doc lint: lint relative section order

Add a linting script to check the relative order of the sections in
the documentation. We should have NAME, then SYNOPSIS, DESCRIPTION,
OPTIONS etc. in that order.

That holds true throughout our documentation, except for a few
exceptions which are hardcoded in the linting script.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/Makefile

@@ -483,7 +483,8 @@ lint-docs::
 		--section=1 $(MAN1_TXT) \
 		--section=5 $(MAN5_TXT) \
 		--section=7 $(MAN7_TXT); \
-	$(PERL_PATH) lint-man-end-blurb.perl $(MAN_TXT)
+	$(PERL_PATH) lint-man-end-blurb.perl $(MAN_TXT); \
+	$(PERL_PATH) lint-man-section-order.perl $(MAN_TXT);
 
 ifeq ($(wildcard po/Makefile),po/Makefile)
 doc-l10n install-l10n::

Documentation/lint-man-section-order.perl

@@ -0,0 +1,125 @@
+#!/usr/bin/perl
+
+use strict;
+use warnings;
+
+my %SECTIONS;
+{
+	my $order = 0;
+	%SECTIONS = (
+		'NAME' => {
+			required => 1,
+			order => $order++,
+		},
+		'SYNOPSIS' => {
+			required => 1,
+			order => $order++,
+		},
+		'DESCRIPTION' => {
+			required => 1,
+			order => $order++,
+			bad => {
+				'git-mktag.txt' => 'OPTIONS',
+				'git-cvsserver.txt' => 'OPTIONS',
+			},
+		},
+		'OPTIONS' => {
+			order => $order++,
+			required => 0,
+			bad => {
+				'git-grep.txt' => 'CONFIGURATION',
+				'git-rebase.txt' => 'CONFIGURATION',
+			},
+		},
+		'CONFIGURATION' => {
+			order => $order++,
+			bad => {
+				'git-svn.txt' => 'BUGS',
+			},
+		},
+		'BUGS' => {
+			order => $order++,
+		},
+		'SEE ALSO' => {
+			order => $order++,
+		},
+		'GIT' => {
+			required => 1,
+			order => $order++,
+		},
+	);
+}
+my $SECTION_RX = do {
+	my ($names) = join "|", keys %SECTIONS;
+	qr/^($names)$/s;
+};
+
+my $exit_code = 0;
+sub report {
+	my ($msg) = @_;
+	print "$ARGV:$.: $msg\n";
+	$exit_code = 1;
+}
+
+my $last_was_section;
+my @actual_order;
+while (my $line = <>) {
+	chomp $line;
+	if ($line =~ $SECTION_RX) {
+		push @actual_order => $line;
+		$last_was_section = 1;
+		# Have no "last" section yet, processing NAME
+		next if @actual_order == 1;
+
+		my @expected_order = sort {
+			$SECTIONS{$a}->{order} <=> $SECTIONS{$b}->{order}
+		} @actual_order;
+
+		my $expected_last = $expected_order[-2];
+		my $actual_last = $actual_order[-2];
+		my $except_last = $SECTIONS{$line}->{bad}->{$ARGV} || '';
+		if (($SECTIONS{$line}->{bad}->{$ARGV} || '') eq $actual_last) {
+			# Either we're whitelisted, or ...
+			next
+		} elsif (exists $SECTIONS{$actual_last}->{bad}->{$ARGV}) {
+			# ... we're complaing about the next section
+			# which is out of order because this one is,
+			# don't complain about that one.
+			next;
+		} elsif ($actual_last ne $expected_last) {
+			report("section '$line' incorrectly ordered, comes after '$actual_last'");
+		}
+		next;
+	}
+	if ($last_was_section) {
+		my $last_section = $actual_order[-1];
+		if (length $last_section ne length $line) {
+			report("dashes under '$last_section' should match its length!");
+		}
+		if ($line !~ /^-+$/) {
+			report("dashes under '$last_section' should be '-' dashes!");
+		}
+		$last_was_section = 0;
+	}
+
+	if (eof) {
+		# We have both a hash and an array to consider, for
+		# convenience
+		my %actual_sections;
+		@actual_sections{@actual_order} = ();
+
+		for my $section (sort keys %SECTIONS) {
+			next if !$SECTIONS{$section}->{required} or exists $actual_sections{$section};
+			report("has no required '$section' section!");
+		}
+
+		# Reset per-file state
+		{
+			@actual_order = ();
+			# this resets our $. for each file
+			close ARGV;
+		}
+	}
+}
+
+exit $exit_code;