filemerge: when using in-memory merge, always put backup files in temp dir

Before calling a merge tool, we create a backup of the local side of
the merge. That file can be put in the working copy or in a temporary
directory, depending on the user's config. When we're merging in
memory, we don't want to write to the actual, on-disk working copy, so
we write the file to the in-memory working copy instead. However,
since we don't support external merge tools with in-memory merge, it
makes no difference where the file is actually stored (and if we ever
do add support for external merge tools, then the file clearly can't
live in the in-memory working-copy object anyway). So, since it
doesn't matter where the file is stored, we can simplify by always
putting them in the system's temp directory.

Differential Revision: https://phab.mercurial-scm.org/D12187

mercurial/filemerge.py

@@ -1,1255 +1,1242 b''
 # filemerge.py - file-level merge handling for Mercurial
 #
 # Copyright 2006, 2007, 2008 Olivia Mackall <[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.
 
 from __future__ import absolute_import
 
 import contextlib
 import os
 import re
 import shutil
 
 from .i18n import _
 from .node import (
     hex,
     short,
 )
 from .pycompat import (
     getattr,
     open,
 )
 
 from . import (
     encoding,
     error,
     formatter,
     match,
     pycompat,
     registrar,
     scmutil,
     simplemerge,
     tagmerge,
     templatekw,
     templater,
     templateutil,
     util,
 )
 
 from .utils import (
     procutil,
     stringutil,
 )
 
 
 def _toolstr(ui, tool, part, *args):
     return ui.config(b"merge-tools", tool + b"." + part, *args)
 
 
 def _toolbool(ui, tool, part, *args):
     return ui.configbool(b"merge-tools", tool + b"." + part, *args)
 
 
 def _toollist(ui, tool, part):
     return ui.configlist(b"merge-tools", tool + b"." + part)
 
 
 internals = {}
 # Merge tools to document.
 internalsdoc = {}
 
 internaltool = registrar.internalmerge()
 
 # internal tool merge types
 nomerge = internaltool.nomerge
 mergeonly = internaltool.mergeonly  # just the full merge, no premerge
 fullmerge = internaltool.fullmerge  # both premerge and merge
 
 # IMPORTANT: keep the last line of this prompt very short ("What do you want to
 # do?") because of issue6158, ideally to <40 English characters (to allow other
 # languages that may take more columns to still have a chance to fit in an
 # 80-column screen).
 _localchangedotherdeletedmsg = _(
     b"file '%(fd)s' was deleted in other%(o)s but was modified in local%(l)s.\n"
     b"You can use (c)hanged version, (d)elete, or leave (u)nresolved.\n"
     b"What do you want to do?"
     b"$$ &Changed $$ &Delete $$ &Unresolved"
 )
 
 _otherchangedlocaldeletedmsg = _(
     b"file '%(fd)s' was deleted in local%(l)s but was modified in other%(o)s.\n"
     b"You can use (c)hanged version, leave (d)eleted, or leave (u)nresolved.\n"
     b"What do you want to do?"
     b"$$ &Changed $$ &Deleted $$ &Unresolved"
 )
 
 
 class absentfilectx(object):
     """Represents a file that's ostensibly in a context but is actually not
     present in it.
 
     This is here because it's very specific to the filemerge code for now --
     other code is likely going to break with the values this returns."""
 
     def __init__(self, ctx, f):
         self._ctx = ctx
         self._f = f
 
     def __bytes__(self):
         return b'absent file %s@%s' % (self._f, self._ctx)
 
     def path(self):
         return self._f
 
     def size(self):
         return None
 
     def data(self):
         return None
 
     def filenode(self):
         return self._ctx.repo().nullid
 
     _customcmp = True
 
     def cmp(self, fctx):
         """compare with other file context
 
         returns True if different from fctx.
         """
         return not (
             fctx.isabsent()
             and fctx.changectx() == self.changectx()
             and fctx.path() == self.path()
         )
 
     def flags(self):
         return b''
 
     def changectx(self):
         return self._ctx
 
     def isbinary(self):
         return False
 
     def isabsent(self):
         return True
 
 
 def _findtool(ui, tool):
     if tool in internals:
         return tool
     cmd = _toolstr(ui, tool, b"executable", tool)
     if cmd.startswith(b'python:'):
         return cmd
     return findexternaltool(ui, tool)
 
 
 def _quotetoolpath(cmd):
     if cmd.startswith(b'python:'):
         return cmd
     return procutil.shellquote(cmd)
 
 
 def findexternaltool(ui, tool):
     for kn in (b"regkey", b"regkeyalt"):
         k = _toolstr(ui, tool, kn)
         if not k:
             continue
         p = util.lookupreg(k, _toolstr(ui, tool, b"regname"))
         if p:
             p = procutil.findexe(p + _toolstr(ui, tool, b"regappend", b""))
             if p:
                 return p
     exe = _toolstr(ui, tool, b"executable", tool)
     return procutil.findexe(util.expandpath(exe))
 
 
 def _picktool(repo, ui, path, binary, symlink, changedelete):
     strictcheck = ui.configbool(b'merge', b'strict-capability-check')
 
     def hascapability(tool, capability, strict=False):
         if tool in internals:
             return strict and internals[tool].capabilities.get(capability)
         return _toolbool(ui, tool, capability)
 
