fix: allow tools to use :linerange, but also run if a file is unchanged

The definition of "unchanged" here is subtle, because pure deletion diff hunks
are ignored. That means this is different from using the --whole flag. This
change allows you to configure, for example, a code formatter that:
 1. Formats specific line ranges if specified via flags
 2. Does not format the entire file when there are no line ranges provided
 3. Performs some other kind of formatting regardless of provided line ranges

This sounds a little far fetched, but it is meant to address a specific corner
case encountered in Google's use of the fix extension. The default behavior is
kept because it exists to prevent mistakes that could erase uncommitted
changes.

Differential Revision: https://phab.mercurial-scm.org/D6723

hgext/fix.py

@@ -36,6 +36,15 b' substituted into the command::'
   {first}   The 1-based line number of the first line in the modified range
   {last}    The 1-based line number of the last line in the modified range
 
+Deleted sections of a file will be ignored by :linerange, because there is no
+corresponding line range in the version being fixed.
+
+By default, tools that set :linerange will only be executed if there is at least
+one changed line range. This is meant to prevent accidents like running a code
+formatter in such a way that it unexpectedly reformats the whole file. If such a
+tool needs to operate on unchanged files, it should set the :skipclean suboption
+to false.
+
 The :pattern suboption determines which files will be passed through each
 configured tool. See :hg:`help patterns` for possible values. If there are file
 arguments to :hg:`fix`, the intersection of these patterns is used.
@@ -126,6 +135,7 b' from mercurial.node import wdirrev'
 
 from mercurial.utils import (
     procutil,
+    stringutil,
 )
 
 from mercurial import (
@@ -162,6 +172,7 b' FIXER_ATTRS = {'
     'pattern': None,
     'priority': 0,
     'metadata': False,
+    'skipclean': 'true',
 }
 
 for key, default in FIXER_ATTRS.items():
@@ -713,6 +724,7 b' def getfixers(ui):'
             setattr(fixers[name], pycompat.sysstr('_' + key),
                     attrs.get(key, default))
         fixers[name]._priority = int(fixers[name]._priority)
+        fixers[name]._skipclean = stringutil.parsebool(fixers[name]._skipclean)
         # Don't use a fixer if it has no pattern configured. It would be
         # dangerous to let it affect all files. It would be pointless to let it
         # affect no files. There is no reasonable subset of files to use as the
@@ -756,7 +768,7 b' class Fixer(object):'
                         {'rootpath': path, 'basename': os.path.basename(path)})]
         if self._linerange:
             ranges = rangesfn()
-            if not ranges:
+            if self._skipclean and not ranges:
                 # No line ranges to fix, so don't run the fixer.
                 return None
             for first, last in ranges:

tests/test-fix.t

@@ -147,6 +147,15 b' Help text for fix.'
     {first}   The 1-based line number of the first line in the modified range
     {last}    The 1-based line number of the last line in the modified range
   
+  Deleted sections of a file will be ignored by :linerange, because there is no
+  corresponding line range in the version being fixed.
+  
+  By default, tools that set :linerange will only be executed if there is at
+  least one changed line range. This is meant to prevent accidents like running
+  a code formatter in such a way that it unexpectedly reformats the whole file.
+  If such a tool needs to operate on unchanged files, it should set the
+  :skipclean suboption to false.
+  
   The :pattern suboption determines which files will be passed through each
   configured tool. See 'hg help patterns' for possible values. If there are file
   arguments to 'hg fix', the intersection of these patterns is used.
@@ -1356,3 +1365,34 b' The way we invoke matching must not proh'
   fixed
 
   $ cd ..
+
+Tools should be able to run on unchanged files, even if they set :linerange.
+This includes a corner case where deleted chunks of a file are not considered
+changes.
+
+  $ hg init skipclean
+  $ cd skipclean
+
+  $ printf "a\nb\nc\n" > foo
+  $ printf "a\nb\nc\n" > bar
+  $ printf "a\nb\nc\n" > baz
+  $ hg commit -Aqm "base"
+
+  $ printf "a\nc\n" > foo
+  $ printf "a\nx\nc\n" > baz
+
+  $ hg fix --working-dir foo bar baz \
+  >        --config 'fix.changedlines:command=printf "Line ranges:\n"; ' \
+  >        --config 'fix.changedlines:linerange=printf "{first} through {last}\n"; ' \
+  >        --config 'fix.changedlines:pattern=rootglob:**' \
+  >        --config 'fix.changedlines:skipclean=false'
+
+  $ cat foo
+  Line ranges:
+  $ cat bar
+  Line ranges:
+  $ cat baz
+  Line ranges:
+  2 through 2
+
+  $ cd ..