Using the Mercurial backend, the lack of `rcs_commit_staged` is noticed
frequently. I couldn't find any tries to update `mercurial.pm`, so not
letting lack of Mercurial AND Perl knowledge bring me down, I copy-pasted
from `git.pm` to mimic its behaviour from a Mercurial perspective. I hope
it can be a foundation for development by those more proficient in
ikiwiki's inner workings. I have doubts that I personally will be able to
revise it more, based on my Perl skills.

I've tested it briefly. `ikiwiki-calendar` and posting of comments now
works with automatic commits, i.e. the `rcs_commit_staged` function works
in those cases. Under my current setup, I don't know where else to expect
it to work. I would be flabberghasted if there wasn't any problems with it,
though.

> Absolutely, the [[rcs]] chart shows mercurial is lagging behind
> nearly everything. 
> 
> I don't think this stuff is hard, or unlikely to work, familiarity with
> the rcs's particular details is the main thing. --[[Joey]] 

Diff follows, for anyone to annotate. Code is also available at [my hg-repo](http://510x.se/hg/program/ikiwiki/file/e741fcfd800f/Plugin/mercurial.pm).

	diff -r 20c61288d7bd Plugin/mercurial.pm
	--- a/Plugin/mercurial.pm	Fri Jul 15 02:55:12 2011 +0200
	+++ b/Plugin/mercurial.pm	Fri Jul 15 03:29:10 2011 +0200
	@@ -7,6 +7,8 @@
	 use Encode;
	 use open qw{:utf8 :std};
	 
	+my $hg_dir=undef;
	+
	 sub import {
		hook(type => "checkconfig", id => "mercurial", call => \&checkconfig);
		hook(type => "getsetup", id => "mercurial", call => \&getsetup);

A corresponding variable is declared for git. It is unused as of yet for
Mercurial, but when more advanced merge features become available for
`mercurial.pm`, I think it will come into play.

> Maybe.. I'd rather avoid unused cruft though. --[[Joey]]

	@@ -69,6 +71,62 @@
			},
	 }
	 
	+sub safe_hg (&@) {
	+	# Start a child process safely without resorting to /bin/sh.
	+	# Returns command output (in list content) or success state
	+	# (in scalar context), or runs the specified data handler.
	+
	+	my ($error_handler, $data_handler, @cmdline) = @_;
	+
	+	my $pid = open my $OUT, "-|";
	+
	+	error("Cannot fork: $!") if !defined $pid;
	+
	+	if (!$pid) {
	+		# In child.
	+		# hg commands want to be in wc.
	+		if (! defined $hg_dir) {
	+			chdir $config{srcdir}
	+			    or error("cannot chdir to $config{srcdir}: $!");
	+		}
	+		else {
	+			chdir $hg_dir
	+			    or error("cannot chdir to $hg_dir: $!");
	+		}

> How can this possibly work, since `$hg_dir` is not set? The code
> that is being replaced seems to use `-R` to make hg use the right
> directory. If it worked for you without specifying the directory,
> it's quite likely this is the place the patch fails in some
> unusual circumstance.. but I think it could easily use `-R` here.

	+		exec @cmdline or error("Cannot exec '@cmdline': $!");
	+	}
	+	# In parent.
	+
	+	# hg output is probably utf-8 encoded, but may contain
	+	# other encodings or invalidly encoded stuff. So do not rely
	+	# on the normal utf-8 IO layer, decode it by hand.
	+	binmode($OUT);

> Is this actually true for hg?

