# Logic for generating Buildkite pipelines from Nix build targets read
# by //nix/readTree.
#
# It outputs a "YAML" (actually JSON) file which is evaluated and
# submitted to Buildkite at the start of each build.
#
# The structure of the file that is being created is documented here:
#   https://buildkite.com/docs/pipelines/defining-steps
{ depot, pkgs, ... }:

let
  inherit (builtins)
    attrValues
    concatLists
    concatStringsSep
    elem
    foldl'
    hasAttr
    hashString
    isNull
    isString
    length
    listToAttrs
    mapAttrs
    toJSON
    unsafeDiscardStringContext;

  inherit (pkgs) lib runCommandNoCC writeText;
  inherit (depot.nix.readTree) mkLabel;
in
rec {
  # Creates a Nix expression that yields the target at the specified
  # location in the repository.
  #
  # This makes a distinction between normal targets (which physically
  # exist in the repository) and subtargets (which are "virtual"
  # targets exposed by a physical one) to make it clear in the build
  # output which is which.
  mkBuildExpr = target:
    let
      descend = expr: attr: "builtins.getAttr \"${attr}\" (${expr})";
      targetExpr = foldl' descend "import ./. {}" target.__readTree;
      subtargetExpr = descend targetExpr target.__subtarget;
    in
    if target ? __subtarget then subtargetExpr else targetExpr;

  # Determine whether to skip a target if it has not diverged from the
  # HEAD branch.
  shouldSkip = parentTargetMap: label: drvPath:
    if (hasAttr label parentTargetMap) && parentTargetMap."${label}".drvPath == drvPath
    then "Target has not changed."
    else false;

  # Create build command for a derivation target.
  mkBuildCommand = target: drvPath: concatStringsSep " " [
    # First try to realise the drvPath of the target so we don't evaluate twice.
    # Nix has no concept of depending on a derivation file without depending on
    # at least one of its `outPath`s, so we need to discard the string context
    # if we don't want to build everything during pipeline construction.
    "(nix-store --realise '${drvPath}' --add-root result --indirect && readlink result)"

    # Since we don't gcroot the derivation files, they may be deleted by the
    # garbage collector. In that case we can reevaluate and build the attribute
    # using nix-build.
    "|| (test ! -f '${drvPath}' && nix-build -E '${mkBuildExpr target}' --show-trace)"
  ];

  # Create a pipeline step from a single target.
  mkStep = headBranch: parentTargetMap: target:
    let
      label = mkLabel target;
      drvPath = unsafeDiscardStringContext target.drvPath;
      shouldSkip' = shouldSkip parentTargetMap;
    in
    {
      label = ":nix: " + label;
      key = hashString "sha1" label;
      skip = shouldSkip' label drvPath;
      command = mkBuildCommand target drvPath;
      env.READTREE_TARGET = label;

      # Add a dependency on the initial static pipeline step which
      # always runs. This allows build steps uploaded in batches to
      # start running before all batches have been uploaded.
      depends_on = ":init:";
    };

  # Helper function to inelegantly divide a list into chunks of at
  # most n elements.
  #
  # This works by assigning each element a chunk ID based on its
  # index, and then grouping all elements by their chunk ID.
  chunksOf = n: list:
    let
      chunkId = idx: toString (idx / n + 1);
      assigned = lib.imap1 (idx: value: { inherit value; chunk = chunkId idx; }) list;
      unchunk = mapAttrs (_: elements: map (e: e.value) elements);
    in
    unchunk (lib.groupBy (e: e.chunk) assigned);

  # Define a build pipeline chunk as a JSON file, using the pipeline
  # format documented on
  # https://buildkite.com/docs/pipelines/defining-steps.
  makePipelineChunk = name: chunkId: chunk: rec {
    filename = "${name}-chunk-${chunkId}.json";
    path = writeText filename (toJSON {
      steps = chunk;
    });
  };

  # Split the pipeline into chunks of at most 192 steps at once, which
  # are uploaded sequentially. This is because of a limitation in the
  # Buildkite backend which struggles to process more than a specific
  # number of chunks at once.
  pipelineChunks = name: steps:
    attrValues (mapAttrs (makePipelineChunk name) (chunksOf 192 steps));

