fix: read the rollback position before the write retry loop

pyrefly-baseline.json

@@ -144,90 +144,6 @@
       "concise_description": "Argument `int | ndarray` is not assignable to parameter `total` with type `int` in function `jamma.core.progress.progress_iterator`",
       "severity": "error"
     },
-    {
-      "line": 178,
-      "column": 23,
-      "stop_line": 178,
-      "stop_column": 38,
-      "path": "src/jamma/lmm/io.py",
-      "code": -2,
-      "name": "missing-attribute",
-      "description": "Object of class `NoneType` has no attribute `tell`",
-      "concise_description": "Object of class `NoneType` has no attribute `tell`",
-      "severity": "error"
-    },
-    {
-      "line": 179,
-      "column": 17,
-      "stop_line": 179,
-      "stop_column": 33,
-      "path": "src/jamma/lmm/io.py",
-      "code": -2,
-      "name": "missing-attribute",
-      "description": "Object of class `NoneType` has no attribute `write`",
-      "concise_description": "Object of class `NoneType` has no attribute `write`",
-      "severity": "error"
-    },
-    {
-      "line": 180,
-      "column": 17,
-      "stop_line": 180,
-      "stop_column": 33,
-      "path": "src/jamma/lmm/io.py",
-      "code": -2,
-      "name": "missing-attribute",
-      "description": "Object of class `NoneType` has no attribute `flush`",
-      "concise_description": "Object of class `NoneType` has no attribute `flush`",
-      "severity": "error"
-    },
-    {
-      "line": 186,
-      "column": 21,
-      "stop_line": 186,
-      "stop_column": 36,
-      "path": "src/jamma/lmm/io.py",
-      "code": -2,
-      "name": "missing-attribute",
-      "description": "Object of class `NoneType` has no attribute `seek`",
-      "concise_description": "Object of class `NoneType` has no attribute `seek`",
-      "severity": "error"
-    },
-    {
-      "line": 186,
-      "column": 37,
-      "stop_line": 186,
-      "stop_column": 40,
-      "path": "src/jamma/lmm/io.py",
-      "code": -2,
-      "name": "unbound-name",
-      "description": "`pos` is uninitialized",
-      "concise_description": "`pos` is uninitialized",
-      "severity": "error"
-    },
-    {
-      "line": 187,
-      "column": 21,
-      "stop_line": 187,
-      "stop_column": 40,
-      "path": "src/jamma/lmm/io.py",
-      "code": -2,
-      "name": "missing-attribute",
-      "description": "Object of class `NoneType` has no attribute `truncate`",
-      "concise_description": "Object of class `NoneType` has no attribute `truncate`",
-      "severity": "error"
-    },
-    {
-      "line": 191,
-      "column": 28,
-      "stop_line": 191,
-      "stop_column": 31,
-      "path": "src/jamma/lmm/io.py",
-      "code": -2,
-      "name": "unbound-name",
-      "description": "`pos` is uninitialized",
-      "concise_description": "`pos` is uninitialized",
-      "severity": "error"
-    },
     {
       "line": 1068,
       "column": 38,

src/jamma/lmm/io.py

@@ -170,21 +170,34 @@ def _write_buf(self, buf: str, count: int) -> None:
             count: Number of logical results in the buffer.
 
         Raises:
-            OSError: After exhausting retries on write failure.
+            RuntimeError: If the writer is not open.
+            OSError: After exhausting retries on write failure, or immediately
+                if the rollback position cannot be read.
         """
+        handle = self._file
+        if handle is None:
+            raise RuntimeError("Writer not opened. Use as context manager.")
+
+        # Read the rollback position once, outside the retry loop. It is the
+        # same on every attempt, since a failed write is truncated back to it.
+        # Taking it inside the try left `pos` unbound whenever tell() itself
+        # raised, so the rollback below raised UnboundLocalError instead --
+        # not an OSError, so it bypassed the retry, the rollback and the
+        # partial-file cleanup, and escaped this method's documented contract.
+        pos = handle.tell()
+
         last_error: OSError | None = None
         for attempt in range(1 + len(_RETRY_BACKOFF)):
             try:
-                pos = self._file.tell()
-                self._file.write(buf)
-                self._file.flush()
+                handle.write(buf)
+                handle.flush()
                 self._count += count
                 return
             except OSError as e:
                 last_error = e
                 try:
-                    self._file.seek(pos)
-                    self._file.truncate()
+                    handle.seek(pos)
+                    handle.truncate()
                 except OSError as seek_err:
                     logger.warning(
                         f"Failed to rollback partial write at position "

tests/test_incremental_writer.py

@@ -633,3 +633,36 @@ def failing_seek(pos):
         assert not output_path.exists(), (
             "Partial output file should be deleted when EPERM + rollback both fail"
         )
+
+    @pytest.mark.tier0
+    def test_write_buf_tell_failure_surfaces_as_oserror(
+        self, tmp_path: Path, sample_result: AssocResult
+    ):
+        """An OSError from tell() propagates as OSError, not UnboundLocalError.
+
+        tell() supplies the rollback position. Reading it inside the retry
+        try-block left the position unbound when tell() itself failed, so the
+        rollback in the except-handler raised UnboundLocalError -- which is not
+        an OSError, so it escaped write()'s documented contract and skipped the
+        partial-file cleanup entirely.
+        """
+        output_path = tmp_path / "test.assoc.txt"
+
+        with patch("jamma.lmm.io.time.sleep"):
+            with pytest.raises(OSError, match="Illegal seek"):
+                with IncrementalAssocWriter(output_path) as writer:
+
+                    def failing_tell():
+                        err = OSError("Illegal seek")
+                        err.errno = errno.ESPIPE
+                        raise err
+
+                    handle = writer._file
+                    assert handle is not None
+                    handle.tell = failing_tell
+
+                    writer.write(sample_result)
+
+        assert not output_path.exists(), (
+            "Partial output file should be deleted when tell() fails"
+        )