[lldb] Fix a crash when tab-completion an empty line in a function with only one local variable

When LLDB sees only one possible completion for an input, it will add a trailing
space to the completion to signal that to the user. If the current argument is
quoted, that also means LLDB needs to add the trailing quote to finish the
current argument first.

In case the user is in a function with only one local variable and is currently
editing an empty line in the multiline expression editor, then we are in the
unique situation where we can have a unique completion for an empty input line.
(In a normal LLDB session this would never occur as empty input would just list
all the possible commands).

In this special situation our check if the current argument needs to receive a
trailing quote will crash LLDB as there is no current argument and the
completion code just unconditionally tries to access the current argument. This
just adds the missing check if we even have a current argument before we check
if we need to add a terminating quote character.

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D85903

lldb/source/Host/common/Editline.cpp

@@ -1004,7 +1004,8 @@ unsigned char Editline::TabCommand(int ch) {
     case CompletionMode::Normal: {
       std::string to_add = completion.GetCompletion();
       to_add = to_add.substr(request.GetCursorArgumentPrefix().size());
-      if (request.GetParsedArg().IsQuoted())
+      // Terminate the current argument with a quote if it started with a quote.
+      if (!request.GetParsedLine().empty() && request.GetParsedArg().IsQuoted())
         to_add.push_back(request.GetParsedArg().GetQuoteChar());
       to_add.push_back(' ');
       el_insertstr(m_editline, to_add.c_str());

lldb/test/API/commands/expression/multiline-completion/TestMultilineCompletion.py

@@ -11,6 +11,21 @@ class MultilineCompletionTest(PExpectTest):
 
     mydir = TestBase.compute_mydir(__file__)
 
+    def start_expression_editor(self):
+        """ Starts the multiline expression editor. """
+        self.child.send("expression\n")
+        self.child.expect_exact("terminate with an empty line to evaluate")
+
+    def exit_expression_editor(self):
+        """ Exits the multiline expression editor. """
+        # Send a newline to finish the current line. The second newline will
+        # finish the new empty line which will exit the editor. The space at the
+        # start prevents that the first newline already exits the editor (in
+        # case the current line of the editor is already empty when this
+        # function is called).
+        self.child.send(" \n\n")
+        self.expect_prompt()
+
     # PExpect uses many timeouts internally and doesn't play well
     # under ASAN on a loaded machine..
     @skipIfAsan
@@ -21,14 +36,23 @@ class MultilineCompletionTest(PExpectTest):
 
         self.launch(executable=self.getBuildArtifact("a.out"), dimensions=(100,500))
         self.expect("b main", substrs=["Breakpoint 1", "address ="])
-        self.expect("run", substrs=["stop reason ="])
+        self.expect("run", substrs=["stop reason = breakpoint 1"])
 
-        self.child.sendline("expr")
-        self.child.expect_exact("terminate with an empty line to evaluate")
+        self.start_expression_editor()
         self.child.send("to_\t")
         self.child.expect_exact("to_complete")
+        self.exit_expression_editor()
 
-        self.child.send("\n\n")
-        self.expect_prompt()
+        # Check that completion empty input in a function with only one
+        # local variable works.
+        self.expect("breakpoint set -p 'break in single_local_func'",
+                    substrs=["Breakpoint 2"])
+        self.expect("continue", substrs=["stop reason = breakpoint 2"])
+        self.start_expression_editor()
+        self.child.send("\t")
+        # Only one local, so this will directly insert 'only_local' with a
+        # trailing space to signal a final completion.
+        self.child.expect_exact("only_local ")
+        self.exit_expression_editor()
 
         self.quit()

lldb/test/API/commands/expression/multiline-completion/main.c

@@ -1,4 +1,11 @@
+int single_local_func() {
+  // This function should always only have a single local variable and no
+  // parameters.
+  int only_local = 3;
+  return only_local; // break in single_local_func
+}
+
 int main(int argc, char **argv) {
   int to_complete = 0;
-  return to_complete;
+  return to_complete + single_local_func();
 }