#!/usr/bin/perl -w
#
# git caching proxy

# usage: run it on some port, and then clone or fetch
#  "git://<realhost>:<realport>/<real-git-url>[ <options>]"
# where <real-git-url> is http://<host>/... or git://<host>/...
# and <options> is zero or more (whitespace-separated) of
#    [<some-option>]      will be ignored if not recognised
#    {<some-option>}      error if not recognised
# options currently known:
#    fetch=must           fail if the fetch/clone from upstream fails
#    fetch=no             just use what is in the cache
#    fetch=try            use what is in the cache if the fetch/clone fails
#    timeout=<seconds>    length of time to allow for fetch/clone

# example inetd.conf line:
#  9419 stream tcp nowait git-cache /usr/bin/git-cache-proxy git-cache-proxy
# you'll need to 
#  adduser git-cache
#  mkdir /var/cache/git-cache-proxy
#  chown git-cache /var/cache/git-cache-proxy

# git-cache-proxy
# Copyright 2010 Tony Finch
# Copyright 2013 Ian Jackson
# 
# git-cache-proxy is free software; you can redistribute it and/or
# modify them under the terms of the GNU General Public License as
# published by the Free Software Foundation; either version 3, or (at
# your option) any later version.
#
# git-cache-proxy is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
# General Public License for more details.
# 
# You should have received a copy of the GNU General Public License along
# with this program; if not, consult the Free Software Foundation's
# website at www.fsf.org, or the GNU Project website at www.gnu.org.
# 
# (Some code taken from userv-utils's git-daemon.in and git-service.in
# which were written by Tony Finch <dot@dotat.at> and subsequently
# heavily modified by Ian Jackson <ijackson@chiark.greenend.org.uk>
# and were released under CC0 1.0.  The whole program is now GPLv3+.)

use strict;
use warnings;

use POSIX;
use Socket;
use Sys::Syslog;
use Fcntl qw(:flock SEEK_SET);
use File::Path qw(remove_tree);

our $us = 'git-cache-proxy';

our $debug = 0;
our $housekeepingeverydays = 1;
our $treeexpiredays = 21;
our $fetchtimeout = 1800;
our $maxfetchtimeout = 3600;
our $cachedir = '/var/cache/git-cache-proxy';
our $housekeepingonly = 0;

#---------- error handling and logging ----------

# This is a bit fiddly, because we want to catch errors sent to stderr
# and dump them to syslog if we can, but only if we are running as an
# inetd service.

our $log; # filehandle (ref), or "1" meaning syslog

sub ntoa {
    my $sockaddr = shift;
    return ('(local)') unless defined $sockaddr;
    my ($port,$addr) = sockaddr_in $sockaddr;
    $addr = inet_ntoa $addr;
    return ("[$addr]:$port",$addr,$port);
}

our ($client) = ntoa getpeername STDIN;
our ($server) = ntoa getsockname STDIN;

sub ensurelog () {
    return if $log;
    openlog $us, qw(pid), 'daemon';
    $log = 1;
}

sub logm ($$) {
    my ($pri, $msg) = @_;
    return if $pri eq 'debug' && !$debug;
    if ($client eq '(local)') {
	print STDERR "$us: $pri: $msg\n" or die $!;
	return;
    }
    ensurelog();
    my $mainmsg = sprintf "%s-%s: %s", $server, $client, $msg;
    if (ref $log) {
	my $wholemsg = sprintf("%s [%d] %s: %s\n",
			       strftime("%Y-%m-%d %H:%M:%S Z", gmtime),
			       $$,
			       $pri eq 'err' ? 'error' : $pri,
			       $mainmsg);
	print $log $wholemsg;
    } else {
	syslog $pri, "%s", "$pri $mainmsg";
    }
}

if ($client ne '(local)') {
    open STDERR, ">/dev/null" or exit 255;
    open TEMPERR, "+>", undef or exit 255;
    open STDERR, ">&TEMPERR" or exit 255;
}

