fix: run fixer tools in the repo root as cwd so they can use the working copy

This lets fixer tools do things like find configuration files, with the caveat
that they'll only see the version of that file in the working copy, regardless
of what revisions are being fixed.

Differential Revision: https://phab.mercurial-scm.org/D6440

hgext/fix.py

@@ -102,6 +102,13 b' perform other post-fixing work. The supp'
     mapping fixer tool names to lists of metadata values returned from
     executions that modified a file. This aggregates the same metadata
     previously passed to the "postfixfile" hook.
+
+Fixer tools are run the in repository's root directory. This allows them to read
+configuration files from the working copy, or even write to the working copy.
+The working copy is not updated to match the revision being fixed. In fact,
+several revisions may be fixed in parallel. Writes to the working copy are not
+amended into the revision being fixed; fixer tools should always write fixed
+file content back to stdout as documented above.
 """
 
 from __future__ import absolute_import
@@ -232,7 +239,7 b' def fix(ui, repo, *pats, **opts):'
             for rev, path in items:
                 ctx = repo[rev]
                 olddata = ctx[path].data()
-                metadata, newdata = fixfile(ui, opts, fixers, ctx, path,
+                metadata, newdata = fixfile(ui, repo, opts, fixers, ctx, path,
                                             basectxs[rev])
                 # Don't waste memory/time passing unchanged content back, but
                 # produce one result per item either way.
@@ -529,7 +536,7 b' def getbasectxs(repo, opts, revstofix):'
                 basectxs[rev].add(pctx)
     return basectxs
 
-def fixfile(ui, opts, fixers, fixctx, path, basectxs):
+def fixfile(ui, repo, opts, fixers, fixctx, path, basectxs):
     """Run any configured fixers that should affect the file in this context
 
     Returns the file content that results from applying the fixers in some order
@@ -538,7 +545,8 b' def fixfile(ui, opts, fixers, fixctx, pa'
     (i.e. they will only avoid lines that are common to all basectxs).
 
     A fixer tool's stdout will become the file's new content if and only if it
-    exits with code zero.
+    exits with code zero. The fixer tool's working directory is the repository's
+    root.
     """
     metadata = {}
     newdata = fixctx[path].data()
@@ -552,7 +560,7 b' def fixfile(ui, opts, fixers, fixctx, pa'
             proc = subprocess.Popen(
                 procutil.tonativestr(command),
                 shell=True,
-                cwd=procutil.tonativestr(b'/'),
+                cwd=repo.root,
                 stdin=subprocess.PIPE,
                 stdout=subprocess.PIPE,
                 stderr=subprocess.PIPE)

tests/test-fix.t

@@ -215,6 +215,13 b' Help text for fix.'
       executions that modified a file. This aggregates the same metadata
       previously passed to the "postfixfile" hook.
   
+  Fixer tools are run the in repository's root directory. This allows them to
+  read configuration files from the working copy, or even write to the working
+  copy. The working copy is not updated to match the revision being fixed. In
+  fact, several revisions may be fixed in parallel. Writes to the working copy
+  are not amended into the revision being fixed; fixer tools should always write
+  fixed file content back to stdout as documented above.
+  
   list of commands:
   
    fix           rewrite file content in changesets or working directory
@@ -1269,6 +1276,41 b' three revisions instead of two.'
 
   $ cd ..
 
+We run fixer tools in the repo root so they can look for config files or other
+important things in the working directory. This does NOT mean we are
+reconstructing a working copy of every revision being fixed; we're just giving
+the tool knowledge of the repo's location in case it can do something
+reasonable with that.
+
+  $ hg init subprocesscwd
+  $ cd subprocesscwd
+
+  $ cat >> .hg/hgrc <<EOF
+  > [fix]
+  > printcwd:command = pwd
+  > printcwd:pattern = path:foo/bar
+  > EOF
+
+  $ mkdir foo
+  $ printf "bar\n" > foo/bar
+  $ hg commit -Aqm blah
+
+  $ hg fix -w -r . foo/bar
+  $ hg cat -r tip foo/bar
+  $TESTTMP/subprocesscwd
+  $ cat foo/bar
+  $TESTTMP/subprocesscwd
+
+  $ cd foo
+
+  $ hg fix -w -r . bar
+  $ hg cat -r tip bar
+  $TESTTMP/subprocesscwd
+  $ cat bar
+  $TESTTMP/subprocesscwd
+
+  $ cd ../..
+
 Tools configured without a pattern are ignored. It would be too dangerous to
 run them on all files, because this might happen while testing a configuration
 that also deletes all of the file content. There is no reasonable subset of the