chainsaw: new extension for dangerous operations

The first provided command is `chainsaw-update`, whose one and single job is
to make sure that it will pull, update and purge the target repository,
no matter what may be in the way (locks, notably), see docstring for rationale.

hgext/chainsaw.py

@@ -0,0 +1,156 b''
+# chainsaw.py
+#
+# Copyright 2022 Georges Racinet <[e-mail]>
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+"""chainsaw is a collection of single-minded and dangerous tools. (EXPERIMENTAL)
+
+  "Don't use a chainsaw to cut your food!"
+
+The chainsaw extension provides commands that are so much geared towards a
+specific use case in a specific context or environment that they are totally
+inappropriate and **really dangerous** in other contexts.
+
+The help text of each command explicitly summarizes its context of application
+and the wanted end result.
+
+It is recommended to run these commands with the ``HGPLAIN`` environment
+variable (see :hg:`help scripting`).
+"""
+
+import shutil
+
+from mercurial.i18n import _
+from mercurial import (
+    cmdutil,
+    commands,
+    error,
+    registrar,
+)
+
+cmdtable = {}
+command = registrar.command(cmdtable)
+# Note for extension authors: ONLY specify testedwith = 'ships-with-hg-core' for
+# extensions which SHIP WITH MERCURIAL. Non-mainline extensions should
+# be specifying the version(s) of Mercurial they are tested with, or
+# leave the attribute unspecified.
+testedwith = b'ships-with-hg-core'
+
+
+@command(
+    b'admin::chainsaw-update',
+    [
+        (
+            b'',
+            b'purge-unknown',
+            True,
+            _(
+                b'Remove unversioned files before update. Disabling this can '
+                b'in some cases interfere with the update.'
+                b'See also :hg:`purge`.'
+            ),
+        ),
+        (
+            b'',
+            b'purge-ignored',
+            True,
+            _(
+                b'Remove ignored files before update. Disable this for '
+                b'instance to reuse previous compiler object files. '
+                b'See also :hg:`purge`.'
+            ),
+        ),
+        (
+            b'',
+            b'rev',
+            b'',
+            _(b'revision to update to'),
+        ),
+        (
+            b'',
+            b'source',
+            b'',
+            _(b'repository to clone from'),
+        ),
+    ],
+    _(b'hg admin::chainsaw-update [OPTION] --rev REV --source SOURCE...'),
+    helpbasic=True,
+)
+def update(ui, repo, **opts):
+    """pull and update to a given revision, no matter what, (EXPERIMENTAL)
+
+    Context of application: *some* Continuous Integration (CI) systems,
+    packaging or deployment tools.
+
+    Wanted end result: clean working directory updated at the given revision.
+
+    chainsaw-update pulls from one source, then updates the working directory
+    to the given revision, overcoming anything that would stand in the way.
+
+    By default, it will:
+
+    - break locks if needed, leading to possible corruption if there
+      is a concurrent write access.
+    - perform recovery actions if needed
+    - revert any local modification.
+    - purge unknown and ignored files.
+    - go as far as to reclone if everything else failed (not implemented yet).
+
+    DO NOT use it for anything else than performing a series
+    of unattended updates, with full exclusive repository access each time
+    and without any other local work than running build scripts.
+    In case the local repository is a share (see :hg:`help share`), exclusive
+    write access to the share source is also mandatory.
+
+    It is recommended to run these commands with the ``HGPLAIN`` environment
+    variable (see :hg:`scripting`).
+
+    Motivation: in Continuous Integration and Delivery systems (CI/CD), the
+    occasional remnant or bogus lock are common sources of waste of time (both
+    working time and calendar time). CI/CD scripts tend to grow with counter-
+    measures, often done in urgency. Also, whilst it is neat to keep
+    repositories from one job to the next (especially with large
+    repositories), an exceptional recloning is better than missing a release
+    deadline.
+    """
+    rev = opts['rev']
+    source = opts['source']
+    if not rev:
+        raise error.InputError(_(b'specify a target revision with --rev'))
+    if not source:
+        raise error.InputError(_(b'specify a pull path with --source'))
+    ui.status(_(b'breaking locks, if any\n'))
+    repo.svfs.tryunlink(b'lock')
+    repo.vfs.tryunlink(b'wlock')
+
+    ui.status(_(b'recovering after interrupted transaction, if any\n'))
+    repo.recover()
+
+    ui.status(_(b'pulling from %s\n') % source)
+    overrides = {(b'ui', b'quiet'): True}
+    with ui.configoverride(overrides, b'chainsaw-update'):
+        pull = cmdutil.findcmd(b'pull', commands.table)[1][0]
+        pull(ui, repo, source, rev=[rev], remote_hidden=False)
+
+    purge = cmdutil.findcmd(b'purge', commands.table)[1][0]
+    purge(
+        ui,
+        repo,
+        dirs=True,
+        all=opts.get('purge_ignored'),
+        files=opts.get('purge_unknown'),
+        confirm=False,
+    )
+
+    ui.status(_(b'updating to revision \'%s\'\n') % rev)
+    update = cmdutil.findcmd(b'update', commands.table)[1][0]
+    update(ui, repo, rev=rev, clean=True)
+
+    ui.status(
+        _(
+            b'chainsaw-update to revision \'%s\' '
+            b'for repository at \'%s\' done\n'
+        )
+        % (rev, repo.root)
+    )