  # Create a pipeline structure for the given targets.
  mkPipeline =
    {
      # HEAD branch of the repository on which release steps, GC
      # anchoring and other "mainline only" steps should run.
      headBranch
    , # List of derivations as read by readTree (in most cases just the
      # output of readTree.gather) that should be built in Buildkite.
      #
      # These are scheduled as the first build steps and run as fast as
      # possible, in order, without any concurrency restrictions.
      drvTargets
    , # Derivation map of a parent commit. Only targets which no longer
      # correspond to the content of this map will be built. Passing an
      # empty map will always build all targets.
      parentTargetMap ? { }
    , # A list of plain Buildkite step structures to run alongside the
      # build for all drvTargets, but before proceeding with any
      # post-build actions such as status reporting.
      #
      # Can be used for things like code formatting checks.
      additionalSteps ? [ ]
    , # A list of plain Buildkite step structures to run after all
      # previous steps succeeded.
      #
      # Can be used for status reporting steps and the like.
      postBuildSteps ? [ ]
    }:
    let
      # List of phases to include. Currently the only phases are 'build'
      # (Nix builds and extra steps that are not post-build steps) and
      # 'post' (all post-build steps).
      #
      # TODO(tazjin): Configurable set of phases.
      phases = [ "build" "release" ];

      # Convert a target into all of its steps, separated by build
      # phase (as phases end up in different chunks).
      targetToSteps = target:
        let
          step = mkStep headBranch parentTargetMap target;

          # Same step, but with an override function applied. This is
          # used in mkExtraStep if the extra step needs to modify the
          # parent derivation somehow.
          #
          # Note that this will never affect the label.
          overridable = f: mkStep headBranch parentTargetMap (f target);

          # Split extra steps by phase.
          splitExtraSteps = lib.groupBy ({ phase, ... }: phase)
            (attrValues (mapAttrs (normaliseExtraStep phases overridable)
              (target.meta.ci.extraSteps or { })));

          extraSteps = mapAttrs (_: steps: map mkExtraStep steps) splitExtraSteps;
        in
        extraSteps // {
          build = [ step ] ++ (extraSteps.build or [ ]);
        };

      # Combine all target steps into step lists per phase.
      #
      # TODO(tazjin): Refactor when configurable phases show up.
      globalSteps = {
        build = additionalSteps;
        release = postBuildSteps;
      };

      phasesWithSteps = lib.zipAttrsWithNames phases (_: concatLists)
        ((map targetToSteps drvTargets) ++ [ globalSteps ]);

      # Generate pipeline chunks for each phase.
      chunks = foldl'
        (acc: phase:
          let phaseSteps = phasesWithSteps.${phase} or [ ]; in
          if phaseSteps == [ ]
          then acc
          else acc ++ (pipelineChunks phase phaseSteps))
        [ ]
        phases;

    in
    runCommandNoCC "buildkite-pipeline" { } ''
      mkdir $out
      echo "Generated ${toString (length chunks)} pipeline chunks"
      ${
        lib.concatMapStringsSep "\n"
          (chunk: "cp ${chunk.path} $out/${chunk.filename}") chunks
      }
    '';

  # Create a drvmap structure for the given targets, containing the
  # mapping of all target paths to their derivations. The mapping can
  # be persisted for future use.
  mkDrvmap = drvTargets: writeText "drvmap.json" (toJSON (listToAttrs (map
    (target: {
      name = mkLabel target;
      value = {
        drvPath = unsafeDiscardStringContext target.drvPath;

        # Include the attrPath in the output to reconstruct the drv
        # without parsing the human-readable label.
        attrPath = target.__readTree ++ lib.optionals (target ? __subtarget) [
          target.__subtarget
        ];
      };
    })
    drvTargets)));

  # Implementation of extra step logic.
  #
  # Each target extra step is an attribute specified in
  # `meta.ci.extraSteps`. Its attribute name will be used as the step
  # name on Buildkite.
  #
  #   command (required): A command that will be run in the depot
  #     checkout when this step is executed. Should be a derivation
  #     resulting in a single executable file, e.g. through
  #     pkgs.writeShellScript.
  #
  #   label (optional): Human-readable label for this step to display
  #     in the Buildkite UI instead of the attribute name.
  #
  #   prompt (optional): Setting this blocks the step until confirmed
  #     by a human. Should be a string which is displayed for
  #     confirmation. These steps always run after the main build is
  #     done and have no influence on CI status.
  #
  #   postBuild (optional): If set to true, this step will run after
  #     all primary build steps (that is, after status has been reported
  #     back to CI).
  #
  #   needsOutput (optional): If set to true, the parent derivation
  #     will be built in the working directory before running the
  #     command. Output will be available as 'result'.
  #     TODO: Figure out multiple-output derivations.
  #
  #   parentOverride (optional): A function (drv -> drv) to override
  #     the parent's target definition when preparing its output. Only
  #     used in extra steps that use needsOutput.
  #
  #   branches (optional): Git references (branches, tags ... ) on
  #     which this step should be allowed to run. List of strings.
  #
  #   alwaysRun (optional): If set to true, this step will always run,
  #     even if its parent has not been rebuilt.
  #
  # Note that gated steps are independent of each other.

