[meld] meldbuffer: Add an option for logging cache inconsistencies

[Kai Willadsen <kaiw src gnome org>]

commit aa01abf92a97b3192e741a50ebf90c51f0a0fa3e
Author: Kai Willadsen <kai willadsen gmail com>
Date:   Sun Aug 8 08:28:58 2021 +1000

    meldbuffer: Add an option for logging cache inconsistencies
    
    This is disabled by default because it's not really that much of a
    check, and also because what is the user going to do here anyway?

 meld/meldbuffer.py        | 14 ++++++++++++--
 test/test_buffer_lines.py | 25 +++++++++++++++++++++++++
 2 files changed, 37 insertions(+), 2 deletions(-)
---
diff --git a/meld/meldbuffer.py b/meld/meldbuffer.py
index fba5675d..55342f17 100644
--- a/meld/meldbuffer.py
+++ b/meld/meldbuffer.py
@@ -15,7 +15,7 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 import logging
-from typing import List, Optional
+from typing import Any, List, Optional
 
 from gi.repository import Gio, GLib, GObject, GtkSource
 
@@ -222,7 +222,7 @@ class BufferLines:
     #: available.
     lines: List[Optional[str]]
 
-    def __init__(self, buf, textfilter=None):
+    def __init__(self, buf, textfilter=None, *, cache_debug: bool = False):
         self.buf = buf
         if textfilter is not None:
             self.textfilter = textfilter
@@ -237,6 +237,16 @@ class BufferLines:
         buf.connect("insert-text", self.on_insert_text),
         buf.connect("delete-range", self.on_delete_range),
         buf.connect_after("insert-text", self.after_insert_text),
+        if cache_debug:
+            buf.connect_after("insert-text", self._check_cache_invariant),
+            buf.connect_after("delete-range", self._check_cache_invariant),
+
+    def _check_cache_invariant(self, *args: Any) -> None:
+        if len(self.lines) != len(self):
+            log.error(
+                "Cache line count does not match buffer line count: "
+                f"{len(self.lines)} != {len(self)}",
+            )
 
     def on_insert_text(self, buf, it, text, textlen):
         buf.move_mark(self.mark, it)
diff --git a/test/test_buffer_lines.py b/test/test_buffer_lines.py
index f5e9b032..0830725d 100644
--- a/test/test_buffer_lines.py
+++ b/test/test_buffer_lines.py
@@ -119,3 +119,28 @@ def test_meld_buffer_delete_range(buffer_setup):
 
     assert buffer_lines[5] == "7"
     assert buffer_lines.lines[4:7] == ["4", "7", "8"]
+
+
+def test_meld_buffer_cache_debug(caplog, buffer_setup):
+
+    buffer, buffer_lines = buffer_setup
+    buffer_lines = BufferLines(buffer, cache_debug=True)
+
+    # Invalidate our line cache...
+    buffer_lines.lines.append("invalid")
+
+    # ...and check that insertion/deletion logs an error
+    buffer.insert(
+        buffer.get_iter_at_line(5),
+        "hey",
+    )
+    assert len(caplog.records) == 1
+    assert caplog.records[0].msg.startswith("Cache line count does not match")
+    caplog.clear()
+
+    buffer.delete(
+        buffer.get_iter_at_line(5),
+        buffer.get_iter_at_line(7),
+    )
+    assert len(caplog.records) == 1
+    assert caplog.records[0].msg.startswith("Cache line count does not match")