censor: use a reasonable amount of memory

Before this change, trying to censor some random revision uses an ever
increasing amount of memory (I stopped at 20GB, but it was by no means
finished), presumably because these contexts have a lot of
information that is kept alive.
After this change, the memory usage plateaus quickly.

Differential Revision: https://phab.mercurial-scm.org/D4582

hgext/censor.py

@@ -1,184 +1,187
 # Copyright (C) 2015 - Mike Edgar <[e-mail]>
 #
 # This extension enables removal of file content at a given revision,
 # rewriting the data/metadata of successive revisions to preserve revision log
 # integrity.
 
 """erase file content at a given revision
 
 The censor command instructs Mercurial to erase all content of a file at a given
 revision *without updating the changeset hash.* This allows existing history to
 remain valid while preventing future clones/pulls from receiving the erased
 data.
 
 Typical uses for censor are due to security or legal requirements, including::
 
  * Passwords, private keys, cryptographic material
  * Licensed data/code/libraries for which the license has expired
  * Personally Identifiable Information or other private data
 
 Censored nodes can interrupt mercurial's typical operation whenever the excised
 data needs to be materialized. Some commands, like ``hg cat``/``hg revert``,
 simply fail when asked to produce censored data. Others, like ``hg verify`` and
 ``hg update``, must be capable of tolerating censored data to continue to
 function in a meaningful way. Such commands only tolerate censored file
 revisions if they are allowed by the "censor.policy=ignore" config option.
 """
 
 from __future__ import absolute_import
 
 from mercurial.i18n import _
 from mercurial.node import short
 
 from mercurial import (
     error,
     pycompat,
     registrar,
     revlog,
     scmutil,
     util,
 )
 
 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 = 'ships-with-hg-core'
 
 @command('censor',
     [('r', 'rev', '', _('censor file from specified revision'), _('REV')),
      ('t', 'tombstone', '', _('replacement tombstone data'), _('TEXT'))],
     _('-r REV [-t TEXT] [FILE]'))
 def censor(ui, repo, path, rev='', tombstone='', **opts):
     with repo.wlock(), repo.lock():
         return _docensor(ui, repo, path, rev, tombstone, **opts)
 
 def _docensor(ui, repo, path, rev='', tombstone='', **opts):
     if not path:
         raise error.Abort(_('must specify file path to censor'))
     if not rev:
         raise error.Abort(_('must specify revision to censor'))
 
     wctx = repo[None]
 
     m = scmutil.match(wctx, (path,))
     if m.anypats() or len(m.files()) != 1:
         raise error.Abort(_('can only specify an explicit filename'))
     path = m.files()[0]
     flog = repo.file(path)
     if not len(flog):
         raise error.Abort(_('cannot censor file with no history'))
 
     rev = scmutil.revsingle(repo, rev, rev).rev()
     try:
         ctx = repo[rev]
     except KeyError:
         raise error.Abort(_('invalid revision identifier %s') % rev)
 
     try:
         fctx = ctx.filectx(path)
     except error.LookupError:
         raise error.Abort(_('file does not exist at revision %s') % rev)
 
     fnode = fctx.filenode()
-    headctxs = [repo[c] for c in repo.heads()]
-    heads = [c for c in headctxs if path in c and c.filenode(path) == fnode]
+    heads = []
+    for headnode in repo.heads():
+        c = repo[headnode]
+        if path in c and c.filenode(path) == fnode:
+            heads.append(c)
     if heads:
         headlist = ', '.join([short(c.node()) for c in heads])
         raise error.Abort(_('cannot censor file in heads (%s)') % headlist,
             hint=_('clean/delete and commit first'))
 
     wp = wctx.parents()
     if ctx.node() in [p.node() for p in wp]:
         raise error.Abort(_('cannot censor working directory'),
             hint=_('clean/delete/update first'))
 
     flogv = flog.version & 0xFFFF
     if flogv != revlog.REVLOGV1:
         raise error.Abort(
             _('censor does not support revlog version %d') % (flogv,))
 
     tombstone = revlog.packmeta({"censored": tombstone}, "")
 
     crev = fctx.filerev()
 
     if len(tombstone) > flog.rawsize(crev):
         raise error.Abort(_(
             'censor tombstone must be no longer than censored data'))
 
     # Using two files instead of one makes it easy to rewrite entry-by-entry
     idxread = repo.svfs(flog.indexfile, 'r')
     idxwrite = repo.svfs(flog.indexfile, 'wb', atomictemp=True)
     if flog.version & revlog.FLAG_INLINE_DATA:
         dataread, datawrite = idxread, idxwrite
     else:
         dataread = repo.svfs(flog.datafile, 'r')
         datawrite = repo.svfs(flog.datafile, 'wb', atomictemp=True)
 
     # Copy all revlog data up to the entry to be censored.
     rio = revlog.revlogio()
     offset = flog.start(crev)
 
     for chunk in util.filechunkiter(idxread, limit=crev * rio.size):
         idxwrite.write(chunk)
     for chunk in util.filechunkiter(dataread, limit=offset):
         datawrite.write(chunk)
 
     def rewriteindex(r, newoffs, newdata=None):
         """Rewrite the index entry with a new data offset and optional new data.
 
         The newdata argument, if given, is a tuple of three positive integers:
         (new compressed, new uncompressed, added flag bits).
         """
         offlags, comp, uncomp, base, link, p1, p2, nodeid = flog.index[r]
         flags = revlog.gettype(offlags)
         if newdata:
             comp, uncomp, nflags = newdata
             flags |= nflags
         offlags = revlog.offset_type(newoffs, flags)
         e = (offlags, comp, uncomp, r, link, p1, p2, nodeid)
         idxwrite.write(rio.packentry(e, None, flog.version, r))
         idxread.seek(rio.size, 1)
 
     def rewrite(r, offs, data, nflags=revlog.REVIDX_DEFAULT_FLAGS):
         """Write the given full text to the filelog with the given data offset.
 
         Returns:
             The integer number of data bytes written, for tracking data offsets.
         """
         flag, compdata = flog.compress(data)
         newcomp = len(flag) + len(compdata)
         rewriteindex(r, offs, (newcomp, len(data), nflags))
         datawrite.write(flag)
         datawrite.write(compdata)
         dataread.seek(flog.length(r), 1)
         return newcomp
 
     # Rewrite censored revlog entry with (padded) tombstone data.
     pad = ' ' * (flog.rawsize(crev) - len(tombstone))
     offset += rewrite(crev, offset, tombstone + pad, revlog.REVIDX_ISCENSORED)
 
     # Rewrite all following filelog revisions fixing up offsets and deltas.
     for srev in pycompat.xrange(crev + 1, len(flog)):
         if crev in flog.parentrevs(srev):
             # Immediate children of censored node must be re-added as fulltext.
             try:
                 revdata = flog.revision(srev)
             except error.CensoredNodeError as e:
                 revdata = e.tombstone
             dlen = rewrite(srev, offset, revdata)
         else:
             # Copy any other revision data verbatim after fixing up the offset.
             rewriteindex(srev, offset)
             dlen = flog.length(srev)
             for chunk in util.filechunkiter(dataread, limit=dlen):
                 datawrite.write(chunk)
         offset += dlen
 
     idxread.close()
     idxwrite.close()
     if dataread is not idxread:
         dataread.close()
         datawrite.close()