feat(gerrit): add git-gc-preserve script

Signed-off-by: Raito Bezarius <masterancpp@gmail.com>

overlays/default.nix

@@ -4,5 +4,6 @@
     u-root = final.callPackage ./u-root {};
     pyroscope = final.callPackage ./pyroscope {};
     s3-revproxy = final.callPackage ./s3-revproxy {};
+    git-gc-preserve = final.callPackage ./git-gc-preserve {};
   })
 ]

overlays/git-gc-preserve/default.nix

@@ -0,0 +1,9 @@
+{ writeShellApplication, git, nettools }:
+
+writeShellApplication {
+  name = "git-gc-preserve";
+
+  runtimeInputs = [ git nettools ];
+
+  text = (builtins.readFile ./script.sh);
+}

overlays/git-gc-preserve/script.sh

@@ -0,0 +1,132 @@
+#!/usr/bin/env bash
+
+set +o errexit
+# Copyright (C) 2022 The Android Open Source Project
+#
+# 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.
+usage() { # exit code
+  cat <<-EOF
+NAME
+    git-gc-preserve - Run git gc and preserve old packs to avoid races for JGit
+SYNOPSIS
+    git gc-preserve
+DESCRIPTION
+    Runs git gc and can preserve old packs to avoid races with concurrently
+    executed commands in JGit.
+    This command uses custom git config options to configure if preserved packs
+    from the last run of git gc should be pruned and if packs should be preserved.
+    This is similar to the implementation in JGit [1] which is used by
+    JGit to avoid errors [2] in such situations.
+    The command prevents concurrent runs of the command on the same repository
+    by acquiring an exclusive file lock on the file
+      "\$repopath/gc-preserve.pid"
+    If it cannot acquire the lock it fails immediately with exit code 3.
+    Failure Exit Codes
+        1: General failure
+        2: Couldn't determine repository path. If the current working directory
+           is outside of the working tree of the git repository use git option
+           --git-dir to pass the root path of the repository.
+           E.g.
+              $ git --git-dir ~/git/foo gc-preserve
+        3: Another process already runs $0 on the same repository
+    [1] https://git.eclipse.org/r/c/jgit/jgit/+/87969
+    [2] https://git.eclipse.org/r/c/jgit/jgit/+/122288
+CONFIGURATION
+    "pack.prunepreserved": if set to "true" preserved packs from the last gc run
+      are pruned before current packs are preserved.
+    "pack.preserveoldpacks": if set to "true" current packs will be hard linked
+      to objects/pack/preserved before git gc is executed. JGit will
+      fallback to the preserved packs in this directory in case it comes
+      across missing objects which might be caused by a concurrent run of
+      git gc.
+EOF
+  exit "$1"
+}
+# acquire file lock, unlock when the script exits
+lock() { # repo
+  readonly LOCKFILE="$1/gc-preserve.pid"
+  test -f "$LOCKFILE" || touch "$LOCKFILE"
+  exec 9> "$LOCKFILE"
+  if flock -nx 9; then
+    echo -n "$$ $USER@$(hostname)" >&9
+    trap unlock EXIT
+  else
+    echo "$0 is already running"
+    exit 3
+  fi
+}
+unlock() {
+  # only delete if the file descriptor 9 is open
+  if { : >&9 ; } &> /dev/null; then
+    rm -f "$LOCKFILE"
+  fi
+  # close the file handle to release file lock
+  exec 9>&-
+}
+# prune preserved packs if pack.prunepreserved == true
+prune_preserved() { # repo
+  configured=$(git --git-dir="$1" config --get pack.prunepreserved)
+  if [ "$configured" != "true" ]; then
+    return 0
+  fi
+  local preserved=$1/objects/pack/preserved
+  if [ -d "$preserved" ]; then
+    printf "Pruning old preserved packs: "
+    count=$(find "$preserved" -name "*.old-pack" | wc -l)
+    rm -rf "$preserved"
+    echo "$count, done."
+  fi
+}
+# preserve packs if pack.preserveoldpacks == true
+preserve_packs() { # repo
+  configured=$(git --git-dir="$1" config --get pack.preserveoldpacks)
+  if [ "$configured" != "true" ]; then
+    return 0
+  fi
+  local packdir=$1/objects/pack
+  pushd "$packdir" >/dev/null || exit 1
+  mkdir -p preserved
+  printf "Preserving packs: "
+  count=0
+  for file in pack-*{.pack,.idx} ; do
+    ln -f "$file" preserved/"$(get_preserved_packfile_name "$file")"
+    if [[ "$file" == pack-*.pack ]]; then
+      ((count++))
+    fi
+  done
+  echo "$count, done."
+  popd >/dev/null || exit 1
+}
+# pack-0...2.pack to pack-0...2.old-pack
+# pack-0...2.idx to pack-0...2.old-idx
+get_preserved_packfile_name() { # packfile > preserved_packfile
+  local old=${1/%\.pack/.old-pack}
+  old=${old/%\.idx/.old-idx}
+  echo "$old"
+}
+# main
+while [ $# -gt 0 ] ; do
+    case "$1" in
+        -u|-h)  usage 0 ;;
+    esac
+    shift
+done
+args=$(git rev-parse --sq-quote "$@")
+repopath=$(git rev-parse --git-dir)
+if [ -z "$repopath" ]; then
+  usage 2
+fi
+lock "$repopath"
+prune_preserved "$repopath"
+preserve_packs "$repopath"
+git gc ${args:+"$args"} || { EXIT_CODE="$?"; echo "git gc failed"; exit "$EXIT_CODE"; }

services/gerrit/default.nix

@@ -47,7 +47,11 @@ in
     networking.firewall.allowedTCPPorts = [ cfg.port ];
     age.secrets.alloy-push-password.file = ../../secrets/metrics-push-password.age;
 
-    environment.systemPackages = [ jdk pkgs.git ];
+    environment.systemPackages = [ jdk
+      pkgs.git
+      # https://gerrit.googlesource.com/gerrit/+/refs/heads/master/contrib/git-gc-preserve
+      pkgs.git-gc-preserve
+    ];
 
     fileSystems."/var/lib/gerrit" = mkIf (cfg.data != "/var/lib/gerrit") {
       device = cfg.data;