[lldb] Handle malformed qfThreadInfo reply

If the remote gdbserver's qfThreadInfo reply has a trailing comma,
GDBRemoteCommunicationClient::GetCurrentProcessAndThreadIDs will return
an empty vector of thread ids. This will cause lldb to recurse through
three functions trying to get the list of threads, until it blows its
stack and crashes.

A trailing comma is a malformed response, but it shouldn't cause lldb to
crash. This patch will return the tids received before the malformed
response.

Reviewed By: clayborg, labath

Differential Revision: https://reviews.llvm.org/D109937

lldb/source/Plugins/Process/gdb-remote/GDBRemoteCommunicationClient.cpp

@@ -2908,8 +2908,12 @@ GDBRemoteCommunicationClient::GetCurrentProcessAndThreadIDs(
       if (ch == 'm') {
         do {
           auto pid_tid = response.GetPidTid(LLDB_INVALID_PROCESS_ID);
+          // If we get an invalid response, break out of the loop.
+          // If there are valid tids, they have been added to ids.
+          // If there are no valid tids, we'll fall through to the
+          // bare-iron target handling below.
           if (!pid_tid)
-            return {};
+            break;
 
           ids.push_back(pid_tid.getValue());
           ch = response.GetChar(); // Skip the command separator

lldb/test/API/functionalities/gdb_remote_client/TestThreadInfoTrailingComma.py

@@ -0,0 +1,27 @@
+import lldb
+from lldbsuite.test.lldbtest import *
+from lldbsuite.test.decorators import *
+from gdbclientutils import *
+
+
+class TestThreadInfoTrailingComma(GDBRemoteTestBase):
+
+    def test(self):
+        class MyResponder(MockGDBServerResponder):
+            def haltReason(self):
+                return "T02thread:1"
+
+            def qfThreadInfo(self):
+                return "m1,2,3,4,"
+
+        self.server.responder = MyResponder()
+        target = self.dbg.CreateTarget('')
+        if self.TraceOn():
+          self.runCmd("log enable gdb-remote packets")
+          self.addTearDownHook(
+                lambda: self.runCmd("log disable gdb-remote packets"))
+        process = self.connect(target)
+        self.assertEqual(process.GetThreadAtIndex(0).GetThreadID(), 1)
+        self.assertEqual(process.GetThreadAtIndex(1).GetThreadID(), 2)
+        self.assertEqual(process.GetThreadAtIndex(2).GetThreadID(), 3)
+        self.assertEqual(process.GetThreadAtIndex(3).GetThreadID(), 4)