stockholm/modules/tv/git/default.nix

{ config, lib, pkgs, ... }:

let
  inherit (builtins)
    attrNames attrValues concatLists getAttr filter hasAttr head lessThan
    removeAttrs tail toJSON typeOf;
  inherit (lib)
    concatMapStringsSep concatStringsSep escapeShellArg hasPrefix
    literalExample makeSearchPath mapAttrsToList mkIf mkOption optionalString
    removePrefix singleton sort types unique;
  inherit (pkgs) linkFarm writeScript writeText;


  ensureList = x:
    if typeOf x == "list" then x else [x];

  getName = x: x.name;

  isPublicRepo = getAttr "public"; # TODO this is also in ./cgit.nix

  makeAuthorizedKey = git-ssh-command: user@{ name, pubkey }:
    # TODO assert name
    # TODO assert pubkey
    let
      options = concatStringsSep "," [
        ''command="exec ${git-ssh-command} ${name}"''
        "no-agent-forwarding"
        "no-port-forwarding"
        "no-pty"
        "no-X11-forwarding"
      ];
    in
    "${options} ${pubkey}";

  # [case-pattern] -> shell-script
  # Create a shell script that succeeds (exit 0) when all its arguments
  # match the case patterns (in the given order).
  makeAuthorizeScript =
    let
      # TODO escape
      to-pattern = x: concatStringsSep "|" (ensureList x);
      go = i: ps:
        if ps == []
          then "exit 0"
          else ''
            case ''$${toString i} in ${to-pattern (head ps)})
            ${go (i + 1) (tail ps)}
            esac'';
    in
    patterns: ''
      #! /bin/sh
      set -euf
      ${concatStringsSep "\n" (map (go 1) patterns)}
      exit -1
    '';

  reponames = rules: sort lessThan (unique (map (x: x.repo.name) rules));

  # TODO makeGitHooks that uses runCommand instead of scriptFarm?
  scriptFarm =
    farm-name: scripts:
    let
      makeScript = script-name: script-string: {
        name = script-name;
        path = writeScript "${farm-name}_${script-name}" script-string;
      };
    in
    linkFarm farm-name (mapAttrsToList makeScript scripts);

  writeJSON = name: data: writeText name (toJSON data);


  cfg = config.services.git;
in

# TODO unify logging of shell scripts to user and journal
# TODO move all scripts to ${etcDir}, so ControlMaster connections
#       immediately pick up new authenticators
# TODO when authorized_keys changes, then restart ssh
#       (or kill already connected users somehow)

{
  imports = [
    ./cgit.nix
  ];

  options.services.git = {
    enable = mkOption {
      type = types.bool;
      default = false;
      description = "Enable Git repository hosting.";
    };
    cgit = mkOption {
      type = types.bool;
      default = true;
      description = "Enable cgit."; # TODO better desc; talk about nginx
    };
    dataDir = mkOption {
      type = types.str;
      default = "/var/lib/git";
      description = "Directory used to store repositories.";
    };
    etcDir = mkOption {
      type = types.str;
      default = "/etc/git";
    };
    rules = mkOption {
      type = types.unspecified;
    };
    repos = mkOption {
      type = types.attrsOf (types.submodule ({
        options = {
          desc = mkOption {
            type = types.nullOr types.str;
            default = null;
            description = ''
              Repository description.
            '';
          };
          section = mkOption {
            type = types.nullOr types.str;
            default = null;
            description = ''
              Repository section.
            '';
          };
          name = mkOption {
            type = types.str;
            description = ''
              Repository name.
            '';
          };
          hooks = mkOption {
            type = types.attrsOf types.str;
            description = ''
              Repository-specific hooks.
            '';
          };
          public = mkOption {
            type = types.bool;
            default = false;
            description = ''
              Allow everybody to read the repository via HTTP if cgit enabled.
            '';
            # TODO allow every configured user to fetch the repository via SSH.
          };
        };
      }));

      default = {};

      example = literalExample ''
        {
          testing = {
            name = "testing";
            hooks.post-update = '''
              #! /bin/sh
              set -euf
              echo post-update hook: $* >&2
            ''';
          };
          testing2 = { name = "testing2"; };
        }
      '';

      description = ''
        Repositories.
      '';
    };
    users = mkOption {
      type = types.unspecified;
    };
  };

  config =
    let
      git-ssh-command = writeScript "git-ssh-command" ''
        #! /bin/sh
        set -euf

        PATH=${makeSearchPath "bin" (with pkgs; [
          coreutils
          git
          gnugrep
          gnused
          systemd
        ])}

        abort() {
          echo "error: $1" >&2
          systemd-cat -p err -t git echo "error: $1"
          exit -1
        }

        GIT_SSH_USER=$1

        systemd-cat -p info -t git echo \
          "authorizing $GIT_SSH_USER $SSH_CONNECTION $SSH_ORIGINAL_COMMAND"

        # References: The Base Definitions volume of
        # POSIX.1‐2013, Section 3.278, Portable Filename Character Set
        portable_filename_bre="^[A-Za-z0-9._-]\\+$"

        command=$(echo "$SSH_ORIGINAL_COMMAND" \
          | sed -n 's/^\([^ ]*\) '"'"'\(.*\)'"'"'/\1/p' \
          | grep "$portable_filename_bre" \
          || abort 'cannot read command')

        GIT_SSH_REPO=$(echo "$SSH_ORIGINAL_COMMAND" \
          | sed -n 's/^\([^ ]*\) '"'"'\(.*\)'"'"'/\2/p' \
          | grep "$portable_filename_bre" \
          || abort 'cannot read reponame')