  # Create a gated step in a step group, independent from any other
  # steps.
  mkGatedStep = { step, label, parent, prompt }: {
    inherit (step) depends_on;
    group = label;
    skip = parent.skip or false;

    steps = [
      {
        inherit (step) branches;
        inherit prompt;
        block = ":radio_button: Run ${label}? (from ${parent.env.READTREE_TARGET})";
      }

      # The explicit depends_on of the wrapped step must be removed,
      # otherwise its dependency relationship with the gate step will
      # break.
      (builtins.removeAttrs step [ "depends_on" ])
    ];
  };

  # Validate and normalise extra step configuration before actually
  # generating build steps, in order to use user-provided metadata
  # during the pipeline generation.
  normaliseExtraStep = knownPhases: overridableParent: key:
    { command
    , label ? key
    , needsOutput ? false
    , parentOverride ? (x: x)
    , branches ? null
    , alwaysRun ? false
    , prompt ? false

      # TODO(tazjin): Default to 'build' after 2022-10-01.
    , phase ? if (isNull postBuild || !postBuild) then "build" else "release"

      # TODO(tazjin): Turn into hard-failure after 2022-10-01.
    , postBuild ? null
    }:
    let
      parent = overridableParent parentOverride;
      parentLabel = parent.env.READTREE_TARGET;

      validPhase = lib.throwIfNot (elem phase knownPhases) ''
        In step '${label}' (from ${parentLabel}):

        Phase '${phase}' is not valid.

        Known phases: ${concatStringsSep ", " knownPhases}
      ''
        phase;
    in
    {
      inherit
        alwaysRun
        branches
        command
        key
        label
        needsOutput
        parent
        parentLabel;

      # //nix/buildkite is growing a new feature for adding different
      # "build phases" which supersedes the previous `postBuild`
      # boolean API.
      #
      # To help users transition, emit warnings if the old API is used.
      phase = lib.warnIfNot (isNull postBuild) ''
        In step '${label}' (from ${parentLabel}):

        Please note: The CI system is introducing support for running
        steps in different build phases.

        The currently supported phases are 'build' (all Nix targets,
        extra steps such as tests that feed into the build results,
        etc.) and 'release' (steps that run after builds and tests
        have already completed).

        This replaces the previous boolean `postBuild` API in extra
        step definitions. Please remove the `postBuild` parameter from
        this step and instead set `phase = ${phase};`.
      ''
        validPhase;

      prompt = lib.throwIf (prompt != false && phase == "build") ''
        In step '${label}' (from ${parentLabel}):

        The 'prompt' feature can only be used by steps in the "release"
        phase, because CI builds should not be gated on manual human
        approvals.
      ''
        prompt;
    };

  # Create the Buildkite configuration for an extra step, optionally
  # wrapping it in a gate group.
  mkExtraStep = cfg:
    let
      step = {
        label = ":gear: ${cfg.label} (from ${cfg.parentLabel})";
        skip = if cfg.alwaysRun then false else cfg.parent.skip or false;
        # TODO(tazjin): Remember to gate this behaviour with active phases.
        depends_on = lib.optional (!cfg.alwaysRun && !cfg.needsOutput) cfg.parent.key;
        branches =
          if cfg.branches != null
          then lib.concatStringsSep " " cfg.branches else null;

        command = pkgs.writeShellScript "${cfg.key}-script" ''
          set -ueo pipefail
          ${lib.optionalString cfg.needsOutput
            "echo '~~~ Preparing build output of ${cfg.parentLabel}'"
          }
          ${lib.optionalString cfg.needsOutput cfg.parent.command}
          echo '+++ Running extra step command'
          exec ${cfg.command}
        '';
      };
    in
    if (isString cfg.prompt)
    then
      mkGatedStep
        {
          inherit step;
          inherit (cfg) label parent prompt;
        }
    else step;
}