mercurial/revlogutils/concurrency_checker.py

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mercurial/revlogutils/concurrency_checker.py	Wed Feb 03 16:33:10 2021 -0800
@@ -0,0 +1,38 @@
+from ..i18n import _
+from .. import error
+
+
+def get_checker(ui, revlog_name=b'changelog'):
+    """Get a function that checks file handle position is as expected.
+
+    This is used to ensure that files haven't been modified outside of our
+    knowledge (such as on a networked filesystem, if `hg debuglocks` was used,
+    or writes to .hg that ignored locks happened).
+
+    Due to revlogs supporting a concept of buffered, delayed, or diverted
+    writes, we're allowing the files to be shorter than expected (the data may
+    not have been written yet), but they can't be longer.
+
+    Please note that this check is not perfect; it can't detect all cases (there
+    may be false-negatives/false-OKs), but it should never claim there's an
+    issue when there isn't (false-positives/false-failures).
+    """
+
+    vpos = ui.config(b'debug', b'revlog.verifyposition.' + revlog_name)
+    # Avoid any `fh.tell` cost if this isn't enabled.
+    if not vpos or vpos not in [b'log', b'warn', b'fail']:
+        return None
+
+    def _checker(fh, fn, expected):
+        if fh.tell() <= expected:
+            return
+
+        msg = _(b'%s: file cursor at position %d, expected %d')
+        # Always log if we're going to warn or fail.
+        ui.log(b'debug', msg + b'\n', fn, fh.tell(), expected)
+        if vpos == b'warn':
+            ui.warn((msg + b'\n') % (fn, fh.tell(), expected))
+        elif vpos == b'fail':
+            raise error.RevlogError(msg % (fn, fh.tell(), expected))
+
+    return _checker