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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;
+}