fsmonitor: add a sample query-fsmonitor hook script for Watchman

This hook script integrates the new fsmonitor capabilities of git with
the cross platform Watchman file watching service. To use the script:

Download and install Watchman from https://facebook.github.io/watchman/
and instruct Watchman to watch your working directory for changes
('watchman watch-project /usr/src/git').

Rename the sample integration hook from query-fsmonitor.sample to
query-fsmonitor.

Configure git to use the extension ('git config core.fsmonitor true')
and optionally turn on the untracked cache for optimal performance
('git config core.untrackedcache true').

Signed-off-by: Ben Peart <[email protected]>
Signed-off-by: Johannes Schindelin <[email protected]>
Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: benpeart

templates/hooks--query-fsmonitor.sample

@@ -0,0 +1,60 @@
+#!/bin/sh
+#
+# An example hook script to integrate Watchman
+# (https://facebook.github.io/watchman/) with git to provide fast
+# git status.
+#
+# 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-fsmonitor"
+
+# check the hook interface version
+if [ $1 -eq 1 ]
+then
+	# convert nanoseconds to seconds
+	time_t=$(($2/1000000000))
+else
+	echo -e "Unsupported query-fsmonitor hook version.\nFalling back to scanning...\n" >&2
+	exit 1;
+fi
+
+# Convert unix style paths to escaped Windows style paths
+case "$(uname -s)" in
+MINGW*|MSYS_NT*)
+  GIT_WORK_TREE="$(cygpath -aw "$PWD" | sed 's,\\,\\\\,g')"
+  ;;
+*)
+  GIT_WORK_TREE="$PWD"
+  ;;
+esac
+
+# Query Watchman for all the changes since the requested time
+echo "[\"query\", \"$GIT_WORK_TREE\", {\"since\": $time_t, \"fields\":[\"name\"]}]" | \
+	watchman -j |
+	perl -0666 -e '
+		use strict;
+		use warnings;
+
+		my $stdin = <>;
+		die "Watchman: command returned no output.\nFalling back to scanning...\n" if $stdin eq "";
+		die "Watchman: command returned invalid output: $stdin\nFalling back to scanning...\n" unless $stdin =~ /^\{/;
+
+		my $json_pkg;
+		eval {
+			require JSON::XS;
+			$json_pkg = "JSON::XS";
+			1;
+		} or do {
+			require JSON::PP;
+			$json_pkg = "JSON::PP";
+		};
+
+		my $o = $json_pkg->new->utf8->decode($stdin);
+		die "Watchman: $o->{error}.\nFalling back to scanning...\n" if $o->{error};
+
+		local $, = "\0";
+		print @{$o->{files}};
+	'