END {
    if ($client ne '(local)') {
	if ($?) { logm 'crit', "crashing ($?)"; }
	seek TEMPERR, 0, SEEK_SET;
	while (<TEMPERR>) {
	    chomp;
	    logm 'crit', $_;
	}
    }
    exit $?;
}

sub fail ($) {
    my ($msg) = @_;
    logm 'err', $msg;
    exit 0;
}

sub gitfail ($) {
    my ($msg) = @_;
    close LOCK;
    alarm 60;
    logm 'notice', $msg;
    my $gitmsg = "ERR $us: $msg";
    $gitmsg = substr($gitmsg,0,65535); # just in case
    printf "%04x%s", length($gitmsg)+4, $gitmsg;
    flush STDOUT;
    exit 0;
}

#---------- argument parsing ----------

for (;;) {
    last unless @ARGV;
    last unless $ARGV[0] =~ m/^-/;
    $_ = shift @ARGV;
    for (;;) {
	last unless m/^-./;
	if (s/^-H/-/) {
	    $housekeepingonly++;
	} elsif (s/^-D/-/) {
	    $debug++;
	} elsif (s/^-L(.*)$//) {
	    my $logfile = $_;
	    open STDERR, ">>", $logfile or fail "open $logfile: $!";
	    $log = \*STDERR;
	} elsif (s/^-d(.*)$//) {
	    $cachedir = $1;
	} elsif (s/^--( max-fetch-timeout
                      | fetch-timeout
                      | tree-expire-days
                      | housekeeping-interval-days
                      )=(\d+)$//x) {
	    my $vn = $1;
	    $vn =~ y/-//d;
	    die $vn unless defined ${ $::{$vn} };
	    ${ $::{$vn} } = $2;
	} else {
	    fail "bad usage: unknown option `$_'";
	}
    }
}

!@ARGV or fail "bad usage: no non-option arguments permitted";

#---------- utility functions ----------

sub lockfile ($$$) {
    my ($fh, $fn, $flockmode) = @_;
    my $what = $fn.(($flockmode & ~LOCK_NB) == LOCK_SH ? " (shared)" : "");
    for (;;) {
	close $fh;
	open $fh, '+>', $fn or fail "open/create $fn for lock: $!";
	logm 'debug', "lock $what: acquiring";
	if (!flock $fh, $flockmode) {
	    if ($flockmode & LOCK_NB && $! == EWOULDBLOCK) {
		return 0; # ok then
	    }
	    fail "lock $what: $!";
	}
	stat $fh or fail "stat opened $fn: $!";
	my $fh_ino = ((stat _)[1]);
	if (!stat $fn) {
	    $! == ENOENT or fail "stat $fn: $!";
	    next;
	}
	my $fn_ino = ((stat _)[1]);
	if ($fn_ino == $fh_ino) {
	    logm 'debug', "lock $what: acquired";
	    return 1;
	}
	logm 'debug', "lock $what: deleted, need to loop again";
	# oh dear
    }
}

sub xread {
    my $length = shift;
    my $buffer = "";
    while ($length > length $buffer) {
        my $ret = sysread STDIN, $buffer, $length, length $buffer;
        fail "expected $length bytes, got ".length $buffer
                            if defined $ret and $ret == 0;
        fail "read: $!" if not defined $ret and $! != EINTR and $! != EAGAIN;
    }
    return $buffer;
}

#---------- main program ----------

chdir $cachedir or fail "chdir $cachedir: $!";

our ($service,$specpath,$spechost,$subdir);
our ($tmpd,$gitd,$lock);
our ($fetch,$url);

sub servinfo ($) {
    my ($msg) = @_;
    logm 'info', "service `$specpath': $msg";
}

sub readcommand () {
    $SIG{ALRM} = sub { fail "timeout" };
    alarm 30;

    my $hex_len = xread 4;
    fail "Bad hex in packet length" unless $hex_len =~ m|^[0-9a-fA-F]{4}$|;
    my $line = xread -4 + hex $hex_len;
    unless (($service,$specpath,$spechost) = $line =~
	    m|^(git-[a-z-]+) /*([!-~ ]+)\0host=([!-~]+)\0$|) {
	$line =~ s|[^ -~]+| |g;
	gitfail "unknown/unsupported instruction `$line'"
    }

    alarm 0;