     def supportscd(tool):
         return tool in internals and internals[tool].mergetype == nomerge
 
     def check(tool, pat, symlink, binary, changedelete):
         tmsg = tool
         if pat:
             tmsg = _(b"%s (for pattern %s)") % (tool, pat)
         if not _findtool(ui, tool):
             if pat:  # explicitly requested tool deserves a warning
                 ui.warn(_(b"couldn't find merge tool %s\n") % tmsg)
             else:  # configured but non-existing tools are more silent
                 ui.note(_(b"couldn't find merge tool %s\n") % tmsg)
         elif symlink and not hascapability(tool, b"symlink", strictcheck):
             ui.warn(_(b"tool %s can't handle symlinks\n") % tmsg)
         elif binary and not hascapability(tool, b"binary", strictcheck):
             ui.warn(_(b"tool %s can't handle binary\n") % tmsg)
         elif changedelete and not supportscd(tool):
             # the nomerge tools are the only tools that support change/delete
             # conflicts
             pass
         elif not procutil.gui() and _toolbool(ui, tool, b"gui"):
             ui.warn(_(b"tool %s requires a GUI\n") % tmsg)
         else:
             return True
         return False
 
     # internal config: ui.forcemerge
     # forcemerge comes from command line arguments, highest priority
     force = ui.config(b'ui', b'forcemerge')
     if force:
         toolpath = _findtool(ui, force)
         if changedelete and not supportscd(toolpath):
             return b":prompt", None
         else:
             if toolpath:
                 return (force, _quotetoolpath(toolpath))
             else:
                 # mimic HGMERGE if given tool not found
                 return (force, force)
 
     # HGMERGE takes next precedence
     hgmerge = encoding.environ.get(b"HGMERGE")
     if hgmerge:
         if changedelete and not supportscd(hgmerge):
             return b":prompt", None
         else:
             return (hgmerge, hgmerge)
 
     # then patterns
 
     # whether binary capability should be checked strictly
     binarycap = binary and strictcheck
 
     for pat, tool in ui.configitems(b"merge-patterns"):
         mf = match.match(repo.root, b'', [pat])
         if mf(path) and check(tool, pat, symlink, binarycap, changedelete):
             if binary and not hascapability(tool, b"binary", strict=True):
                 ui.warn(
                     _(
                         b"warning: check merge-patterns configurations,"
                         b" if %r for binary file %r is unintentional\n"
                         b"(see 'hg help merge-tools'"
                         b" for binary files capability)\n"
                     )
                     % (pycompat.bytestr(tool), pycompat.bytestr(path))
                 )
             toolpath = _findtool(ui, tool)
             return (tool, _quotetoolpath(toolpath))
 
     # then merge tools
     tools = {}
     disabled = set()
     for k, v in ui.configitems(b"merge-tools"):
         t = k.split(b'.')[0]
         if t not in tools:
             tools[t] = int(_toolstr(ui, t, b"priority"))
         if _toolbool(ui, t, b"disabled"):
             disabled.add(t)
     names = tools.keys()
     tools = sorted(
         [(-p, tool) for tool, p in tools.items() if tool not in disabled]
     )
     uimerge = ui.config(b"ui", b"merge")
     if uimerge:
         # external tools defined in uimerge won't be able to handle
         # change/delete conflicts
         if check(uimerge, path, symlink, binary, changedelete):
             if uimerge not in names and not changedelete:
                 return (uimerge, uimerge)
             tools.insert(0, (None, uimerge))  # highest priority
     tools.append((None, b"hgmerge"))  # the old default, if found
     for p, t in tools:
         if check(t, None, symlink, binary, changedelete):
             toolpath = _findtool(ui, t)
             return (t, _quotetoolpath(toolpath))
 
     # internal merge or prompt as last resort
     if symlink or binary or changedelete:
         if not changedelete and len(tools):
             # any tool is rejected by capability for symlink or binary
             ui.warn(_(b"no tool found to merge %s\n") % path)
         return b":prompt", None
     return b":merge", None
 
 
 def _eoltype(data):
     """Guess the EOL type of a file"""
     if b'\0' in data:  # binary
         return None
     if b'\r\n' in data:  # Windows
         return b'\r\n'
     if b'\r' in data:  # Old Mac
         return b'\r'
     if b'\n' in data:  # UNIX
         return b'\n'
     return None  # unknown
 
 
 def _matcheol(file, backup):
     """Convert EOL markers in a file to match origfile"""
     tostyle = _eoltype(backup.data())  # No repo.wread filters?
     if tostyle:
         data = util.readfile(file)
         style = _eoltype(data)
         if style:
             newdata = data.replace(style, tostyle)
             if newdata != data:
                 util.writefile(file, newdata)
 
 
 @internaltool(b'prompt', nomerge)
 def _iprompt(repo, mynode, local, other, base, toolconf):
     """Asks the user which of the local `p1()` or the other `p2()` version to
     keep as the merged version."""
     ui = repo.ui
     fd = local.fctx.path()
     uipathfn = scmutil.getuipathfn(repo)
 
