Add a sample ref-updated hook to repack geometrically

Make a simple "least effort" attempt to run geometric repacking after
every known update which may have written git objects, all while
avoiding overloading a server with too much repacking work.

Change-Id: I7b67bedb47e3e27923932c2a959bda32c930a84a
Release-Notes: Added sample ref-updated hook to repack repositories geometrically

contrib/hooks/ref-updated_repack-geometric.sh

diff --git a/contrib/hooks/ref-updated_repack-geometric.sh b/contrib/hooks/ref-updated_repack-geometric.sh
new file mode 100755
index 0000000..0fe640d
--- /dev/null
+++ b/contrib/hooks/ref-updated_repack-geometric.sh
@@ -0,0 +1,117 @@
+#!/bin/bash -e
+#
+# SPDX-FileCopyrightText: Copyright (c) 2024 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+# SPDX-License-Identifier: Apache-2.0
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+#
+# Best run from the Gerrit ref-updated hook
+#
+
+# Make a simple "least effort" attempt to run geometric repacking after every
+# known update which may have written git objects, all while avoiding overloading
+# a server with too much repacking work.
+
+# The least effort avoids running more than one git repack on the same repo at a
+# time, or while a git gc is already running on a repo (by using .git/gc.pid as
+# a lock). To avoid overloading the server, it also avoids running more than 3
+# git repacks total across all repos. If any of these conditions would be violated,
+# this script simply does nothing and exits. The intention is to avoid doing too
+# much work during a burst, assuming that future updates will likely be good enough
+# to service the repos which were missed.
+#
+# Since this is an event based approach to repository maintenance, it is
+# recommended that another time based GC approach, perhaps a more significant and
+# costly one, repacking refs, creating bitmaps... be used in parallel with this
+# script. This simple policy of "least effort" should keep most repos from
+# degrading much even with very infrequent time based GCs.
+#
+# Since this script uses gc.pid to lock the repo against other git gcs, it means
+# that this script could potentially starve any time based gc maintenance from
+# happening on busy repos. It is therefore advisable for any such time based gc
+# jobs to spin for a while attempting to run if the job cannot acquire the gc.pid
+# lock to help ensure that time based gc also gets a chance to run.
+#
+# In order to be able to skip repacking for each update happening during repacking,
+# this script returns immediately after starting repacking in the background. If
+# this script were to instead block during repacking, it would simply delay
+# repacking for those updates instead of having a consolidating effect. That being
+# said, a smarter script might consider tracking that some updates happened after
+# repacking started and ensure that it gets repacked once again (while still
+# consolidating many updates), but that would likely no longer qualify as least
+# effort.
+#
+
+[ -z "$GERRIT_SITE" ] && { echo "ERROR: GERRIT_SITE not set" ; exit 1 ; }
+[ -z "$GIT_DIR" ] && { echo "ERROR: GIT_DIR not set" ; exit 2 ; }
+
+# ---- Generic ----
+
+debug() { true || echo "---- debug: $@" ; }
+
+cleanup() { [ -n "$GC_LOCK" ] && rm -- "$GCLOCK" ; }
+
+exec_locked() { # <lock> <cmd> [<args>...]
+    local lock=$1 rtn=0
+    shift
+    if ( set -o noclobber ; echo $$ > "$lock" ) > /dev/null 2>&1 ; then
+        GC_LOCK=$lock
+        debug "locked $lock"
+        "$@" || rtn=$?
+        rm -- "$lock" && unset GC_LOCK
+        debug "unlocked $lock"
+        return $rtn
+    fi
+    debug "already locked $lock"
+    return 20
+}
+
+exec_acquired() { # <lock> <max> <cmd> [<args>...]
+    local semaphore=$1 max=$2 rtn=0 slot lock
+    shift 2
+    mkdir -p -- "$semaphore"
+    for slot in $(seq "$max") ; do
+        lock="$semaphore/$slot"
+        touch -- "$lock"
+        exec 3<> "$lock"
+        if flock -n 3 ; then
+            debug "acquired semaphore $slot"
+            "$@" || rtn=$?
+            flock -o 3
+            debug "released semaphore $slot"
+            return $rtn
+        fi
+    done
+    debug "semaphore loaded $semaphore"
+    return 30
+}
+
+# ---- Policy ----
+
+gc_lock() { # <cmd> [<args>...]
+    exec_locked "$LOCK" "$@"
+}
+
+gc_runner() { # <cmd> [<args>...]
+    exec_acquired "$SEMAPHORE" "$MAX_RUNNERS" "$@"
+}
+
+trap cleanup EXIT
+
+MAX_RUNNERS=3
+SEMAPHORE=$GERRIT_SITE/logs/git-geometric.semaphore
+LOCK=$GIT_DIR/gc.pid
+
+gc_runner gc_lock git repack -n -d --no-write-bitmap-index --geometric=2 &
+