	+	my @lines;
	+	while (<$OUT>) {
	+		$_=decode_utf8($_, 0);
	+
	+		chomp;
	+
	+		if (! defined $data_handler) {
	+			push @lines, $_;
	+		}
	+		else {
	+			last unless $data_handler->($_);
	+		}
	+	}
	+
	+	close $OUT;
	+
	+	$error_handler->("'@cmdline' failed: $!") if $? && $error_handler;
	+
	+	return wantarray ? @lines : ($? == 0);
	+}
	+# Convenient wrappers.
	+sub run_or_die ($@) { safe_hg(\&error, undef, @_) }
	+sub run_or_cry ($@) { safe_hg(sub { warn @_ }, undef, @_) }
	+sub run_or_non ($@) { safe_hg(undef, undef, @_) }
	+
	 sub mercurial_log ($) {
		my $out = shift;
		my @infos;
	@@ -116,10 +174,7 @@
	 }
	 
	 sub rcs_update () {
	-	my @cmdline = ("hg", "-q", "-R", "$config{srcdir}", "update");
	-	if (system(@cmdline) != 0) {
	-		warn "'@cmdline' failed: $!";
	-	}
	+	run_or_cry('hg', '-q', 'update');
	 }
	 
	 sub rcs_prepedit ($) {

With the `run_or_{die,cry,non}()` functions defined as in `git.pm`, some old Mercurial functions can be rewritten more compactly.

	@@ -129,6 +184,14 @@
	 sub rcs_commit (@) {
		my %params=@_;
	 
	+	return rcs_commit_helper(@_);
	+}
	+
	+sub rcs_commit_helper (@) {
	+	my %params=@_;
	+
	+	my %env=%ENV;

> This `%env` stash is unused; `%ENV` is never modified.

	+
		my $user="Anonymous";
		if (defined $params{session}) {
			if (defined $params{session}->param("name")) {

Here comes the `rcs_commit{,_staged}` part. It is modeled on a `rcs_commit_helper` function, as in `git.pm`.

Some old `mercurial.pm` logic concerning commiter name is kept instead of transplanting the more elaborate logic from `git.pm`. Maybe it is better to "steal" that as well.

> Exactly how to encode the nickname from openid in the commit metadata,
> and get it back out in `rcs_recentchanges`, would probably vary from git.

	@@ -143,43 +206,45 @@
			$params{message} = "no message given";
		}
	 
	-	my @cmdline = ("hg", "-q", "-R", $config{srcdir}, "commit", 
	-	               "-m", IkiWiki::possibly_foolish_untaint($params{message}),
	-	               "-u", IkiWiki::possibly_foolish_untaint($user));
	-	if (system(@cmdline) != 0) {
	-		warn "'@cmdline' failed: $!";
	+	$params{message} = IkiWiki::possibly_foolish_untaint($params{message});
	+
	+	my @opts;
	+	
	+	if (exists $params{file}) {
	+		push @opts, '--', $params{file};
		}
	-
	+	# hg commit returns non-zero if nothing really changed.
	+	# So we should ignore its exit status (hence run_or_non).
	+	run_or_non('hg', 'commit', '-m', $params{message}, '-q', @opts);
	+	
	+	%ENV=%env;
		return undef; # success
	 }
	 
	 sub rcs_commit_staged (@) {
		# Commits all staged changes. Changes can be staged using rcs_add,
		# rcs_remove, and rcs_rename.
	-	my %params=@_;
	-	
	-	error("rcs_commit_staged not implemented for mercurial"); # TODO
	+	return rcs_commit_helper(@_);
	 }
	 
	 sub rcs_add ($) {
		my ($file) = @_;
	 
	-	my @cmdline = ("hg", "-q", "-R", "$config{srcdir}", "add", "$config{srcdir}/$file");
	-	if (system(@cmdline) != 0) {
	-		warn "'@cmdline' failed: $!";
	-	}
	+	run_or_cry('hg', 'add', $file);
	 }
	 
	 sub rcs_remove ($) {
	+	# Remove file from archive.
	+
		my ($file) = @_;
	 
	-	error("rcs_remove not implemented for mercurial"); # TODO
	+	run_or_cry('hg', 'remove', '-f', $file);
	 }
	 
	 sub rcs_rename ($$) {
		my ($src, $dest) = @_;
	 
	-	error("rcs_rename not implemented for mercurial"); # TODO
	+	run_or_cry('hg', 'rename', '-f', $src, $dest);
	 }
	 
	 sub rcs_recentchanges ($) {

> Remainder seems ok to me. Should probably test that the remove plugin 
> works, since this implements `rcs_remove` too and you didn't mention
> any tests that would run that code path. --[[Joey]]