# 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;

  # 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);

  # 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 -trimpath=${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
    '';

  # 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=$PWD -trimpath=${go} -p ${path} ${includeSources uniqueDeps} ${spaceOut srcs}
        ${asmPack}
      '').overrideAttrs (_: {
        passthru = {
          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;
}