        ${cfg.etcDir}/authorize-command \
            "$GIT_SSH_USER" "$GIT_SSH_REPO" "$command" \
          || abort 'access denied'

        repodir=${escapeShellArg cfg.dataDir}/$GIT_SSH_REPO

        systemd-cat -p info -t git \
          echo "authorized exec $command $repodir"

        export GIT_SSH_USER
        export GIT_SSH_REPO
        exec "$command" "$repodir"
      '';

      init-script = writeScript "git-init" ''
        #! /bin/sh
        set -euf

        PATH=${makeSearchPath "bin" (with pkgs; [
          coreutils
          findutils
          gawk
          git
          gnugrep
          gnused
        ])}

        dataDir=${escapeShellArg cfg.dataDir}
        mkdir -p "$dataDir"

        # Notice how the presence of hooks symlinks determine whether
        # we manage a repositry or not.

        # Make sure that no existing repository has hooks.  We can delete
        # symlinks because we assume we created them.
        find "$dataDir" -mindepth 2 -maxdepth 2 -name hooks -type l -delete
        bad_hooks=$(find "$dataDir" -mindepth 2 -maxdepth 2 -name hooks)
        if echo "$bad_hooks" | grep -q .; then
          printf 'error: unknown hooks:\n%s\n' \
            "$(echo "$bad_hooks" | sed 's/^/  /')" \
            >&2
          exit -1
        fi

        # Initialize repositories.
        ${concatMapStringsSep "\n" (repo:
          let
            hooks = scriptFarm "git-hooks" (makeHooks repo);
          in
          ''
            reponame=${escapeShellArg repo.name}
            repodir=$dataDir/$reponame
            mode=${toString (if isPublicRepo repo then 0711 else 0700)}
            if ! test -d "$repodir"; then
              mkdir -m "$mode" "$repodir"
              git init --bare --template=/var/empty "$repodir"
              chown -R git:nogroup "$repodir"
            fi
            ln -s ${hooks} "$repodir/hooks"
          ''
        ) (attrValues cfg.repos)}

        # Warn about repositories that exist but aren't mentioned in the
        # current configuration (and thus didn't receive a hooks symlink).
        unknown_repos=$(find "$dataDir" -mindepth 1 -maxdepth 1 \
          -type d \! -exec test -e '{}/hooks' \; -print)
        if echo "$unknown_repos" | grep -q .; then
          printf 'warning: stale repositories:\n%s\n' \
            "$(echo "$unknown_repos" | sed 's/^/  /')" \
            >&2
        fi
      '';

      makeHooks = repo: removeAttrs repo.hooks [ "pre-receive" ] // {
        pre-receive = ''
          #! /bin/sh
          set -euf

          PATH=${makeSearchPath "bin" (with pkgs; [
            coreutils # env
            git
            systemd
          ])}

          accept() {
            #systemd-cat -p info -t git echo "authorized $1"
            accept_string="''${accept_string+$accept_string
          }authorized $1"
          }
          reject() {
            #systemd-cat -p err -t git echo "denied $1"
            #echo 'access denied' >&2
            #exit_code=-1
            reject_string="''${reject_string+$reject_string
          }access denied: $1"
          }

          empty=0000000000000000000000000000000000000000

          accept_string=
          reject_string=
          while read oldrev newrev ref; do

            if [ $oldrev = $empty ]; then
              receive_mode=create
            elif [ $newrev = $empty ]; then
              receive_mode=delete
            elif [ "$(git merge-base $oldrev $newrev)" = $oldrev ]; then
              receive_mode=fast-forward
            else
              receive_mode=non-fast-forward
            fi

            if ${cfg.etcDir}/authorize-push \
                "$GIT_SSH_USER" "$GIT_SSH_REPO" "$ref" "$receive_mode"; then
              accept "$receive_mode $ref"
            else
              reject "$receive_mode $ref"
            fi
          done

          if [ -n "$reject_string" ]; then
            systemd-cat -p err -t git echo "$reject_string"
            exit -1
          fi

          systemd-cat -p info -t git echo "$accept_string"

          ${optionalString (hasAttr "post-receive" repo.hooks) ''
            # custom post-receive hook
            ${repo.hooks.post-receive}''}
        '';
      };

      etc-base =
        assert (hasPrefix "/etc/" cfg.etcDir);
        removePrefix "/etc/" cfg.etcDir;
    in
    mkIf cfg.enable {
      system.activationScripts.git-init = "${init-script}";

      # TODO maybe put all scripts here and then use PATH?
      environment.etc."${etc-base}".source =
        scriptFarm "git-ssh-authorizers" {
          authorize-command = makeAuthorizeScript (map ({ repo, user, perm }: [
            (map getName (ensureList user))
            (map getName (ensureList repo))
            (map getName perm.allow-commands)
          ]) cfg.rules);

          authorize-push = makeAuthorizeScript (map ({ repo, user, perm }: [
            (map getName (ensureList user))
            (map getName (ensureList repo))
            (ensureList perm.allow-receive-ref)
            (map getName perm.allow-receive-modes)
          ]) (filter (x: hasAttr "allow-receive-ref" x.perm) cfg.rules));
        };

      users.extraUsers = singleton {
        description = "Git repository hosting user";
        name = "git";
        shell = "/bin/sh";
        openssh.authorizedKeys.keys =
          mapAttrsToList (_: makeAuthorizedKey git-ssh-command) cfg.users;
        uid = 112606723; # genid git
      };
    };
}