     # Avoid prompting during an in-memory merge since it doesn't support merge
     # conflicts.
     if local.fctx.changectx().isinmemory():
         raise error.InMemoryMergeConflictsError(
             b'in-memory merge does not support file conflicts'
         )
 
     prompts = partextras([local.label, other.label])
     prompts[b'fd'] = uipathfn(fd)
     try:
         if other.fctx.isabsent():
             index = ui.promptchoice(_localchangedotherdeletedmsg % prompts, 2)
             choice = [b'local', b'other', b'unresolved'][index]
         elif local.fctx.isabsent():
             index = ui.promptchoice(_otherchangedlocaldeletedmsg % prompts, 2)
             choice = [b'other', b'local', b'unresolved'][index]
         else:
             # IMPORTANT: keep the last line of this prompt ("What do you want to
             # do?") very short, see comment next to _localchangedotherdeletedmsg
             # at the top of the file for details.
             index = ui.promptchoice(
                 _(
                     b"file '%(fd)s' needs to be resolved.\n"
                     b"You can keep (l)ocal%(l)s, take (o)ther%(o)s, or leave "
                     b"(u)nresolved.\n"
                     b"What do you want to do?"
                     b"$$ &Local $$ &Other $$ &Unresolved"
                 )
                 % prompts,
                 2,
             )
             choice = [b'local', b'other', b'unresolved'][index]
 
         if choice == b'other':
             return _iother(repo, mynode, local, other, base, toolconf)
         elif choice == b'local':
             return _ilocal(repo, mynode, local, other, base, toolconf)
         elif choice == b'unresolved':
             return _ifail(repo, mynode, local, other, base, toolconf)
     except error.ResponseExpected:
         ui.write(b"\n")
         return _ifail(repo, mynode, local, other, base, toolconf)
 
 
 @internaltool(b'local', nomerge)
 def _ilocal(repo, mynode, local, other, base, toolconf):
     """Uses the local `p1()` version of files as the merged version."""
     return 0, local.fctx.isabsent()
 
 
 @internaltool(b'other', nomerge)
 def _iother(repo, mynode, local, other, base, toolconf):
     """Uses the other `p2()` version of files as the merged version."""
     if other.fctx.isabsent():
         # local changed, remote deleted -- 'deleted' picked
         _underlyingfctxifabsent(local.fctx).remove()
         deleted = True
     else:
         _underlyingfctxifabsent(local.fctx).write(
             other.fctx.data(), other.fctx.flags()
         )
         deleted = False
     return 0, deleted
 
 
 @internaltool(b'fail', nomerge)
 def _ifail(repo, mynode, local, other, base, toolconf):
     """
     Rather than attempting to merge files that were modified on both
     branches, it marks them as unresolved. The resolve command must be
     used to resolve these conflicts."""
     # for change/delete conflicts write out the changed version, then fail
     if local.fctx.isabsent():
         _underlyingfctxifabsent(local.fctx).write(
             other.fctx.data(), other.fctx.flags()
         )
     return 1, False
 
 
 def _underlyingfctxifabsent(filectx):
     """Sometimes when resolving, our fcd is actually an absentfilectx, but
     we want to write to it (to do the resolve). This helper returns the
     underyling workingfilectx in that case.
     """
     if filectx.isabsent():
         return filectx.changectx()[filectx.path()]
     else:
         return filectx
 
 
 def _verifytext(input, ui):
     """verifies that text is non-binary"""
     if stringutil.binary(input.text()):
         msg = _(b"%s looks like a binary file.") % input.fctx.path()
         ui.warn(_(b'warning: %s\n') % msg)
         raise error.Abort(msg)
 
 
 def _premerge(repo, local, other, base, toolconf):
     tool, toolpath, binary, symlink, scriptfn = toolconf
     if symlink or local.fctx.isabsent() or other.fctx.isabsent():
         return 1
 
     ui = repo.ui
 
     validkeep = [b'keep', b'keep-merge3', b'keep-mergediff']
 
     # do we attempt to simplemerge first?
     try:
         premerge = _toolbool(ui, tool, b"premerge", not binary)
     except error.ConfigError:
         premerge = _toolstr(ui, tool, b"premerge", b"").lower()
         if premerge not in validkeep:
             _valid = b', '.join([b"'" + v + b"'" for v in validkeep])
             raise error.ConfigError(
                 _(b"%s.premerge not valid ('%s' is neither boolean nor %s)")
                 % (tool, premerge, _valid)
             )
 
     if premerge:
         mode = b'merge'
         if premerge == b'keep-mergediff':
             mode = b'mergediff'
         elif premerge == b'keep-merge3':
             mode = b'merge3'
         if any(
             stringutil.binary(input.text()) for input in (local, base, other)
         ):
             return 1  # continue merging
         merged_text, conflicts = simplemerge.simplemerge(
             local, base, other, mode=mode
         )
         if not conflicts or premerge in validkeep:
             # fcd.flags() already has the merged flags (done in
             # mergestate.resolve())
             local.fctx.write(merged_text, local.fctx.flags())
         if not conflicts:
             ui.debug(b" premerge successful\n")
             return 0
     return 1  # continue merging
 
 
 def _mergecheck(repo, mynode, fcd, fco, fca, toolconf):
     tool, toolpath, binary, symlink, scriptfn = toolconf
     uipathfn = scmutil.getuipathfn(repo)
     if symlink:
         repo.ui.warn(
             _(b'warning: internal %s cannot merge symlinks for %s\n')
             % (tool, uipathfn(fcd.path()))
         )
         return False
     if fcd.isabsent() or fco.isabsent():
         repo.ui.warn(
             _(
                 b'warning: internal %s cannot merge change/delete '
                 b'conflict for %s\n'
             )
             % (tool, uipathfn(fcd.path()))
         )
         return False
     return True
 
