# Copyright 2019 Google LLC.
# SPDX-License-Identifier: Apache-2.0
#
# buildGo provides Nix functions to build Go packages in the style of Bazel's
# rules_go.

{ pkgs ? import <nixpkgs> {}
, ... }:

let
  inherit (builtins)
    attrNames
    baseNameOf
    dirOf
    elemAt
    filter
    listToAttrs
    map
    match
    readDir
    replaceStrings
    toString;

  inherit (pkgs) lib go runCommand fetchFromGitHub protobuf symlinkJoin removeReferencesTo;

  # Helpers for low-level Go compiler invocations
  spaceOut = lib.concatStringsSep " ";

  includeDepSrc = dep: "-I ${dep}";
  includeSources = deps: spaceOut (map includeDepSrc deps);

  includeDepLib = dep: "-L ${dep}";
  includeLibs = deps: spaceOut (map includeDepLib deps);

  srcBasename = src: elemAt (match "([a-z0-9]{32}\-)?(.*\.go)" (baseNameOf src)) 1;
  srcCopy = path: src: "cp ${src} $out/${path}/${srcBasename src}";
  srcList = path: srcs: lib.concatStringsSep "\n" (map (srcCopy path) srcs);

  allDeps = deps: lib.unique (lib.flatten (deps ++ (map (d: d.goDeps) deps)));

  xFlags = x_defs: spaceOut (map (k: "-X ${k}=${x_defs."${k}"}") (attrNames x_defs));

  pathToName = p: replaceStrings ["/"] ["_"] (toString p);

  removeRefs = refs: "${removeReferencesTo}/bin/remove-references-to ${lib.concatMapStrings (ref: " -t ${ref}") ([ go ] ++ refs)}";

  # Add an `overrideGo` attribute to a function result that works
  # similar to `overrideAttrs`, but is used specifically for the
  # arguments passed to Go builders.
  makeOverridable = f: orig: (f orig) // {
    overrideGo = new: makeOverridable f (orig // (new orig));
  };

  # High-level build functions

  # Build a Go program out of the specified files and dependencies.
  program = { name, srcs, deps ? [], x_defs ? {} }:
  let uniqueDeps = allDeps (map (d: d.gopkg) deps);
  in runCommand name {} ''
    ${go}/bin/go tool compile -o ${name}.a -trimpath="$PWD;${go}" ${includeSources uniqueDeps} ${spaceOut srcs}
    mkdir -p $out/bin
    export GOROOT_FINAL=go
    ${go}/bin/go tool link -o $out/bin/${name} -buildid nix ${xFlags x_defs} ${includeLibs uniqueDeps} ${name}.a
    ${removeRefs srcs} $out/bin/${name}
  '';

  # Build a Go library assembled out of the specified files.
  #
  # This outputs both the sources and compiled binary, as both are
  # needed when downstream packages depend on it.
  package = { name, srcs, deps ? [], path ? name, sfiles ? [] }:
  let
    uniqueDeps = allDeps (map (d: d.gopkg) deps);

    # The build steps below need to be executed conditionally for Go
    # assembly if the analyser detected any *.s files.
    #
    # This is required for several popular packages (e.g. x/sys).
    ifAsm = do: lib.optionalString (sfiles != []) do;
    asmBuild = ifAsm ''
      ${go}/bin/go tool asm -trimpath $PWD -I $PWD -I ${go}/share/go/pkg/include -D GOOS_linux -D GOARCH_amd64 -gensymabis -o ./symabis ${spaceOut sfiles}
      ${go}/bin/go tool asm -trimpath $PWD -I $PWD -I ${go}/share/go/pkg/include -D GOOS_linux -D GOARCH_amd64 -o ./asm.o ${spaceOut sfiles}
    '';
    asmLink = ifAsm "-symabis ./symabis -asmhdr $out/go_asm.h";
    asmPack = ifAsm ''
      ${go}/bin/go tool pack r $out/${path}.a ./asm.o
    '';

    gopkg = (runCommand "golib-${name}" {} ''
      mkdir -p $out/${path}
      ${srcList path (map (s: "${s}") srcs)}
      ${asmBuild}
      ${go}/bin/go tool compile -pack ${asmLink} -o $out/${path}.a -trimpath="$PWD;${go}" -p ${path} ${includeSources uniqueDeps} ${spaceOut srcs}
      ${asmPack}
      ${removeRefs srcs} $out/${path}.a
    '') // {
      inherit gopkg;
      goDeps = uniqueDeps;
      goImportPath = path;
    };
  in gopkg;

  # Build a tree of Go libraries out of an external Go source
  # directory that follows the standard Go layout and was not built
  # with buildGo.nix.
  #
  # The derivation for each actual package will reside in an attribute
  # named "gopkg", and an attribute named "gobin" for binaries.
  external = import ./external { inherit pkgs program package; };

  # Import support libraries needed for protobuf & gRPC support
  protoLibs = import ./proto.nix {
    inherit external;
  };

  # Build a Go library out of the specified protobuf definition.
  proto = { name, proto, path ? name, goPackage ? name, extraDeps ? [] }: (makeOverridable package) {
    inherit name path;
    deps = [ protoLibs.goProto.proto.gopkg ] ++ extraDeps;
    srcs = lib.singleton (runCommand "goproto-${name}.pb.go" {} ''
      cp ${proto} ${baseNameOf proto}
      ${protobuf}/bin/protoc --plugin=${protoLibs.goProto.protoc-gen-go.gopkg}/bin/protoc-gen-go \
        --go_out=plugins=grpc,import_path=${baseNameOf path}:. ${baseNameOf proto}
      mv ./${goPackage}/*.pb.go $out
    '');
  };

  # Build a Go library out of the specified gRPC definition.
  grpc = args: proto (args // { extraDeps = [ protoLibs.goGrpc.gopkg ]; });

in {
  # Only the high-level builder functions are exposed, but made
  # overrideable.
  program = makeOverridable program;
  package = makeOverridable package;
  proto = makeOverridable proto;
  grpc = makeOverridable grpc;
  external = makeOverridable external;
}