Bug#26686 mysql-test-run.pl aborts when waitpid returns -1

 - Add error handling for waitpid returns -1 for "simple run of command"


mysql-test/lib/mtr_process.pl:
  - Add error handling for waitpid returns -1
  when a simple command is run.
  - Add missing return
  - Add mtr_errors where the program should never come 
  - Remove an else to improve program readability
  - Change mtr_debug to mtr_warning for "Got EAGAIN from fork()..."

mysql-test/lib/mtr_process.pl

@@ -125,19 +125,18 @@ sub spawn_impl ($$$$$$$$) {
     {
       if ( $! == $!{EAGAIN} )           # See "perldoc Errno"
       {
-        mtr_debug("Got EAGAIN from fork(), sleep 1 second and redo");
+        mtr_warning("Got EAGAIN from fork(), sleep 1 second and redo");
         sleep(1);
         redo FORK;
       }
-      else
-      {
-        mtr_error("$path ($pid) can't be forked");
-      }
+
+      mtr_error("$path ($pid) can't be forked, error: $!");
+
     }
 
     if ( $pid )
     {
-      spawn_parent_impl($pid,$mode,$path);
+      return spawn_parent_impl($pid,$mode,$path);
     }
     else
     {
@@ -202,8 +201,11 @@ sub spawn_impl ($$$$$$$$) {
       {
         mtr_child_error("failed to execute \"$path\": $!");
       }
+      mtr_error("Should never come here 1!");
     }
+    mtr_error("Should never come here 2!");
   }
+  mtr_error("Should never come here 3!");
 }
 
 
@@ -216,12 +218,21 @@ sub spawn_parent_impl {
   {
     if ( $mode eq 'run' )
     {
-      # Simple run of command, we wait for it to return
+      # Simple run of command, wait blocking for it to return
       my $ret_pid= waitpid($pid,0);
       if ( $ret_pid != $pid )
       {
-        mtr_error("waitpid($pid, 0) returned $ret_pid " .
-		  "when waiting for '$path'");
+	# The "simple" waitpid has failed, print debug info
+	# and try to handle the error
+        mtr_warning("waitpid($pid, 0) returned $ret_pid " .
+		    "when waiting for '$path', error: '$!'");
+	if ( $ret_pid == -1 )
+	{
+	  # waitpid returned -1, that would indicate the process
+	  # no longer exist and waitpid couldn't wait for it.
+	  return 1;
+	}
+	mtr_error("Error handling failed");
       }
 
       return mtr_process_exit_status($?);