[PATCH v3] debug: automate the process of truncating a damaged obsstore (issue5265)

[Simon Farnsworth]

# HG changeset patch
# User Simon Farnsworth <simonfar at fb.com>
# Date 1467646999 25200
#      Mon Jul 04 08:43:19 2016 -0700
# Node ID 31eea7188911ca17b36e69170dfcec07f4da37ae
# Parent  fd93b15b5c30d16fd9c9eba61402d07fc4085db3
debug: automate the process of truncating a damaged obsstore (issue5265)

We occasionally see users who've had a system crash damage the obsstore file
in their .hg/store directory; this makes all `hg` commands fail until we go
in and remove the damaged section of the obsstore by hand.

Automate the process we use when this happens, as a debug command because it
loses the corrupted data. We only use it in rare circumstances when it's
important to retrieve a user's work and apply it to a fresh clone.

diff --git a/mercurial/commands.py b/mercurial/commands.py
--- a/mercurial/commands.py
+++ b/mercurial/commands.py
@@ -15,6 +15,7 @@
 import re
 import shlex
 import socket
+import struct
 import sys
 import tempfile
 import time
@@ -3702,6 +3703,61 @@
             displayer.show(repo[r], **props)
         displayer.close()
 
+ at command('debugtruncatestore',
+    [('', 'obsolete', None, _('truncate bad markers in obsstore'))],
+    _('[OPTION]'))
+def debugtruncatestore(ui, repo, **opts):
+    """Fix up repository corruption by truncating damaged files
+
+    Most on-disk data structures are designed to be append-only. A failed write
+    (e.g. due to an unexpected power failure) can leave the file corrupted.
+
+    This command attempts to recover from that situation by replacing the
+    corrupted file with a version that only contains the valid records from the
+    broken file. It is not guaranteed to remove all corrupt records - it will
+    only remove corrupt records where normal use of the repo would result in a
+    crash.
+
+    Corrupt files will be renamed with a .corrupt extension before the fixed
+    version is written out, so that you can examine the corruption and/or undo
+    this command.
+
+    You should normally use :hg:`recover` before resorting to this command.
+    """
+
+    if 'obsolete' in opts:
+        data = repo.svfs.tryread('obsstore')
+        if data:
+            # Slow algorithm - but this is an emergency debug operation
+            version = None
+            corrupt = False
+            while version is None and len(data) > 0:
+                try:
+                    (version, markers) = obsolete._readmarkers(data)
+                    # Force evaluation of all the markers - the pure
+                    # implementation returns a generator which won't detonate
+                    # until you evaluate the bad marker.
+                    for marker in markers:
+                        pass
+                except (ValueError, struct.error, error.Abort):
+                    corrupt = True
+                    version = None
+                    data = data[:-1]
+                    continue
+                break
+            if corrupt:
+                with repo.lock():
+                    repo.svfs.rename('obsstore', 'obsstore.corrupt')
+                    if len(data) > 0:
+                        repo.svfs.write('obsstore', data)
+                        ui.write(_('truncated damaged obsstore\n'))
+                    else:
+                        ui.write(_('deleted unreadable obsstore\n'))
+            else:
+                ui.write(_('no damage to obsstore\n'))
+        else:
+            ui.write(_('no obsstore\n'))
+
 @command('debugwalk', walkopts, _('[OPTION]... [FILE]...'), inferrepo=True)
 def debugwalk(ui, repo, *pats, **opts):
     """show how files match on given patterns"""
diff --git a/tests/test-completion.t b/tests/test-completion.t
--- a/tests/test-completion.t
+++ b/tests/test-completion.t
@@ -109,6 +109,7 @@
   debugsub
   debugsuccessorssets
   debugtemplate
+  debugtruncatestore
   debugwalk
   debugwireargs
 
@@ -274,6 +275,7 @@
   debugsub: rev
   debugsuccessorssets: 
   debugtemplate: rev, define
+  debugtruncatestore: obsolete
   debugwalk: include, exclude
   debugwireargs: three, four, five, ssh, remotecmd, insecure
   files: rev, print0, include, exclude, template, subrepos
diff --git a/tests/test-debugcommands.t b/tests/test-debugcommands.t
--- a/tests/test-debugcommands.t
+++ b/tests/test-debugcommands.t
@@ -126,3 +126,45 @@
    debugstacktrace.py:7 *in * (glob)
    debugstacktrace.py:6 *in g (glob)
    */util.py:* in debugstacktrace (glob)
+
+Test corruption-fixing debugtruncatestore command
+
+  $ hg init corrupt-obsstore
+  $ cd corrupt-obsstore
+  $ cat >> .hg/hgrc << EOF
+  > [experimental]
+  > evolution = all
+  > [extensions]
+  > evolve =
+  > EOF
+  $ echo a > file
+  $ hg commit -qAm file-a -d 1/1/2001
+  $ hg debugobsolete
+
+  $ echo corrupt > .hg/store/obsstore
+  $ hg debugobsolete 2> /dev/null
+  [255]
+  $ hg debugtruncatestore --obsolete
+  deleted unreadable obsstore
+  $ hg debugobsolete
+
+  $ echo bee > file
+  $ hg commit -qAm file-b -d 1/1/2001
+  $ echo b > file
+  $ hg commit -qAm file-b -d 1/1/2001
+  $ hg fold -m file-b -r '.^::.' -d 1/1/2001
+  2 changesets folded
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+
+  $ hg debugobsolete
+  b429646ef7b48810f0aeaceb257eb589ff2e7e07 a7a6f057a0dda49d17b8ddb53d228494f939fed3 0 \(.*\) {'user': 'test'} (re)
+  12af3ef7db84073f5b5802cb875af46c6fc8f420 a7a6f057a0dda49d17b8ddb53d228494f939fed3 0 \(.*\) {'user': 'test'} (re)
+  $ python -c 'print("CORRUPT" * 120)' >> .hg/store/obsstore
+  $ hg debugobsolete 2> /dev/null
+  [1]
+  $ hg debugtruncatestore --obsolete
+  truncated damaged obsstore
+  $ hg debugobsolete
+  b429646ef7b48810f0aeaceb257eb589ff2e7e07 a7a6f057a0dda49d17b8ddb53d228494f939fed3 0 \(.*\) {'user': 'test'} (re)
+  12af3ef7db84073f5b5802cb875af46c6fc8f420 a7a6f057a0dda49d17b8ddb53d228494f939fed3 0 \(.*\) {'user': 'test'} (re)
+  $ cd ..
diff --git a/tests/test-help.t b/tests/test-help.t
--- a/tests/test-help.t
+++ b/tests/test-help.t
@@ -917,6 +917,8 @@
                  show set of successors for revision
    debugtemplate
                  parse and apply a template
+   debugtruncatestore
+                 Fix up repository corruption by truncating damaged files
    debugwalk     show how files match on given patterns
    debugwireargs
                  (no help text available)