Keep MemoryFileSystem append writes private until transaction commit

fsspec/implementations/memory.py

@@ -303,6 +303,10 @@ def _open(
         if mode in ["rb", "ab", "r+b", "a+b"]:
             if path in self.store:
                 f = self.store[path]
+                if self._intrans and "a" in mode:
+                    # append to a copy so the store is unchanged until commit
+                    f = MemoryFile(self, path, f.getvalue())
+                    f.created = self.store[path].created
                 if "a" in mode:
                     # position at the end of file
                     f.seek(0, 2)

fsspec/implementations/tests/test_memory.py

@@ -1,5 +1,6 @@
 import os
 import pickle
+from contextlib import nullcontext
 from datetime import datetime, timezone
 from pathlib import PurePosixPath, PureWindowsPath
 
@@ -396,6 +397,25 @@ def test_append_creates_missing_file(m, mode):
     assert m.cat(filename) == b"data"
 
 
+@pytest.mark.parametrize("global_store", [False, True])
+@pytest.mark.parametrize("mode", ["ab", "a+b"])
+@pytest.mark.parametrize("rollback", [False, True])
+def test_transaction_append_existing_file(m, global_store, mode, rollback):
+    fs = filesystem("memory", global_store=global_store, skip_instance_cache=True)
+    fs.pipe_file("file", b"original")
+
+    with pytest.raises(RuntimeError) if rollback else nullcontext():
+        with fs.transaction:
+            with fs.open("file", mode) as f:
+                f.write(b"-appended")
+            assert fs.cat_file("file") == b"original"
+            if rollback:
+                raise RuntimeError("discard transaction")
+
+    expected = b"original" if rollback else b"original-appended"
+    assert fs.cat_file("file") == expected
+
+
 def test_moves(m):
     m.touch("source.txt")
     m.mv("source.txt", "target.txt")