source: SaveSpace.git/hooks/fsmonitor-watchman.sample

#!/usr/bin/perl

use strict;
use warnings;
use IPC::Open2;

# An example hook script to integrate Watchman
# (https://facebook.github.io/watchman/) with git to speed up detecting
# new and modified files.
#
# The hook is passed a version (currently 2) and last update token
# formatted as a string and outputs to stdout a new update token and
# all files that have been modified since the update token. Paths must
# be relative to the root of the working tree and separated by a single NUL.
#
# To enable this hook, rename this file to "query-watchman" and set
# 'git config core.fsmonitor .git/hooks/query-watchman'
#
my ($version, $last_update_token) = @ARGV;

# Uncomment for debugging
# print STDERR "$0 $version $last_update_token\n";

# Check the hook interface version
if ($version ne 2) {
        die "Unsupported query-fsmonitor hook version '$version'.\n" .
            "Falling back to scanning...\n";
}

my $git_work_tree = get_working_dir();

my $retry = 1;

my $json_pkg;
eval {
        require JSON::XS;
        $json_pkg = "JSON::XS";
        1;
} or do {
        require JSON::PP;
        $json_pkg = "JSON::PP";
};

launch_watchman();

sub launch_watchman {
        my $o = watchman_query();
        if (is_work_tree_watched($o)) {
                output_result($o->{clock}, @{$o->{files}});
        }
}

sub output_result {
        my ($clockid, @files) = @_;

        # Uncomment for debugging watchman output
        # open (my $fh, ">", ".git/watchman-output.out");
        # binmode $fh, ":utf8";
        # print $fh "$clockid\n@files\n";
        # close $fh;

        binmode STDOUT, ":utf8";
        print $clockid;
        print "\0";
        local $, = "\0";
        print @files;
}

sub watchman_clock {
        my $response = qx/watchman clock "$git_work_tree"/;
        die "Failed to get clock id on '$git_work_tree'.\n" .
                "Falling back to scanning...\n" if $? != 0;

        return $json_pkg->new->utf8->decode($response);
}

sub watchman_query {
        my $pid = open2(\*CHLD_OUT, \*CHLD_IN, 'watchman -j --no-pretty')
        or die "open2() failed: $!\n" .
        "Falling back to scanning...\n";

        # In the query expression below we're asking for names of files that
        # changed since $last_update_token but not from the .git folder.
        #
        # To accomplish this, we're using the "since" generator to use the
        # recency index to select candidate nodes and "fields" to limit the
        # output to file names only. Then we're using the "expression" term to
        # further constrain the results.
        if (substr($last_update_token, 0, 1) eq "c") {
                $last_update_token = "\"$last_update_token\"";
        }
        my $query = <<" END";
                ["query", "$git_work_tree", {
                        "since": $last_update_token,
                        "fields": ["name"],
                        "expression": ["not", ["dirname", ".git"]]
                }]
        END

        # Uncomment for debugging the watchman query
        # open (my $fh, ">", ".git/watchman-query.json");
        # print $fh $query;
        # close $fh;

        print CHLD_IN $query;
        close CHLD_IN;
        my $response = do {local $/; <CHLD_OUT>};

        # Uncomment for debugging the watch response
        # open ($fh, ">", ".git/watchman-response.json");
        # print $fh $response;
        # close $fh;

        die "Watchman: command returned no output.\n" .
        "Falling back to scanning...\n" if $response eq "";
        die "Watchman: command returned invalid output: $response\n" .
        "Falling back to scanning...\n" unless $response =~ /^\{/;

        return $json_pkg->new->utf8->decode($response);
}

sub is_work_tree_watched {
        my ($output) = @_;
        my $error = $output->{error};
        if ($retry > 0 and $error and $error =~ m/unable to resolve root .* directory (.*) is not watched/) {
                $retry--;
                my $response = qx/watchman watch "$git_work_tree"/;
                die "Failed to make watchman watch '$git_work_tree'.\n" .
                    "Falling back to scanning...\n" if $? != 0;
                $output = $json_pkg->new->utf8->decode($response);
                $error = $output->{error};
                die "Watchman: $error.\n" .
                "Falling back to scanning...\n" if $error;

                # Uncomment for debugging watchman output
                # open (my $fh, ">", ".git/watchman-output.out");
                # close $fh;

                # Watchman will always return all files on the first query so
                # return the fast "everything is dirty" flag to git and do the
                # Watchman query just to get it over with now so we won't pay
                # the cost in git to look up each individual file.
                my $o = watchman_clock();
                $error = $output->{error};

                die "Watchman: $error.\n" .
                "Falling back to scanning...\n" if $error;

                output_result($o->{clock}, ("/"));
                $last_update_token = $o->{clock};

                eval { launch_watchman() };
                return 0;
        }

        die "Watchman: $error.\n" .
        "Falling back to scanning...\n" if $error;

        return 1;
}

sub get_working_dir {
        my $working_dir;
        if ($^O =~ 'msys' || $^O =~ 'cygwin') {
                $working_dir = Win32::GetCwd();
                $working_dir =~ tr/\\/\//;
        } else {
                require Cwd;
                $working_dir = Cwd::cwd();
        }

        return $working_dir;
}