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Filename : fsmonitor-watchman.sample
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#!/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.
	my $last_update_line = "";
	if (substr($last_update_token, 0, 1) eq "c") {
		$last_update_token = "\"$last_update_token\"";
		$last_update_line = qq[\n"since": $last_update_token,];
	}
	my $query = <<"	END";
		["query", "$git_work_tree", {$last_update_line
			"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;
}