[lldb/driver] Fix SIGTSTP handling

Our SIGTSTP handler was working, but that was mostly accidental.

The reason it worked is because lldb is multithreaded for most of its
lifetime and the OS is reasonably fast at responding to signals. So,
what happened was that the kill(SIGTSTP) which we sent from inside the
handler was delivered to another thread while the handler was still set
to SIG_DFL (which then correctly put the entire process to sleep).

Sometimes it happened that the other thread got the second signal after
the first thread had already restored the handler, in which case the
signal handler would run again, and it would again attempt to send the
SIGTSTP signal back to itself.

Normally it didn't take many iterations for the signal to be delivered
quickly enough. However, if you were unlucky (or were playing around
with pexpect) you could get SIGTSTP while lldb was single-threaded, and
in that case, lldb would go into an endless loop because the second
SIGTSTP could only be handled on the main thread, and only after the
handler for the first signal returned (and re-installed itself). In that
situation the handler would keep re-sending the signal to itself.

This patch fixes the issue by implementing the handler the way it
supposed to be done:
- before sending the second SIGTSTP, we unblock the signal (it gets
  automatically blocked upon entering the handler)
- we use raise to send the signal, which makes sure it gets delivered to
  the thread which is running the handler

This also means we don't need the SIGCONT handler, as our TSTP handler
resumes right after the entire process is continued, and we can do the
required work there.

I also include a test case for the SIGTSTP flow. It uses pexpect, but it
includes a couple of extra twists. Specifically, I needed to create an
extra process on top of lldb, which will run lldb in a separate process
group and simulate the role of the shell. This is needed because SIGTSTP
is not effective on a session leader (the signal gets delivered, but it
does not cause a stop) -- normally there isn't anyone to notice the
stop.

Differential Revision: https://reviews.llvm.org/D120320

lldb/packages/Python/lldbsuite/test/lldbpexpect.py

@@ -23,11 +23,15 @@ class PExpectTest(TestBase):
     def expect_prompt(self):
         self.child.expect_exact(self.PROMPT)
 
-    def launch(self, executable=None, extra_args=None, timeout=60, dimensions=None):
+    def launch(self, executable=None, extra_args=None, timeout=60,
+               dimensions=None, run_under=None, post_spawn=None):
         logfile = getattr(sys.stdout, 'buffer',
                             sys.stdout) if self.TraceOn() else None
 
-        args = ['--no-lldbinit', '--no-use-colors']
+        args = []
+        if run_under is not None:
+            args += run_under
+        args += [lldbtest_config.lldbExec, '--no-lldbinit', '--no-use-colors']
         for cmd in self.setUpCommands():
             args += ['-O', cmd]
         if executable is not None:
@@ -41,8 +45,11 @@ class PExpectTest(TestBase):
 
         import pexpect
         self.child = pexpect.spawn(
-                lldbtest_config.lldbExec, args=args, logfile=logfile,
+                args[0], args=args[1:], logfile=logfile,
                 timeout=timeout, dimensions=dimensions, env=env)
+
+        if post_spawn is not None:
+            post_spawn()
         self.expect_prompt()
         for cmd in self.setUpCommands():
             self.child.expect_exact(cmd)

lldb/test/API/driver/job_control/TestJobControl.py

@@ -0,0 +1,32 @@
+"""
+Test lldb's handling of job control signals (SIGTSTP, SIGCONT).
+"""
+
+
+from lldbsuite.test.lldbtest import *
+from lldbsuite.test.lldbpexpect import PExpectTest
+
+
+class JobControlTest(PExpectTest):
+
+    mydir = TestBase.compute_mydir(__file__)
+
+    def test_job_control(self):
+        def post_spawn():
+            self.child.expect("PID=([0-9]+)")
+            self.lldb_pid = int(self.child.match[1])
+
+        run_under = [sys.executable, self.getSourcePath('shell.py')]
+        self.launch(run_under=run_under, post_spawn=post_spawn)
+
+        os.kill(self.lldb_pid, signal.SIGTSTP)
+        self.child.expect("STATUS=([0-9]+)")
+        status = int(self.child.match[1])
+
+        self.assertTrue(os.WIFSTOPPED(status))
+        self.assertEquals(os.WSTOPSIG(status), signal.SIGTSTP)
+
+        os.kill(self.lldb_pid, signal.SIGCONT)
+
+        self.child.sendline("quit")
+        self.child.expect("RETURNCODE=0")

lldb/test/API/driver/job_control/shell.py

@@ -0,0 +1,34 @@
+"""
+Launch a process (given through argv) similar to how a shell would do it.
+"""
+
+import signal
+import subprocess
+import sys
+import os
+
+
+def preexec_fn():
+    # Block SIGTTOU generated by the tcsetpgrp call
+    orig_mask = signal.pthread_sigmask(signal.SIG_BLOCK, [signal.SIGTTOU])
+
+    # Put us in a new process group.
+    os.setpgid(0, 0)
+
+    # And put it in the foreground.
+    fd = os.open("/dev/tty", os.O_RDONLY)
+    os.tcsetpgrp(fd, os.getpgid(0))
+    os.close(fd)
+
+    signal.pthread_sigmask(signal.SIG_SETMASK, orig_mask)
+
+
+if __name__ == "__main__":
+    child = subprocess.Popen(sys.argv[1:], preexec_fn=preexec_fn)
+    print("PID=%d" % child.pid)
+
+    _, status = os.waitpid(child.pid, os.WUNTRACED)
+    print("STATUS=%d" % status)
+
+    returncode = child.wait()
+    print("RETURNCODE=%d" % returncode)

lldb/tools/driver/Driver.cpp

@@ -671,23 +671,30 @@ void sigint_handler(int signo) {
   _exit(signo);
 }
 
-void sigtstp_handler(int signo) {
+#ifndef _WIN32
+static void sigtstp_handler(int signo) {
   if (g_driver != nullptr)
     g_driver->GetDebugger().SaveInputTerminalState();
 
+  // Unblock the signal and remove our handler.
+  sigset_t set;
+  sigemptyset(&set);
+  sigaddset(&set, signo);
+  pthread_sigmask(SIG_UNBLOCK, &set, nullptr);
   signal(signo, SIG_DFL);
-  kill(getpid(), signo);
-  signal(signo, sigtstp_handler);
-}
 
-void sigcont_handler(int signo) {
+  // Now re-raise the signal. We will immediately suspend...
+  raise(signo);
+  // ... and resume after a SIGCONT.
+
+  // Now undo the modifications.
+  pthread_sigmask(SIG_BLOCK, &set, nullptr);
+  signal(signo, sigtstp_handler);
+
   if (g_driver != nullptr)
     g_driver->GetDebugger().RestoreInputTerminalState();
-
-  signal(signo, SIG_DFL);
-  kill(getpid(), signo);
-  signal(signo, sigcont_handler);
 }
+#endif
 
 static void printHelp(LLDBOptTable &table, llvm::StringRef tool_name) {
   std::string usage_str = tool_name.str() + " [options]";
@@ -826,7 +833,6 @@ int main(int argc, char const *argv[]) {
   signal(SIGPIPE, SIG_IGN);
   signal(SIGWINCH, sigwinch_handler);
   signal(SIGTSTP, sigtstp_handler);
-  signal(SIGCONT, sigcont_handler);
 #endif
 
   int exit_code = 0;