 
 def _merge(repo, local, other, base, mode):
     """
     Uses the internal non-interactive simple merge algorithm for merging
     files. It will fail if there are any conflicts and leave markers in
     the partially merged file. Markers will have two sections, one for each side
     of merge, unless mode equals 'union' which suppresses the markers."""
     ui = repo.ui
 
     try:
         _verifytext(local, ui)
         _verifytext(base, ui)
         _verifytext(other, ui)
     except error.Abort:
         return True, True, False
     else:
         merged_text, conflicts = simplemerge.simplemerge(
             local, base, other, mode=mode
         )
         # fcd.flags() already has the merged flags (done in
         # mergestate.resolve())
         local.fctx.write(merged_text, local.fctx.flags())
         return True, conflicts, False
 
 
 @internaltool(
     b'union',
     fullmerge,
     _(
         b"warning: conflicts while merging %s! "
         b"(edit, then use 'hg resolve --mark')\n"
     ),
     precheck=_mergecheck,
 )
 def _iunion(repo, mynode, local, other, base, toolconf, backup):
     """
     Uses the internal non-interactive simple merge algorithm for merging
     files. It will use both left and right sides for conflict regions.
     No markers are inserted."""
     return _merge(repo, local, other, base, b'union')
 
 
 @internaltool(
     b'merge',
     fullmerge,
     _(
         b"warning: conflicts while merging %s! "
         b"(edit, then use 'hg resolve --mark')\n"
     ),
     precheck=_mergecheck,
 )
 def _imerge(repo, mynode, local, other, base, toolconf, backup):
     """
     Uses the internal non-interactive simple merge algorithm for merging
     files. It will fail if there are any conflicts and leave markers in
     the partially merged file. Markers will have two sections, one for each side
     of merge."""
     return _merge(repo, local, other, base, b'merge')
 
 
 @internaltool(
     b'merge3',
     fullmerge,
     _(
         b"warning: conflicts while merging %s! "
         b"(edit, then use 'hg resolve --mark')\n"
     ),
     precheck=_mergecheck,
 )
 def _imerge3(repo, mynode, local, other, base, toolconf, backup):
     """
     Uses the internal non-interactive simple merge algorithm for merging
     files. It will fail if there are any conflicts and leave markers in
     the partially merged file. Marker will have three sections, one from each
     side of the merge and one for the base content."""
     return _merge(repo, local, other, base, b'merge3')
 
 
 @internaltool(
     b'merge3-lie-about-conflicts',
     fullmerge,
     b'',
     precheck=_mergecheck,
 )
 def _imerge3alwaysgood(*args, **kwargs):
     # Like merge3, but record conflicts as resolved with markers in place.
     #
     # This is used for `diff.merge` to show the differences between
     # the auto-merge state and the committed merge state. It may be
     # useful for other things.
     b1, junk, b2 = _imerge3(*args, **kwargs)
     # TODO is this right? I'm not sure what these return values mean,
     # but as far as I can tell this will indicate to callers tha the
     # merge succeeded.
     return b1, False, b2
 
 
 @internaltool(
     b'mergediff',
     fullmerge,
     _(
         b"warning: conflicts while merging %s! "
         b"(edit, then use 'hg resolve --mark')\n"
     ),
     precheck=_mergecheck,
 )
 def _imerge_diff(repo, mynode, local, other, base, toolconf, backup):
     """
     Uses the internal non-interactive simple merge algorithm for merging
     files. It will fail if there are any conflicts and leave markers in
     the partially merged file. The marker will have two sections, one with the
     content from one side of the merge, and one with a diff from the base
     content to the content on the other side. (experimental)"""
     return _merge(repo, local, other, base, b'mergediff')
 
 
 @internaltool(b'merge-local', mergeonly, precheck=_mergecheck)
 def _imergelocal(repo, mynode, local, other, base, toolconf, backup):
     """
     Like :merge, but resolve all conflicts non-interactively in favor
     of the local `p1()` changes."""
     return _merge(repo, local, other, base, b'local')
 
 
 @internaltool(b'merge-other', mergeonly, precheck=_mergecheck)
 def _imergeother(repo, mynode, local, other, base, toolconf, backup):
     """
     Like :merge, but resolve all conflicts non-interactively in favor
     of the other `p2()` changes."""
     return _merge(repo, local, other, base, b'other')
 
 
 @internaltool(
     b'tagmerge',
     mergeonly,
     _(
         b"automatic tag merging of %s failed! "
         b"(use 'hg resolve --tool :merge' or another merge "
         b"tool of your choice)\n"
     ),
 )
 def _itagmerge(repo, mynode, local, other, base, toolconf, backup):
     """
     Uses the internal tag merge algorithm (experimental).
     """
     success, status = tagmerge.merge(repo, local.fctx, other.fctx, base.fctx)
     return success, status, False
 
 
 @internaltool(b'dump', fullmerge, binary=True, symlink=True)
 def _idump(repo, mynode, local, other, base, toolconf, backup):
     """
     Creates three versions of the files to merge, containing the
     contents of local, other and base. These files can then be used to
     perform a merge manually. If the file to be merged is named
     ``a.txt``, these files will accordingly be named ``a.txt.local``,
     ``a.txt.other`` and ``a.txt.base`` and they will be placed in the
     same directory as ``a.txt``.
 