tests/test-chainsaw-update.t

@@ -0,0 +1,105 b''
+============================================
+Tests for the admin::chainsaw-update command
+============================================
+
+setup
+=====
+
+  $ cat >> $HGRCPATH << EOF
+  > [extensions]
+  > chainsaw=
+  > EOF
+
+  $ hg init src
+  $ cd src
+  $ echo 1 > foo
+  $ hg ci -Am1
+  adding foo
+  $ cd ..
+
+Actual tests
+============
+
+Simple invocation
+-----------------
+
+  $ hg init repo
+  $ cd repo
+  $ hg admin::chainsaw-update --rev default --source ../src
+  breaking locks, if any
+  recovering after interrupted transaction, if any
+  no interrupted transaction available
+  pulling from ../src
+  updating to revision 'default'
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  chainsaw-update to revision 'default' for repository at '$TESTTMP/repo' done
+
+  $ cat foo
+  1
+
+Test lock breacking capabilities
+--------------------------------
+
+Demonstrate lock-breaking capabilities with locks that regular Mercurial
+operation would not break, because the hostnames registered in locks differ
+from the current hostname (happens a lot with succesive containers):
+
+  $ ln -s invalid.host.test/effffffc:171814 .hg/store/lock
+  $ ln -s invalid.host.test/effffffc:171814 .hg/wlock
+  $ hg debuglock
+  lock:  (.*?), process 171814, host invalid.host.test/effffffc \((\d+)s\) (re)
+  wlock: (.*?), process 171814, host invalid.host.test/effffffc \((\d+)s\) (re)
+  [2]
+
+  $ hg admin::chainsaw-update --no-purge-ignored --rev default --source ../src -q
+  no interrupted transaction available
+
+Test file purging capabilities
+------------------------------
+
+Let's also add local modifications (tracked and untracked) to demonstrate the
+purging.
+
+  $ echo untracked > bar
+  $ echo modified > foo
+  $ hg status -A
+  M foo
+  ? bar
+
+  $ echo 2 > ../src/foo
+  $ hg -R ../src commit -m2
+  $ hg admin::chainsaw-update --rev default --source ../src -q
+  no interrupted transaction available
+  $ hg status -A
+  C foo
+  $ cat foo
+  2
+
+Now behaviour with respect to ignored files: they are not purged if
+the --no-purge-ignored flag is passed, but they are purged by default
+
+  $ echo bar > .hgignore
+  $ hg ci -Aqm hgignore
+  $ echo ignored > bar
+  $ hg status --all
+  I bar
+  C .hgignore
+  C foo
+
+  $ hg admin::chainsaw-update --no-purge-ignored --rev default --source ../src -q
+  no interrupted transaction available
+  $ hg status --all
+  I bar
+  C .hgignore
+  C foo
+  $ cat bar
+  ignored
+
+  $ hg admin::chainsaw-update --rev default --source ../src -q
+  no interrupted transaction available
+  $ hg status --all
+  C .hgignore
+  C foo
+  $ test -f bar
+  [1]
+