[lldb][Editline] Fix crash when navigating through empty command history.

[rupprecht]

Summary

An empty history entry can happen by entering the expression evaluator an immediately hitting enter:

$ lldb
(lldb) e
Enter expressions, then terminate with an empty line to evaluate:
  1:  <hit enter>

The next time the user enters the expression evaluator, if they hit the up arrow to load the previous expression, lldb crashes. This patch treats empty history sessions as a single expression of zero length, instead of an empty list of expressions.

Fixes http://llvm.org/PR49845.

[JDevlieghere]

LGTM. Thanks!

[iii]

lldb-arm-ubuntu (https://lab.llvm.org/buildbot/#/builders/17/builds/9015) fails with:

FAIL: LLDB (/home/tcwg-buildslave/worker/lldb-arm-ubuntu/build/bin/clang-arm) :: test_nav_arrow_up_empty_pr49845 (TestMultilineNavigation.TestCase)
======================================================================
ERROR: test_nav_arrow_up_empty_pr49845 (TestMultilineNavigation.TestCase)
   Tests that navigating with the up arrow doesn't crash.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/tcwg-buildslave/worker/lldb-arm-ubuntu/llvm-project/lldb/third_party/Python/module/pexpect-4.6/pexpect/expect.py", line 111, in expect_loop
    incoming = spawn.read_nonblocking(spawn.maxread, timeout)
  File "/home/tcwg-buildslave/worker/lldb-arm-ubuntu/llvm-project/lldb/third_party/Python/module/pexpect-4.6/pexpect/pty_spawn.py", line 482, in read_nonblocking
    raise TIMEOUT('Timeout exceeded.')
pexpect.exceptions.TIMEOUT: Timeout exceeded.

The bot attributed this to my changes (https://reviews.llvm.org/D105629), but they have nothing to do with lldb or arm. I'm not sure whether it's a real issue or just a laggy builder, in which case increasing a timeout might be worth considering?

@rupprecht could you take a look please?

[rupprecht]

In D100048#2879669, @iii wrote:

lldb-arm-ubuntu (https://lab.llvm.org/buildbot/#/builders/17/builds/9015) fails with:

FAIL: LLDB (/home/tcwg-buildslave/worker/lldb-arm-ubuntu/build/bin/clang-arm) :: test_nav_arrow_up_empty_pr49845 (TestMultilineNavigation.TestCase)
======================================================================
ERROR: test_nav_arrow_up_empty_pr49845 (TestMultilineNavigation.TestCase)
   Tests that navigating with the up arrow doesn't crash.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/tcwg-buildslave/worker/lldb-arm-ubuntu/llvm-project/lldb/third_party/Python/module/pexpect-4.6/pexpect/expect.py", line 111, in expect_loop
    incoming = spawn.read_nonblocking(spawn.maxread, timeout)
  File "/home/tcwg-buildslave/worker/lldb-arm-ubuntu/llvm-project/lldb/third_party/Python/module/pexpect-4.6/pexpect/pty_spawn.py", line 482, in read_nonblocking
    raise TIMEOUT('Timeout exceeded.')
pexpect.exceptions.TIMEOUT: Timeout exceeded.

The bot attributed this to my changes (https://reviews.llvm.org/D105629), but they have nothing to do with lldb or arm. I'm not sure whether it's a real issue or just a laggy builder, in which case increasing a timeout might be worth considering?

@rupprecht could you take a look please?

It passed on the next build attempt. I don't think this is a real issue. Maybe the arm buildbot is just not provisioned very well.

The timeout appears to be 30, per the failing test logs. I don't know if raising further it is the right call, because this seems to happen so rarely, and you probably want to keep the value low so you can iterate on tests faster.

lldb/source/Host/common/Editline.cpp

while (start < input.length()) {
     size_t end = input.find('\n', start);
     if (end == std::string::npos) {
       result.push_back(input.substr(start));
       break;
     }
     result.push_back(input.substr(start, end - start));
     start = end + 1;
   }
+  // Treat an empty history session as a single command of zero-length instead
+  // of returning an empty vector.
+  if (result.empty()) {
+    result.emplace_back();
+  }
   return result;
 }
 
 EditLineStringType FixIndentation(const EditLineStringType &line,
                                   int indent_correction) {
   if (indent_correction == 0)
     return line;
   if (indent_correction < 0)

lldb/test/API/commands/expression/multiline-navigation/TestMultilineNavigation.py

def test_nav_arrow_down(self):
         # Then press enter twice to evaluate the expression.
         self.child.send(self.arrow_down + "\b3\n\n")
         # The result of our expression '111 + 223' should be '334'.
         # If the expression is '333' then arrow down failed to get
         # us back to the second line.
         self.child.expect_exact("(int) $0 = 334")
 
         self.quit()
+
+    @skipIfAsan
+    @skipIfEditlineSupportMissing
+    @expectedFailureAll(oslist=['freebsd'], bugnumber='llvm.org/pr48316')
+    def test_nav_arrow_up_empty_pr49845(self):
+        """Tests that navigating with the up arrow doesn't crash."""
+        self.launch()
+
+        # Create an empty history session by only entering a newline.
+        self.child.sendline("expr")
+        self.child.expect_exact("terminate with an empty line to evaluate")
+        self.child.send("\n")
+        self.expect_prompt()
+
+        # Send just the up arrow in the expression evaluator. This should bring up the previous empty expression.
+        self.child.sendline("expr")
+        self.child.expect_exact("terminate with an empty line to evaluate")
+        self.child.send(self.arrow_up)
+        self.child.expect_exact("1: ")
+        self.child.send("\n")
+        self.expect_prompt()
+
+        self.quit()