sys/wait.h>
 #include <pwd.h>
+#include <fcntl.h>
 
 #include "osiProcess.h"
 #include "errlog.h"
@@ -61,12 +63,18 @@ LIBCOM_API osiSpawnDetachedProcessReturn epicsStdCall osiSpawnDetachedProcess
     (const char *pProcessName, const char *pBaseExecutableName)
 {
     int status;
+    int fds[2]; /* [reader, writer] */
+
+    if(pipe(fds))
+        return osiSpawnDetachedProcessFail;
 
     /*
      * create a duplicate process
      */
     status = fork ();
     if (status < 0) {
+        close(fds[0]);
+        close(fds[1]);
         return osiSpawnDetachedProcessFail;
     }
 
@@ -75,11 +83,29 @@ LIBCOM_API osiSpawnDetachedProcessReturn epicsStdCall osiSpawnDetachedProcess
      * in the initiating (parent) process
      */
     if (status) {
-        return osiSpawnDetachedProcessSuccess;
-    }
+        osiSpawnDetachedProcessReturn ret = osiSpawnDetachedProcessSuccess;
+        char buf;
+        ssize_t n;
+        close(fds[1]);
 
-    /*
-     * This is executed only by the new child process.
+        n = read(fds[0], &buf, 1);
+        /* Success if child exec'd without sending a '!'.
+         * Of course child may crash soon after, but can't
+         * wait around for this to happen.
+         */
+        if(n!=0) {
+            ret = osiSpawnDetachedProcessFail;
+        }
+
+        close(fds[0]);
+        return ret;
+    }
+    close(fds[0]);
+    (void)fcntl ( fds[1], F_SETFD, FD_CLOEXEC );
+
+    /* This is executed only by the new child process.
+     * Since we may be called from a library, we don't assume that
+     * all other code has set properly set FD_CLOEXEC.
      * Close all open files except for STDIO, so they will not
      * be inherited by the new program.
      */
@@ -89,6 +115,8 @@ LIBCOM_API osiSpawnDetachedProcessReturn epicsStdCall osiSpawnDetachedProcess
             if (fd==STDIN_FILENO) continue;
             if (fd==STDOUT_FILENO) continue;
             if (fd==STDERR_FILENO) continue;
+            /* pipe to our parent will be closed automatically via FD_CLOEXEC */
+            if (fd==fds[1]) continue;
             close (fd);
         }
     }
@@ -115,6 +143,10 @@ LIBCOM_API osiSpawnDetachedProcessReturn epicsStdCall osiSpawnDetachedProcess
         fprintf ( stderr, "**** You may need to modify your PATH environment variable.\n" );
         fprintf ( stderr, "**** Unable to start \"%s\" process.\n", pProcessName);
     }
+    /* signal error to parent */
+    ssize_t ret = write(fds[1], "!", 1);
+    (void)ret; /* not much we could do about this */
+    close(fds[1]);
     /* Don't run our parent's atexit() handlers */
     _exit ( -1 );
 }