On Linux, clear the signal mask of the launched inferior

[ybelkadi]

Summary

Due to fork()/execve(), the launched inferior inherits the signal mask of its parent (lldb-server). But because lldb-server modifies its signal mask (It blocks SIGCHLD, for example), the inferior starts with some signals being initially blocked.

One consequence is that TestCallThatRestarts.ExprCommandThatRestartsTestCase (test/expression_command/call-restarts) fails because sigchld_handler() in lotta-signals.c is not called, due to the SIGCHLD signal being blocked.

To prevent the signal masking done by lldb-server from affecting the created inferior, the signal mask of the inferior is now cleared before the execve().

[labath]

Looks good. Thanks for catching this.

Does the test you mentioned pass after this, or does it still fail intermittently as the original bug indicates? If it seems to pass, you should also enable the test.

[ybelkadi]

Yes, the test passes, but as PR19246 reported intermittent failures, I thought I'd wait for the results of the tests run by the bots.

Here is an updated patch also enabling the test. Could you please commit it if it's accepted?

[labath]

Ok, I see what you mean. I will commit it with the test enabled, and I will keep an eye out on the buildbots.

[ybelkadi]

Thanks.

source/Plugins/Process/Linux/NativeProcessLinux.cpp

NativeProcessLinux::Launch(LaunchArgs *args, Error &error)
     // Recognized child exit status codes.
     enum {
         ePtraceFailed = 1,
         eDupStdinFailed,
         eDupStdoutFailed,
         eDupStderrFailed,
         eChdirFailed,
         eExecFailed,
-        eSetGidFailed
+        eSetGidFailed,
+        eSetSigMaskFailed
     };
 
     // Child process.
     if (pid == 0)
     {
         // FIXME consider opening a pipe between parent/child and have this forked child
         // send log info to parent re: launch status, in place of the log lines removed here.
 
if (pid == 0)
                 }
                 else
                 {
                     // Disabling ASLR succeeded.
                 }
             }
         }
 
+        // Clear the signal mask to prevent the child from being affected by
+        // any masking done by the parent.
+        sigset_t set;
+        if (sigemptyset(&set) != 0 || pthread_sigmask(SIG_SETMASK, &set, nullptr) != 0)
+            exit(eSetSigMaskFailed);
+
         // Execute.  We should never return...
         execve(argv[0],
                const_cast<char *const *>(argv),
                const_cast<char *const *>(envp));
 
         // ...unless exec fails.  In which case we definitely need to end the child here.
         exit(eExecFailed);
     }
else if (WIFEXITED(status))
                 error.SetErrorString("Child failed to set working directory.");
                 break;
             case eExecFailed:
                 error.SetErrorString("Child exec failed.");
                 break;
             case eSetGidFailed:
                 error.SetErrorString("Child setgid failed.");
                 break;
+            case eSetSigMaskFailed:
+                error.SetErrorString("Child failed to set signal mask.");
+                break;
             default:
                 error.SetErrorString("Child returned unknown exit status.");
                 break;
         }
 
         if (log)
         {
             log->Printf ("NativeProcessLinux::%s inferior exited with status %d before issuing a STOP",