     This implies premerge. Therefore, files aren't dumped, if premerge
     runs successfully. Use :forcedump to forcibly write files out.
     """
     a = _workingpath(repo, local.fctx)
     fd = local.fctx.path()
 
     from . import context
 
     if isinstance(local.fctx, context.overlayworkingfilectx):
         raise error.InMemoryMergeConflictsError(
             b'in-memory merge does not support the :dump tool.'
         )
 
     util.writefile(a + b".local", local.fctx.decodeddata())
     repo.wwrite(fd + b".other", other.fctx.data(), other.fctx.flags())
     repo.wwrite(fd + b".base", base.fctx.data(), base.fctx.flags())
     return False, 1, False
 
 
 @internaltool(b'forcedump', mergeonly, binary=True, symlink=True)
 def _forcedump(repo, mynode, local, other, base, toolconf, backup):
     """
     Creates three versions of the files as same as :dump, but omits premerge.
     """
     return _idump(repo, mynode, local, other, base, toolconf, backup)
 
 
 def _xmergeimm(repo, mynode, local, other, base, toolconf, backup):
     # In-memory merge simply raises an exception on all external merge tools,
     # for now.
     #
     # It would be possible to run most tools with temporary files, but this
     # raises the question of what to do if the user only partially resolves the
     # file -- we can't leave a merge state. (Copy to somewhere in the .hg/
     # directory and tell the user how to get it is my best idea, but it's
     # clunky.)
     raise error.InMemoryMergeConflictsError(
         b'in-memory merge does not support external merge tools'
     )
 
 
 def _describemerge(ui, repo, mynode, fcl, fcb, fco, env, toolpath, args):
     tmpl = ui.config(b'command-templates', b'pre-merge-tool-output')
     if not tmpl:
         return
 
     mappingdict = templateutil.mappingdict
     props = {
         b'ctx': fcl.changectx(),
         b'node': hex(mynode),
         b'path': fcl.path(),
         b'local': mappingdict(
             {
                 b'ctx': fcl.changectx(),
                 b'fctx': fcl,
                 b'node': hex(mynode),
                 b'name': _(b'local'),
                 b'islink': b'l' in fcl.flags(),
                 b'label': env[b'HG_MY_LABEL'],
             }
         ),
         b'base': mappingdict(
             {
                 b'ctx': fcb.changectx(),
                 b'fctx': fcb,
                 b'name': _(b'base'),
                 b'islink': b'l' in fcb.flags(),
                 b'label': env[b'HG_BASE_LABEL'],
             }
         ),
         b'other': mappingdict(
             {
                 b'ctx': fco.changectx(),
                 b'fctx': fco,
                 b'name': _(b'other'),
                 b'islink': b'l' in fco.flags(),
                 b'label': env[b'HG_OTHER_LABEL'],
             }
         ),
         b'toolpath': toolpath,
         b'toolargs': args,
     }
 
     # TODO: make all of this something that can be specified on a per-tool basis
     tmpl = templater.unquotestring(tmpl)
 
     # Not using cmdutil.rendertemplate here since it causes errors importing
     # things for us to import cmdutil.
     tres = formatter.templateresources(ui, repo)
     t = formatter.maketemplater(
         ui, tmpl, defaults=templatekw.keywords, resources=tres
     )
     ui.status(t.renderdefault(props))
 
 
 def _xmerge(repo, mynode, local, other, base, toolconf, backup):
     fcd = local.fctx
     fco = other.fctx
     fca = base.fctx
     tool, toolpath, binary, symlink, scriptfn = toolconf
     uipathfn = scmutil.getuipathfn(repo)
     if fcd.isabsent() or fco.isabsent():
         repo.ui.warn(
             _(b'warning: %s cannot merge change/delete conflict for %s\n')
             % (tool, uipathfn(fcd.path()))
         )
         return False, 1, None
     localpath = _workingpath(repo, fcd)
     args = _toolstr(repo.ui, tool, b"args")
 
     with _maketempfiles(
         fco, fca, repo.wvfs.join(backup.path()), b"$output" in args
     ) as temppaths:
         basepath, otherpath, localoutputpath = temppaths
         outpath = b""
 
         def format_label(input):
             if input.label_detail:
                 return b'%s: %s' % (input.label, input.label_detail)
             else:
                 return input.label
 
         env = {
             b'HG_FILE': fcd.path(),
             b'HG_MY_NODE': short(mynode),
             b'HG_OTHER_NODE': short(fco.changectx().node()),
             b'HG_BASE_NODE': short(fca.changectx().node()),
             b'HG_MY_ISLINK': b'l' in fcd.flags(),
             b'HG_OTHER_ISLINK': b'l' in fco.flags(),
             b'HG_BASE_ISLINK': b'l' in fca.flags(),
             b'HG_MY_LABEL': format_label(local),
             b'HG_OTHER_LABEL': format_label(other),
             b'HG_BASE_LABEL': format_label(base),
         }
         ui = repo.ui
 
