[lldb] Prevent crash when completing ambiguous subcommands

[JDevlieghere]

Summary

Fix a crash when trying to complete an ambiguous subcommand. Take set s tar for example: here s is ambiguous between set and show. Pressing TAB after this input currently crashes LLDB because we go look for the subcommand and fail to find one, but still add the original command (s) as a completion. This causes problems later on when we're trying to insert the completion and find that the "completed string" is shorter than the input string and call std::string::substr to eliminate the common prefix.

frame #12: 0x0000000133fafb44 liblldb.17.0.0git.dylib`lldb_private::Editline::TabCommand(this=0x000000010a645aa0, ch=9) at Editline.cpp:1044:24
   1041   std::string longest_prefix = completions.LongestCommonPrefix();
   1042   if (!longest_prefix.empty())
   1043     longest_prefix =
-> 1044         longest_prefix.substr(request.GetCursorArgumentPrefix().size());
(lldb) v longest_prefix
(std::string) longest_prefix = "s"
(lldb) p request.GetCursorArgumentPrefix().size()
(size_t) 3

rdar://111848598

[jingham]

This looks like it might be the place where we also say "set s could either be set or show". But that's actually done elsewhere so this change is okay. We should maybe make sure we have a test for setting s<TAB> -> {set or show} options.

lldb/source/Commands/CommandObjectMultiword.cpp

if (new_matches.GetSize() == 1 &&
         }
       }
     }
     return;
   }
 
   StringList new_matches;
   CommandObject *sub_command_object = GetSubcommandObject(arg0, &new_matches);
-  if (sub_command_object == nullptr) {
-    request.AddCompletions(new_matches);
+
+  // The subcommand is ambiguous. The completion isn't meaningful.
+  if (!sub_command_object)
     return;
-  }
 
   // Remove the one match that we got from calling GetSubcommandObject.
   new_matches.DeleteStringAtIndex(0);
   request.AddCompletions(new_matches);
   request.ShiftArguments();
   sub_command_object->HandleCompletion(request);
 }
 

lldb/test/API/functionalities/completion/TestCompletion.py

def test_complete_breakpoint_with_names(self):
 
         # test common breapoint name completion
         bp1 = target.BreakpointCreateByName("main", "a.out")
         self.assertTrue(bp1)
         self.assertEqual(bp1.GetNumLocations(), 1)
         self.complete_from_to("breakpoint set -N n", "breakpoint set -N n")
         self.assertTrue(bp1.AddNameWithErrorHandling("nn"))
         self.complete_from_to("breakpoint set -N ", "breakpoint set -N nn")
+
+    def test_ambiguous_command(self):
+        """Test completing an ambiguous commands"""
+        self.complete_from_to("settings s", ['set', 'show'])
+
+    def test_ambiguous_subcommand(self):
+        """Test completing a subcommand of an ambiguous command"""
+        self.complete_from_to("settings s ta", [])