[LLDB] Fix completion of space-only lines in the REPL on Linux (llvm#83203)

modular/modular#1796 discovered that if you try
to complete a space-only line in the REPL on Linux, LLDB crashes. I
suspect that editline doesn't behave the same way on linux and on
darwin, because I can't replicate this on darwin.

Adding a boundary check in the completion code prevents the crash from
happening.

Author: walter-erquinigo

lldb/source/Host/common/Editline.cpp

@@ -1029,8 +1029,11 @@ unsigned char Editline::TabCommand(int ch) {
     case CompletionMode::Normal: {
       std::string to_add = completion.GetCompletion();
       // Terminate the current argument with a quote if it started with a quote.
-      if (!request.GetParsedLine().empty() && request.GetParsedArg().IsQuoted())
+      Args &parsedLine = request.GetParsedLine();
+      if (!parsedLine.empty() && request.GetCursorIndex() < parsedLine.size() &&
+          request.GetParsedArg().IsQuoted()) {
         to_add.push_back(request.GetParsedArg().GetQuoteChar());
+      }
       to_add.push_back(' ');
       el_deletestr(m_editline, request.GetCursorArgumentPrefix().size());
       el_insertstr(m_editline, to_add.c_str());

lldb/test/API/repl/clang/TestClangREPL.py

@@ -1,7 +1,6 @@
-import lldb
 from lldbsuite.test.decorators import *
-from lldbsuite.test.lldbtest import *
 from lldbsuite.test.lldbpexpect import PExpectTest
+from lldbsuite.test.lldbtest import *
 
 
 class TestCase(PExpectTest):
@@ -17,13 +16,7 @@ def expect_repl(self, expr, substrs=[]):
         self.current_repl_line_number += 1
         self.child.expect_exact(str(self.current_repl_line_number) + ">")
 
-    # PExpect uses many timeouts internally and doesn't play well
-    # under ASAN on a loaded machine..
-    @skipIfAsan
-    @skipIf(oslist=["linux"], archs=["arm", "aarch64"])  # Randomly fails on buildbot
-    @skipIfEditlineSupportMissing
-    def test_basic_completion(self):
-        """Test that we can complete a simple multiline expression"""
+    def start_repl(self):
         self.build()
         self.current_repl_line_number = 1
 
@@ -41,6 +34,14 @@ def test_basic_completion(self):
         self.child.send("expression --repl -l c --\n")
         self.child.expect_exact("1>")
 
+    # PExpect uses many timeouts internally and doesn't play well
+    # under ASAN on a loaded machine..
+    @skipIfAsan
+    @skipIf(oslist=["linux"], archs=["arm", "aarch64"])  # Randomly fails on buildbot
+    @skipIfEditlineSupportMissing
+    def test_basic_completion(self):
+        """Test that we can complete a simple multiline expression"""
+        self.start_repl()
         # Try evaluating a simple expression.
         self.expect_repl("3 + 3", substrs=["(int) $0 = 6"])
 
@@ -54,3 +55,16 @@ def test_basic_completion(self):
         self.expect_repl("$persistent + 10", substrs=["(long) $2 = 17"])
 
         self.quit()
+
+    # PExpect uses many timeouts internally and doesn't play well
+    # under ASAN on a loaded machine..
+    @skipIfAsan
+    @skipIf(oslist=["linux"], archs=["arm", "aarch64"])  # Randomly fails on buildbot
+    @skipIfEditlineSupportMissing
+    def test_completion_with_space_only_line(self):
+        """Test that we don't crash when completing lines with spaces only"""
+        self.start_repl()
+
+        self.child.send("   ")
+        self.child.send("\t")
+        self.expect_repl("3 + 3", substrs=["(int) $0 = 6"])