         if b"$output" in args:
             # read input from backup, write to original
             outpath = localpath
             localpath = localoutputpath
         replace = {
             b'local': localpath,
             b'base': basepath,
             b'other': otherpath,
             b'output': outpath,
             b'labellocal': format_label(local),
             b'labelother': format_label(other),
             b'labelbase': format_label(base),
         }
         args = util.interpolate(
             br'\$',
             replace,
             args,
             lambda s: procutil.shellquote(util.localpath(s)),
         )
         if _toolbool(ui, tool, b"gui"):
             repo.ui.status(
                 _(b'running merge tool %s for file %s\n')
                 % (tool, uipathfn(fcd.path()))
             )
         if scriptfn is None:
             cmd = toolpath + b' ' + args
             repo.ui.debug(b'launching merge tool: %s\n' % cmd)
             _describemerge(ui, repo, mynode, fcd, fca, fco, env, toolpath, args)
             r = ui.system(
                 cmd, cwd=repo.root, environ=env, blockedtag=b'mergetool'
             )
         else:
             repo.ui.debug(
                 b'launching python merge script: %s:%s\n' % (toolpath, scriptfn)
             )
             r = 0
             try:
                 # avoid cycle cmdutil->merge->filemerge->extensions->cmdutil
                 from . import extensions
 
                 mod = extensions.loadpath(toolpath, b'hgmerge.%s' % tool)
             except Exception:
                 raise error.Abort(
                     _(b"loading python merge script failed: %s") % toolpath
                 )
             mergefn = getattr(mod, scriptfn, None)
             if mergefn is None:
                 raise error.Abort(
                     _(b"%s does not have function: %s") % (toolpath, scriptfn)
                 )
             argslist = procutil.shellsplit(args)
             # avoid cycle cmdutil->merge->filemerge->hook->extensions->cmdutil
             from . import hook
 
             ret, raised = hook.pythonhook(
                 ui, repo, b"merge", toolpath, mergefn, {b'args': argslist}, True
             )
             if raised:
                 r = 1
         repo.ui.debug(b'merge tool returned: %d\n' % r)
         return True, r, False
 
 
 def _populate_label_detail(input, template):
     """Applies the given template to the ctx and stores it in the input."""
     ctx = input.fctx.changectx()
     if ctx.node() is None:
         ctx = ctx.p1()
 
     props = {b'ctx': ctx}
     templateresult = template.renderdefault(props)
     input.label_detail = templateresult.splitlines()[0]  # split for safety
 
 
 def _populate_label_details(repo, inputs, tool=None):
     """Populates the label details using the conflict marker template."""
     ui = repo.ui
     template = ui.config(b'command-templates', b'mergemarker')
     if tool is not None:
         template = _toolstr(ui, tool, b'mergemarkertemplate', template)
     template = templater.unquotestring(template)
     tres = formatter.templateresources(ui, repo)
     tmpl = formatter.maketemplater(
         ui, template, defaults=templatekw.keywords, resources=tres
     )
 
     for input in inputs:
         _populate_label_detail(input, tmpl)
 
 
 def partextras(labels):
     """Return a dictionary of extra labels for use in prompts to the user
 
     Intended use is in strings of the form "(l)ocal%(l)s".
     """
     if labels is None:
         return {
             b"l": b"",
             b"o": b"",
         }
 
     return {
         b"l": b" [%s]" % labels[0],
         b"o": b" [%s]" % labels[1],
     }
 
 
-def _makebackup(repo, ui, wctx, fcd):
+def _makebackup(repo, ui, fcd):
     """Makes and returns a filectx-like object for ``fcd``'s backup file.
 
     In addition to preserving the user's pre-existing modifications to `fcd`
     (if any), the backup is used to undo certain premerges, confirm whether a
     merge changed anything, and determine what line endings the new file should
     have.
 
     Backups only need to be written once since their content doesn't change
     afterwards.
     """
     if fcd.isabsent():
         return None
     # TODO: Break this import cycle somehow. (filectx -> ctx -> fileset ->
     # merge -> filemerge). (I suspect the fileset import is the weakest link)
     from . import context
 
+    if isinstance(fcd, context.overlayworkingfilectx):
+        # If we're merging in-memory, we're free to put the backup anywhere.
+        fd, backup = pycompat.mkstemp(b'hg-merge-backup')
+        with os.fdopen(fd, 'wb') as f:
+            f.write(fcd.data())
+    else:
     backup = scmutil.backuppath(ui, repo, fcd.path())
-    inworkingdir = backup.startswith(repo.wvfs.base) and not backup.startswith(
-        repo.vfs.base
-    )
-    if isinstance(fcd, context.overlayworkingfilectx) and inworkingdir:
-        # If the backup file is to be in the working directory, and we're
-        # merging in-memory, we must redirect the backup to the memory context
-        # so we don't disturb the working directory.
-        relpath = backup[len(repo.wvfs.base) + 1 :]
-        wctx[relpath].write(fcd.data(), fcd.flags())
-        return wctx[relpath]
-    else:
-        # Otherwise, write to wherever path the user specified the backups
-        # should go. We still need to switch based on whether the source is
-        # in-memory so we can use the fast path of ``util.copy`` if both are
-        # on disk.
-        if isinstance(fcd, context.overlayworkingfilectx):
-            util.writefile(backup, fcd.data())
-        else:
             a = _workingpath(repo, fcd)
             util.copyfile(a, backup)
-        # A arbitraryfilectx is returned, so we can run the same functions on
-        # the backup context regardless of where it lives.
+
         return context.arbitraryfilectx(backup, repo=repo)
 