    $service eq 'git-upload-pack'
	or gitfail "unknown/unsupported service `$service'";

    $fetch = 2; # 0:don't; 1:try; 2:force
    $url = $specpath;

    while ($url =~ s#\s+(\[)([^][{}]+)\]$## ||
	   $url =~ s#\s+(\{)([^][{}]+)\}$##) {
	$_ = $2;
	my $must = $1 eq '{';
	if (m/^fetch=try$/) {
	    $fetch = 1;
	} elsif (m/^fetch=no$/) {
	    $fetch = 0;
	} elsif (m/^fetch=must$/) {
	    $fetch = 2; # the default
	} elsif (m/^timeout=(\d+)$/ && $1 >= 1) {
	    $fetchtimeout = $1 <= $maxfetchtimeout ? $1 : $maxfetchtimeout;
	} elsif ($must) {
	    gitfail "unknown/unsupported option `$_'";
	}
    }

    $url =~ m{^(?:https?|git)://[-.0-9a-z]+/}
        or gitfail "unknown/unsupported url scheme or format `$url'";

    $subdir = $url;
    $subdir =~ s|\\|\\\\|g;
    $subdir =~ s|,|\\,|g;
    $subdir =~ s|/|,|g;

    $tmpd= "$subdir\\.tmp";
    $gitd= "$subdir\\.git";
    $lock = "$subdir\\.lock";

    servinfo "locking";
}

sub clonefetch () {
    lockfile \*LOCK, $lock, LOCK_EX;

    my $exists = lstat $gitd;
    $exists or $!==ENOENT or fail "lstat $gitd: $!";

    our $fetchfail = '';

    if ($fetch) {

	our @cmd;

	if (!$exists) {
	    system qw(rm -rf --), $tmpd;
	    @cmd = (qw(git clone -q --mirror), $url, $tmpd);
	    servinfo "cloning";
	} else {
	    @cmd = (qw(git remote update --prune));
	    servinfo "fetching";
	}
	my $cmd = "@cmd[0..1]";

	my $child = open FETCHERR, "-|";
	defined $child or fail "fork: $!";
	if (!$child) {
	    if ($exists) {
		chdir $gitd or fail "chdir $gitd: $!";
	    }
	    setpgrp or fail "setpgrp: $!";
	    open STDERR, ">&STDOUT" or fail "redirect stderr: $!";
	    exec @cmd or fail "exec $cmd[0]: $!";
	}

	my $fetcherr = '';
	my $timedout = 0;
	{
	    local $SIG{ALRM} = sub {
		servinfo "fetch/clone timeout";
		$timedout=1; kill 9, -$child;
	    };
	    alarm($fetchtimeout);
	    $!=0; { local $/=undef; $fetcherr = <FETCHERR>; }
	    !FETCHERR->error or fail "read pipe from fetch/clone: $!";
	    alarm(10);
	}

	kill -9, $child or fail "kill fetch/clone: $!";
	$!=0; $?=0; if (!close FETCHERR) {
	    fail "reap fetch/clone: $!" if $!;
	    my $fetchfail =
		!($? & 255) ? "$cmd died with error exit code ".($? >> 8) :
		$? != 9 ? "$cmd died due to fatal signa, status $?" :
		$timedout ? "$cmd timed out (${fetchtimeout}s)" :
		"$cmd died due to unexpected SIGKILL";
	    if (length $fetcherr) {
		$fetchfail .= "\n$fetcherr";
		$fetchfail =~ s/\n$//;
		$fetchfail =~ s{\n}{ // }g;
	    }
	    if ($fetch >= 2) {
		gitfail $fetchfail;
	    } else {
		servinfo "fetch/clone failed: $fetchfail";
	    }
	}

