--- /dev/null
+#!/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 1) and a time in nanoseconds
+# formatted as a string and outputs to stdout all files that have been
+# modified since the given time. 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, $time) = @ARGV;
+
+# Check the hook interface version
+
+if ($version == 1) {
+ # convert nanoseconds to seconds
+ $time = int $time / 1000000000;
+} else {
+ die "Unsupported query-fsmonitor hook version '$version'.\n" .
+ "Falling back to scanning...\n";
+}
+
+# Convert unix style paths to escaped Windows style paths when running
+# in Windows command prompt
+
+my $system = `uname -s`;
+$system =~ s/[\r\n]+//g;
+my $git_work_tree;
+
+if ($system =~ m/^MSYS_NT/) {
+ $git_work_tree = `cygpath -aw "\$PWD"`;
+ $git_work_tree =~ s/[\r\n]+//g;
+ $git_work_tree =~ s,\\,/,g;
+} else {
+ $git_work_tree = $ENV{'PWD'};
+}
+
+my $retry = 1;
+
+launch_watchman();
+
+sub launch_watchman {
+
+ # Set input record separator
+ local $/ = 0666;
+
+ my $pid = open2(\*CHLD_OUT, \*CHLD_IN, 'watchman -j')
+ 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 $time but were not transient (ie created after
+ # $time but no longer exist).
+ #
+ # 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.
+ #
+ # The category of transient files that we want to ignore will have a
+ # creation clock (cclock) newer than $time_t value and will also not
+ # currently exist.
+
+ my $query = <<" END";
+ ["query", "$git_work_tree", {
+ "since": $time,
+ "fields": ["name"],
+ "expression": ["not", ["allof", ["since", $time, "cclock"], ["not", "exists"]]]
+ }]
+ END
+
+ print CHLD_IN $query;
+ my $response = <CHLD_OUT>;
+
+ 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 =~ /^\{/;
+
+ my $json_pkg;
+ eval {
+ require JSON::XS;
+ $json_pkg = "JSON::XS";
+ 1;
+ } or do {
+ require JSON::PP;
+ $json_pkg = "JSON::PP";
+ };
+
+ my $o = $json_pkg->new->utf8->decode($response);
+
+ if ($retry > 0 and $o->{error} and $o->{error} =~ m/unable to resolve root .* directory (.*) is not watched/) {
+ print STDERR "Adding '$git_work_tree' to watchman's watch list.\n";
+ $retry--;
+ qx/watchman watch "$git_work_tree"/;
+ die "Failed to make watchman watch '$git_work_tree'.\n" .
+ "Falling back to scanning...\n" if $? != 0;
+
+ # 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.
+ print "/\0";
+ eval { launch_watchman() };
+ exit 0;
+ }
+
+ die "Watchman: $o->{error}.\n" .
+ "Falling back to scanning...\n" if $o->{error};
+
+ binmode STDOUT, ":utf8";
+ local $, = "\0";
+ print @{$o->{files}};
+}