 
 @contextlib.contextmanager
 def _maketempfiles(fco, fca, localpath, uselocalpath):
     """Writes out `fco` and `fca` as temporary files, and (if uselocalpath)
     copies `localpath` to another temporary file, so an external merge tool may
     use them.
     """
     tmproot = pycompat.mkdtemp(prefix=b'hgmerge-')
 
     def maketempfrompath(prefix, path):
         fullbase, ext = os.path.splitext(path)
         pre = b"%s~%s" % (os.path.basename(fullbase), prefix)
         name = os.path.join(tmproot, pre)
         if ext:
             name += ext
         f = open(name, "wb")
         return f, name
 
     def tempfromcontext(prefix, ctx):
         f, name = maketempfrompath(prefix, ctx.path())
         data = ctx.decodeddata()
         f.write(data)
         f.close()
         return name
 
     b = tempfromcontext(b"base", fca)
     c = tempfromcontext(b"other", fco)
     d = localpath
     if uselocalpath:
         # We start off with this being the backup filename, so remove the .orig
         # to make syntax-highlighting more likely.
         if d.endswith(b'.orig'):
             d, _ = os.path.splitext(d)
         f, d = maketempfrompath(b"local", d)
         with open(localpath, b'rb') as src:
             f.write(src.read())
         f.close()
 
     try:
         yield b, c, d
     finally:
         shutil.rmtree(tmproot)
 
 
 def filemerge(repo, wctx, mynode, orig, fcd, fco, fca, labels=None):
     """perform a 3-way merge in the working directory
 
     mynode = parent node before merge
     orig = original local filename before merge
     fco = other file context
     fca = ancestor file context
     fcd = local file context for current/destination file
 
     Returns whether the merge is complete, the return value of the merge, and
     a boolean indicating whether the file was deleted from disk."""
     ui = repo.ui
     fd = fcd.path()
     uipathfn = scmutil.getuipathfn(repo)
     fduipath = uipathfn(fd)
     binary = fcd.isbinary() or fco.isbinary() or fca.isbinary()
     symlink = b'l' in fcd.flags() + fco.flags()
     changedelete = fcd.isabsent() or fco.isabsent()
     tool, toolpath = _picktool(repo, ui, fd, binary, symlink, changedelete)
     scriptfn = None
     if tool in internals and tool.startswith(b'internal:'):
         # normalize to new-style names (':merge' etc)
         tool = tool[len(b'internal') :]
     if toolpath and toolpath.startswith(b'python:'):
         invalidsyntax = False
         if toolpath.count(b':') >= 2:
             script, scriptfn = toolpath[7:].rsplit(b':', 1)
             if not scriptfn:
                 invalidsyntax = True
             # missing :callable can lead to spliting on windows drive letter
             if b'\\' in scriptfn or b'/' in scriptfn:
                 invalidsyntax = True
         else:
             invalidsyntax = True
         if invalidsyntax:
             raise error.Abort(_(b"invalid 'python:' syntax: %s") % toolpath)
         toolpath = script
     ui.debug(
         b"picked tool '%s' for %s (binary %s symlink %s changedelete %s)\n"
         % (
             tool,
             fduipath,
             pycompat.bytestr(binary),
             pycompat.bytestr(symlink),
             pycompat.bytestr(changedelete),
         )
     )
 
     if tool in internals:
         func = internals[tool]
         mergetype = func.mergetype
         onfailure = func.onfailure
         precheck = func.precheck
         isexternal = False
     else:
         if wctx.isinmemory():
             func = _xmergeimm
         else:
             func = _xmerge
         mergetype = fullmerge
         onfailure = _(b"merging %s failed!\n")
         precheck = None
         isexternal = True
 
     toolconf = tool, toolpath, binary, symlink, scriptfn
 
     if not labels:
         labels = [b'local', b'other']
     if len(labels) < 3:
         labels.append(b'base')
     local = simplemerge.MergeInput(fcd, labels[0])
     other = simplemerge.MergeInput(fco, labels[1])
     base = simplemerge.MergeInput(fca, labels[2])
     if mergetype == nomerge:
         return func(
             repo,
             mynode,
             local,
             other,
             base,
             toolconf,
         )
 
     if orig != fco.path():
         ui.status(
             _(b"merging %s and %s to %s\n")
             % (uipathfn(orig), uipathfn(fco.path()), fduipath)
         )
     else:
         ui.status(_(b"merging %s\n") % fduipath)
 
     ui.debug(b"my %s other %s ancestor %s\n" % (fcd, fco, fca))
 
     if precheck and not precheck(repo, mynode, fcd, fco, fca, toolconf):
         if onfailure:
             if wctx.isinmemory():
                 raise error.InMemoryMergeConflictsError(
                     b'in-memory merge does not support merge conflicts'
                 )
             ui.warn(onfailure % fduipath)
         return 1, False
 
