[lldb] Fix crash when tab-completing in multi-line expr

Summary:
Tab completing inside the multiline expression command can cause LLDB to crash. The easiest way
to do this is to go inside a frame with at least one local variable and then try to complete:

    (lldb) expr
    1. a[tab]

Reason for this was some mixup when we calculate the cursor position. Obviously we should calculate
the offset inside the string by doing 'end - start', but we are doing 'start - end' (which causes the offset to
become -1 which will lead to some out-of-bounds reading).

Fixes rdar://51754005

I don't see any way to test this as the *multiline* expression completion is completely untested at the moment
and I don't think we have any existing code for testing infrastructure for it.

Reviewers: shafik, davide, labath

Reviewed By: labath

Subscribers: abidh, lldb-commits, davide, clayborg, labath

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D64995

llvm-svn: 367308

lldb/packages/Python/lldbsuite/test/expression_command/multiline-completion/Makefile

@@ -0,0 +1,3 @@
+LEVEL = ../../make
+C_SOURCES := main.c
+include $(LEVEL)/Makefile.rules

lldb/packages/Python/lldbsuite/test/expression_command/multiline-completion/TestMultilineCompletion.py

@@ -0,0 +1,52 @@
+"""
+Test completion for multiline expressions.
+"""
+
+import lldb
+from lldbsuite.test.decorators import *
+from lldbsuite.test.lldbtest import *
+
+class MultilineCompletionTest(TestBase):
+
+    mydir = TestBase.compute_mydir(__file__)
+    NO_DEBUG_INFO_TESTCASE = True
+
+    def setUp(self):
+        TestBase.setUp(self)
+        self.source = 'main.c'
+
+    def expect_string(self, string):
+        import pexpect
+        """This expects for "string", with timeout & EOF being test fails."""
+        try:
+            self.child.expect_exact(string)
+        except pexpect.EOF:
+            self.fail("Got EOF waiting for '%s'" % (string))
+        except pexpect.TIMEOUT:
+            self.fail("Timed out waiting for '%s'" % (string))
+
+    @expectedFailureAll(
+        oslist=["windows"],
+        bugnumber="llvm.org/pr22274: need a pexpect replacement for windows")
+    def test_basic_completion(self):
+        """Test that we can complete a simple multiline expression"""
+        self.build()
+        self.setTearDownCleanup()
+
+        import pexpect
+        exe = self.getBuildArtifact("a.out")
+        prompt = "(lldb) "
+
+        run_commands = ' -o "b main" -o "r"'
+        self.child = pexpect.spawn(
+            '%s %s %s %s' %
+            (lldbtest_config.lldbExec, self.lldbOption, run_commands, exe))
+        child = self.child
+
+        self.expect_string(prompt)
+        self.child.sendline("expr")
+        self.expect_string("terminate with an empty line to evaluate")
+        self.child.send("to_\t")
+        self.expect_string("to_complete")
+
+        self.deletePexpectChild()

lldb/packages/Python/lldbsuite/test/expression_command/multiline-completion/main.c

@@ -0,0 +1,5 @@
+int main(int argc, char **argv) {
+  lldb_enable_attach();
+  int to_complete = 0;
+  return to_complete;
+}

lldb/source/Core/IOHandler.cpp

@@ -233,7 +233,7 @@ int IOHandlerDelegate::IOHandlerComplete(
         matches, descriptions);
   case Completion::Expression: {
     CompletionResult result;
-    CompletionRequest request(current_line, current_line - cursor,
+    CompletionRequest request(current_line, cursor - current_line,
                               skip_first_n_matches, max_matches, result);
     CommandCompletions::InvokeCommonCompletionCallbacks(
         io_handler.GetDebugger().GetCommandInterpreter(),