[lldb] Detach the child process when stepping over a fork

Step over thread plans were claiming to explain the fork stop reasons,
which prevented the default fork logic (detaching from the child
process) from kicking in. This patch changes that.

Differential Revision: https://reviews.llvm.org/D141605

lldb/source/Target/ThreadPlan.cpp

@@ -171,6 +171,9 @@ bool ThreadPlan::IsUsuallyUnexplainedStopReason(lldb::StopReason reason) {
   case eStopReasonExec:
   case eStopReasonThreadExiting:
   case eStopReasonInstrumentation:
+  case eStopReasonFork:
+  case eStopReasonVFork:
+  case eStopReasonVForkDone:
     return true;
   default:
     return false;

lldb/test/API/functionalities/fork/resumes-child/Makefile

@@ -0,0 +1,3 @@
+CXX_SOURCES := main.cpp
+
+include Makefile.rules

lldb/test/API/functionalities/fork/resumes-child/TestForkResumesChild.py

@@ -0,0 +1,22 @@
+"""
+Make sure that the fork child keeps running.
+"""
+
+
+
+import lldb
+import lldbsuite.test.lldbutil as lldbutil
+from lldbsuite.test.lldbtest import *
+from lldbsuite.test.decorators import *
+
+
+class TestForkResumesChild(TestBase):
+
+    NO_DEBUG_INFO_TESTCASE = True
+
+    @skipIfWindows
+    def test_step_over_fork(self):
+        self.build()
+        lldbutil.run_to_source_breakpoint(self, "// break here", lldb.SBFileSpec("main.cpp"))
+        self.runCmd("next")
+        self.expect("continue", substrs = ["exited with status = 0"])

lldb/test/API/functionalities/fork/resumes-child/main.cpp

@@ -0,0 +1,29 @@
+#include <cassert>
+#include <chrono>
+#include <cstdlib>
+#include <sys/wait.h>
+#include <thread>
+#include <unistd.h>
+
+int main() {
+  pid_t fork_result = fork(); // break here
+  assert(fork_result >= 0);
+  if (fork_result == 0) {
+    // child
+    _exit(47);
+  }
+  // parent
+  // Use polling to avoid blocking if the child is not actually resumed.
+  auto deadline = std::chrono::steady_clock::now() + std::chrono::seconds(10);
+  std::chrono::milliseconds poll_interval{10};
+  while (std::chrono::steady_clock::now() < deadline) {
+    int status;
+    pid_t waitpid_result = waitpid(fork_result, &status, WNOHANG);
+    if (waitpid_result == fork_result)
+      return 0;
+    assert(waitpid_result == 0);
+    std::this_thread::sleep_for(poll_interval);
+    poll_interval *= 2;
+  }
+  abort();
+}