[lldb] Don't save empty expressions in the multiline editor history

[teemperor]

Summary

Right now running expr to start the multiline expression editor and then pressing enter
causes an empty history empty to be created for the multiline editor. That doesn't seem
very useful for users as pressing the 'up' key will now also bring up these empty expressions.

I don't think there is ever a use case for recalling a completely empty expression from the
history, so instead don't save those entries to the history file and make sure we never recall
them when navigating over the expression history.

Note: This is actually a Swift downstream patch that got shipped with Apple's LLDB for many years.
However, this recently started conflicting with upstream LLDB as D100048 added a test that
made sure that empty expression entries don't crash LLDB. Apple's LLDB was never affected by
this crash as it never saved empty expressions in the first place.

[augusto2112]

LGTM!

[aprantl]

Thanks!

lldb/source/Host/common/Editline.cpp

while (m_editor_status == EditorStatus::Editing) {
     m_current_line_rows = -1;
     el_wpush(m_editline, EditLineConstString(
                              "\x1b[^")); // Revert to the existing line content
     el_wgets(m_editline, &count);
   }
 
   interrupted = m_editor_status == EditorStatus::Interrupted;
   if (!interrupted) {
-    // Save the completed entry in history before returning
+    // Save the completed entry in history before returning. Don't save empty
+    // input as that just clutters the command history.
+    if (m_input_lines.size() > 1 || !m_input_lines.front().empty())
       m_history_sp->Enter(CombineLines(m_input_lines).c_str());
 
     lines = GetInputAsStringList();
   }
   return m_editor_status != EditorStatus::EndOfInput;
 }
 
 void Editline::PrintAsync(Stream *stream, const char *s, size_t len) {
   std::lock_guard<std::mutex> guard(m_output_mutex);

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

def test_nav_arrow_down(self):
         self.child.expect_exact("(int) $0 = 334")
 
         self.quit()
 
     @skipIfAsan
     @skipIfEditlineSupportMissing
     @expectedFailureAll(oslist=['freebsd'], bugnumber='llvm.org/pr48316')
     @skipIf(oslist=["linux"], archs=["arm", "aarch64"]) # Randomly fails on buildbot
-    def test_nav_arrow_up_empty_pr49845(self):
-        """Tests that navigating with the up arrow doesn't crash."""
+    def test_nav_arrow_up_empty(self):
+        """
+        Tests that navigating with the up arrow doesn't crash and skips
+        empty history entries.
+        """
         self.launch()
 
-        # Create an empty history session by only entering a newline.
+        # Create a real history entry '456' and then follow up with an
+        # empty entry (that shouldn't be saved).
+        self.child.sendline("expr")
+        self.child.expect_exact("terminate with an empty line to evaluate")
+        self.child.send("456\n\n")
+        self.expect_prompt()
+
         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.
+        # The up arrow should recall the actual history entry and not the
+        # the empty entry (as that one shouldn't have been saved).
         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.child.expect_exact("456")
+        self.child.send("\n\n")
         self.expect_prompt()
 
         self.quit()