-    backup = _makebackup(repo, ui, wctx, fcd)
+    backup = _makebackup(repo, ui, fcd)
     r = 1
     try:
         internalmarkerstyle = ui.config(b'ui', b'mergemarkers')
         if isexternal:
             markerstyle = _toolstr(ui, tool, b'mergemarkers')
         else:
             markerstyle = internalmarkerstyle
 
         if mergetype == fullmerge:
             # conflict markers generated by premerge will use 'detailed'
             # settings if either ui.mergemarkers or the tool's mergemarkers
             # setting is 'detailed'. This way tools can have basic labels in
             # space-constrained areas of the UI, but still get full information
             # in conflict markers if premerge is 'keep' or 'keep-merge3'.
             labeltool = None
             if markerstyle != b'basic':
                 # respect 'tool's mergemarkertemplate (which defaults to
                 # command-templates.mergemarker)
                 labeltool = tool
             if internalmarkerstyle != b'basic' or markerstyle != b'basic':
                 _populate_label_details(
                     repo, [local, other, base], tool=labeltool
                 )
 
             r = _premerge(
                 repo,
                 local,
                 other,
                 base,
                 toolconf,
             )
             # we're done if premerge was successful (r is 0)
             if not r:
                 return r, False
 
             # Reset to basic labels
             local.label_detail = None
             other.label_detail = None
             base.label_detail = None
 
         if markerstyle != b'basic':
             _populate_label_details(repo, [local, other, base], tool=tool)
 
         needcheck, r, deleted = func(
             repo,
             mynode,
             local,
             other,
             base,
             toolconf,
             backup,
         )
 
         if needcheck:
             r = _check(repo, r, ui, tool, fcd, backup)
 
         if r:
             if onfailure:
                 if wctx.isinmemory():
                     raise error.InMemoryMergeConflictsError(
                         b'in-memory merge '
                         b'does not support '
                         b'merge conflicts'
                     )
                 ui.warn(onfailure % fduipath)
             _onfilemergefailure(ui)
 
         return r, deleted
     finally:
         if not r and backup is not None:
             backup.remove()
 
 
 def _haltmerge():
     msg = _(b'merge halted after failed merge (see hg resolve)')
     raise error.InterventionRequired(msg)
 
 
 def _onfilemergefailure(ui):
     action = ui.config(b'merge', b'on-failure')
     if action == b'prompt':
         msg = _(b'continue merge operation (yn)?$$ &Yes $$ &No')
         if ui.promptchoice(msg, 0) == 1:
             _haltmerge()
     if action == b'halt':
         _haltmerge()
     # default action is 'continue', in which case we neither prompt nor halt
 
 
 def hasconflictmarkers(data):
     # Detect lines starting with a string of 7 identical characters from the
     # subset Mercurial uses for conflict markers, followed by either the end of
     # line or a space and some text. Note that using [<>=+|-]{7} would detect
     # `<><><><><` as a conflict marker, which we don't want.
     return bool(
         re.search(
             br"^([<>=+|-])\1{6}( .*)$",
             data,
             re.MULTILINE,
         )
     )
 
 
 def _check(repo, r, ui, tool, fcd, backup):
     fd = fcd.path()
     uipathfn = scmutil.getuipathfn(repo)
 
     if not r and (
         _toolbool(ui, tool, b"checkconflicts")
         or b'conflicts' in _toollist(ui, tool, b"check")
     ):
         if hasconflictmarkers(fcd.data()):
             r = 1
 
     checked = False
     if b'prompt' in _toollist(ui, tool, b"check"):
         checked = True
         if ui.promptchoice(
             _(b"was merge of '%s' successful (yn)?$$ &Yes $$ &No")
             % uipathfn(fd),
             1,
         ):
             r = 1
 
     if (
         not r
         and not checked
         and (
             _toolbool(ui, tool, b"checkchanged")
             or b'changed' in _toollist(ui, tool, b"check")
         )
     ):
         if backup is not None and not fcd.cmp(backup):
             if ui.promptchoice(
                 _(
                     b" output file %s appears unchanged\n"
                     b"was merge successful (yn)?"
                     b"$$ &Yes $$ &No"
                 )
                 % uipathfn(fd),
                 1,
             ):
                 r = 1
 
     if backup is not None and _toolbool(ui, tool, b"fixeol"):
         _matcheol(_workingpath(repo, fcd), backup)
 
     return r
 
 
 def _workingpath(repo, ctx):
     return repo.wjoin(ctx.path())
 
 
 def loadinternalmerge(ui, extname, registrarobj):
     """Load internal merge tool from specified registrarobj"""
     for name, func in pycompat.iteritems(registrarobj._table):
         fullname = b':' + name
         internals[fullname] = func
         internals[b'internal:' + name] = func
         internalsdoc[fullname] = func
 
         capabilities = sorted([k for k, v in func.capabilities.items() if v])
         if capabilities:
             capdesc = b"    (actual capabilities: %s)" % b', '.join(
                 capabilities
             )
             func.__doc__ = func.__doc__ + pycompat.sysstr(b"\n\n%s" % capdesc)
 
     # to put i18n comments into hg.pot for automatically generated texts
 
     # i18n: "binary" and "symlink" are keywords
     # i18n: this text is added automatically
     _(b"    (actual capabilities: binary, symlink)")
     # i18n: "binary" is keyword
     # i18n: this text is added automatically
     _(b"    (actual capabilities: binary)")
     # i18n: "symlink" is keyword
     # i18n: this text is added automatically
     _(b"    (actual capabilities: symlink)")
 
 
 # load built-in merge tools explicitly to setup internalsdoc
 loadinternalmerge(None, None, internaltool)
 
 # tell hggettext to extract docstrings from these functions:
 i18nfunctions = internals.values()