	if (!$exists) {
	    rename $tmpd, $gitd or fail "rename fresh $tmpd to $gitd: $!";
	    $exists = 1;
	}
    } else {
	$fetchfail = 'not attempted';
    }

    if (!$exists) {
	gitfail "no cached data, and not cloned: $fetchfail";
    }

    servinfo "sharing";
    lockfile \*LOCK, $lock, LOCK_SH; # NB releases and relocks

    if (stat $gitd) {
	return 1;
    }
    $!==ENOENT or fail "stat $gitd: $!";

    # Well, err, someone must have taken the lock in between
    # and garbage collected it.  How annoying.
    return 0;
}

sub hkfail ($) { my ($msg) = @_; fail "housekeeping: $msg"; }

sub housekeeping () {
    logm 'info', "housekeeping started";
    foreach $lock (<[a-z]*\\.lock>) {
	my $subdir = $lock;  $subdir =~ s/\\.lock$//;
	if (!lstat $lock) {
	    $! == ENOENT or hkfail "$lock: lstat: $!";
	    next;
	}
	if (-M _ <= $treeexpiredays) {
	    logm 'debug', "housekeeping: subdirs $subdir: touched recently";
	    next;
	}
	if (!lockfile \*LOCK, $lock, LOCK_EX|LOCK_NB) {
	    logm 'info', "housekeeping: subdirs $subdir: lock busy, skipping";
	    next;
	}
	logm 'info', "housekeeping: subdirs $subdir: cleaning";
	my $ok = 1;
	foreach my $suffix (qw(tmp git)) {
	    my $dir = "${subdir}\\.$suffix";
	    my $errs;
	    remove_tree($dir, { safe=>1, error=>\$errs });
	    if (stat $dir) {
		$ok = 0;
		logm 'warning', "housekeeping: $dir: problems with".
		    "deletion prevent cleanup:";
		foreach my $err (@$errs) {
		    logm 'info', "problem deleting: $err->[0]: $err->[1]";
		}
	    }
	}
	if ($ok) {
	    unlink $lock or hkfail "remove $lock: $!";
	}
    }
    open HS, ">", "Housekeeping.stamp" or hkfail "touch Housekeeping.stamp: $!";
    close HS or hkfail "close Housekeeping.stamp: $!";
    logm 'info', "housekeeping finished";
}

sub housekeepingcheck ($$) {
    my ($dofork, $force) = @_;
    if (!$force) {
	if (!lockfile \*HLOCK, "Housekeeping.lock", LOCK_EX|LOCK_NB) {
	    logm 'debug', "housekeeping lock taken, not running";
	    close HLOCK;
	    return 0;
	}
    }
    if ($force) {
	logm 'info', "housekeeping forced";
    } elsif (!lstat "Housekeeping.stamp") {
	$! == ENOENT or fail "lstat Housekeeping.stamp: $!";
	logm 'info', "housekeeping not done yet, will run";
    } elsif (-M _ <= $housekeepingeverydays) {
	logm 'debug', "housekeeping done recently";
	close HLOCK;
	return 0;
    }
    if ($dofork) {
	my $child = fork;
	defined $child or hkfail "fork: $!";
	if (!$child) {
	    housekeeping();
	    exit 0;
	}
    } else {
	housekeeping();
    }
    close HLOCK;
    return 1;
}

sub runcommand () {
    servinfo "serving";

    chdir $gitd or fail "chdir $gitd: $!";

    exec qw(git-upload-pack --strict --timeout=1000 .)
	or fail "exec git-upload-pack: $!";
}

sub daemonservice () {
    readcommand();
    while (!clonefetch()) { }
    housekeepingcheck(1,0);
    runcommand();
}

if ($housekeepingonly) {
    housekeepingcheck(0, $housekeepingonly>=2);
} else {
    daemonservice();
}