--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,17 @@
+2011-11-02  Jim Meyering  <meyering@redhat.com>
+
+	gitlog-to-changelog: provide a ChangeLog-repair mechanism
+	Git logs are often treated as immutable, because editing them
+	changes the SHA1 checksums of all descendants.  Thus, errors in
+	git logs tend to stay there forever.  However, when we generate
+	a ChangeLog file -- typically for distribution -- from that git log,
+	we can actually make corrections in the generated file.  The key
+	lies in recording in machine-readable/applicable form the desired
+	corrections.  See --help for description and an example.
+	* build-aux/gitlog-to-changelog (parse_amend_file): New function.
+	(usage): Describe it; alphabetize option descriptions.
+	(main): Honor the new option, carefully.
+
 2011-11-01  Jim Meyering  <meyering@redhat.com>
 
 	gitlog-to-changelog: avoid an infloop

--- a/build-aux/gitlog-to-changelog
+++ b/build-aux/gitlog-to-changelog
@@ -3,7 +3,7 @@
     if 0;
 # Convert git log output to ChangeLog format.
 
-my $VERSION = '2011-10-31 16:06'; # UTC
+my $VERSION = '2011-11-02 07:53'; # UTC
 # The definition above must lie within the first 8 lines in order
 # for the Emacs time-stamp write hook (at end) to update it.
 # If you change this file with Emacs, please let the write hook
@@ -60,13 +60,15 @@
 
 OPTIONS:
 
+   --amend=FILE FILE maps from an SHA1 to perl code (i.e., s/old/new/) that
+                  makes a change to SHA1's commit log text or metadata.
+   --append-dot append a dot to the first line of each commit message if
+                  there is no other punctuation or blank at the end.
    --since=DATE convert only the logs since DATE;
                   the default is to convert all log entries.
    --format=FMT set format string for commit subject and body;
                   see 'man git-log' for the list of format metacharacters;
                   the default is '%s%n%b%n'
-   --append-dot append a dot to the first line of each commit message if
-                  there is no other punctuation or blank at the end.
 
    --help       display this help and exit
    --version    output version information and exit
@@ -76,6 +78,23 @@
   $ME --since=2008-01-01 > ChangeLog
   $ME -- -n 5 foo > last-5-commits-to-branch-foo
 
+In a FILE specified via --amend, comment lines (starting with "#") are ignored.
+FILE must consist of <SHA,CODE+> pairs where SHA is a 40-byte SHA1 (alone on
+a line) referring to a commit in the current project, and CODE refers to one
+or more consecutive lines of Perl code.  Pairs must be separated by one or
+more blank line.
+
+Here is sample input for use with --amend=FILE, from coreutils:
+
+3a169f4c5d9159283548178668d2fae6fced3030
+# fix typo in title:
+s/all tile types/all file types/
+
+1379ed974f1fa39b12e2ffab18b3f7a607082202
+# Due to a bug in vc-dwim, I mis-attributed a patch by Paul to myself.
+# Change the author to be Paul.  Note the escaped "@":
+s,Jim .*>,Paul Eggert <eggert\@cs.ucla.edu>,
+
 EOF
     }
   exit $exit_code;
@@ -101,9 +120,60 @@
   return join (' ', map {shell_quote $_} @_);
 }
 
+# Parse file F.
+# Comment lines (starting with "#") are ignored.
+# F must consist of <SHA,CODE+> pairs where SHA is a 40-byte SHA1
+# (alone on a line) referring to a commit in the current project, and
+# CODE refers to one or more consecutive lines of Perl code.
+# Pairs must be separated by one or more blank line.
+sub parse_amend_file($)
+{
+  my ($f) = @_;
+
+  open F, '<', $f
+    or die "$ME: $f: failed to open for reading: $!\n";
+
+  my $fail;
+  my $h = {};
+  my $in_code = 0;
+  my $sha;
+  while (defined (my $line = <F>))
+    {
+      $line =~ /^\#/
+        and next;
+      chomp $line;
+      $line eq ''
+        and $in_code = 0, next;
+
+      if (!$in_code)
+        {
+          $line =~ /^([0-9a-fA-F]{40})$/
+            or (warn "$ME: $f:$.: invalid line; expected an SHA1\n"),
+              $fail = 1, next;
+          $sha = lc $1;
+          $in_code = 1;
+          exists $h->{$sha}
+            and (warn "$ME: $f:$.: duplicate SHA1\n"),
+              $fail = 1, next;
+        }
+      else
+        {
+          $h->{$sha} ||= '';
+          $h->{$sha} .= "$line\n";
+        }
+    }
+  close F;
+
+  $fail
+    and exit 1;
+
+  return $h;
+}
+
 {
   my $since_date;
   my $format_string = '%s%n%b%n';
+  my $amend_file;
   my $append_dot = 0;
   GetOptions
     (
@@ -111,14 +181,20 @@
      version => sub { print "$ME version $VERSION\n"; exit },
      'since=s' => \$since_date,
      'format=s' => \$format_string,
+     'amend=s' => \$amend_file,
      'append-dot' => \$append_dot,
     ) or usage 1;
 
+
   defined $since_date
     and unshift @ARGV, "--since=$since_date";
 
+  # This is a hash that maps an SHA1 to perl code (i.e., s/old/new/)
+  # that makes a correction in the log or attribution of that commit.
+  my $amend_code = defined $amend_file ? parse_amend_file $amend_file : {};
+
   my @cmd = (qw (git log --log-size),
-             '--pretty=format:%ct  %an  <%ae>%n%n'.$format_string, @ARGV);
+             '--pretty=format:%H:%ct  %an  <%ae>%n%n'.$format_string, @ARGV);
   open PIPE, '-|', @cmd
     or die ("$ME: failed to run `". quoted_cmd (@cmd) ."': $!\n"
             . "(Is your Git too old?  Version 1.5.1 or later is required.)\n");
@@ -137,7 +213,34 @@
       $n_read == $log_nbytes
         or die "$ME:$.: unexpected EOF\n";
 
-      my @line = split "\n", $log;
+      # Extract leading hash.
+      my ($sha, $rest) = split ':', $log, 2;
+      defined $sha
+        or die "$ME:$.: malformed log entry\n";
+      $sha =~ /^[0-9a-fA-F]{40}$/
+        or die "$ME:$.: invalid SHA1: $sha\n";
+
+      # If this commit's log requires any transformation, do it now.
+      my $code = $amend_code->{$sha};
+      if (defined $code)
+        {
+          eval 'use Safe';
+          my $s = new Safe;
+          # Put the unpreprocessed entry into "$_".
+          $_ = $rest;
+
+          # Let $code operate on it, safely.
+          my $r = $s->reval("$code")
+            or die "$ME:$.:$sha: failed to eval \"$code\":\n$@\n";
+
+          # Note that we've used this entry.
+          delete $amend_code->{$sha};
+
+          # Update $rest upon success.
+          $rest = $_;
+        }
+
+      my @line = split "\n", $rest;
       my $author_line = shift @line;
       defined $author_line
         or die "$ME:$.: unexpected EOF\n";
@@ -200,6 +303,16 @@
   close PIPE
     or die "$ME: error closing pipe from " . quoted_cmd (@cmd) . "\n";
   # FIXME-someday: include $PROCESS_STATUS in the diagnostic
+
+  # Complain about any unused entry in the --amend=F specified file.
+  my $fail = 0;
+  foreach my $sha (keys %$amend_code)
+    {
+      warn "$ME:$amend_file: unused entry: $sha\n";
+      $fail = 1;
+    }
+
+  exit $fail;
 }
 
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