Honor build tags when listing files

This is done using a brand new build context that is configured using
the regular environment variables.

Signed-off-by: Steeve Morin <[email protected]>

go/tools/gopackagesdriver/BUILD.bazel

@@ -5,6 +5,7 @@ go_library(
     srcs = [
         "bazel.go",
         "bazel_json_builder.go",
+        "build_context.go",
         "driver_request.go",
         "flatpackage.go",
         "json_packages_driver.go",

go/tools/gopackagesdriver/build_context.go

@@ -0,0 +1,38 @@
+package main
+
+import (
+	"go/build"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+var buildContext = makeBuildContext()
+
+func makeBuildContext() *build.Context {
+	bctx := &build.Context{
+		GOOS:        getenvDefault("GOOS", build.Default.GOOS),
+		GOARCH:      getenvDefault("GOARCH", build.Default.GOARCH),
+		GOROOT:      getenvDefault("GOROOT", build.Default.GOROOT),
+		GOPATH:      getenvDefault("GOPATH", build.Default.GOPATH),
+		BuildTags:   strings.Split(getenvDefault("GOTAGS", ""), ","),
+		ReleaseTags: build.Default.ReleaseTags[:],
+	}
+	if v, ok := os.LookupEnv("CGO_ENABLED"); ok {
+		bctx.CgoEnabled = v == "1"
+	} else {
+		bctx.CgoEnabled = build.Default.CgoEnabled
+	}
+	return bctx
+}
+
+func filterSourceFilesForTags(files []string) []string {
+	ret := make([]string, 0, len(files))
+	for _, f := range files {
+		dir, filename := filepath.Split(f)
+		if match, _ := buildContext.MatchFile(dir, filename); match {
+			ret = append(ret, f)
+		}
+	}
+	return ret
+}

go/tools/gopackagesdriver/flatpackage.go

@@ -102,6 +102,13 @@ func (fp *FlatPackage) ResolvePaths(prf PathResolverFunc) error {
 	return nil
 }
 
+// FilterFilesForBuildTags filters the source files given the current build
+// tags.
+func (fp *FlatPackage) FilterFilesForBuildTags() {
+	fp.GoFiles = filterSourceFilesForTags(fp.GoFiles)
+	fp.CompiledGoFiles = filterSourceFilesForTags(fp.CompiledGoFiles)
+}
+
 func (fp *FlatPackage) IsStdlib() bool {
 	return fp.Standard
 }

go/tools/gopackagesdriver/packageregistry.go

@@ -56,6 +56,10 @@ func (pr *PackageRegistry) Remove(pkgs ...*FlatPackage) *PackageRegistry {
 func (pr *PackageRegistry) ResolvePaths(prf PathResolverFunc) error {
 	for _, pkg := range pr.packagesByImportPath {
 		pkg.ResolvePaths(prf)
+		pkg.FilterFilesForBuildTags()
+		for _, f := range pkg.CompiledGoFiles {
+			pr.packagesByFile[f] = pkg
+		}
 		for _, f := range pkg.CompiledGoFiles {
 			pr.packagesByFile[f] = pkg
 		}