cmd/go: for generate, use build context values for GOOS/GOARCH

Fixes #16120

Change-Id: Ia352558231e00baab5c698e93d7267564c07ec0c
Reviewed-on: https://go-review.googlesource.com/24242Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Andrew Gerrand <adg@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/generate.go

@@ -14,7 +14,6 @@ import (
 	"os/exec"
 	"path/filepath"
 	"regexp"
-	"runtime"
 	"strconv"
 	"strings"
 )
@@ -276,8 +275,8 @@ func isGoGenerate(buf []byte) bool {
 // single go:generate command.
 func (g *Generator) setEnv() {
 	g.env = []string{
-		"GOARCH=" + runtime.GOARCH,
-		"GOOS=" + runtime.GOOS,
+		"GOARCH=" + buildContext.GOARCH,
+		"GOOS=" + buildContext.GOOS,
 		"GOFILE=" + g.file,
 		"GOLINE=" + strconv.Itoa(g.lineNum),
 		"GOPACKAGE=" + g.pkg,

src/cmd/go/go_test.go

@@ -2920,3 +2920,27 @@ func TestAlwaysLinkSysoFiles(t *testing.T) {
 	tg.run("list", "-f", "{{.SysoFiles}}", "syso")
 	tg.grepStdout("a.syso", "missing syso file with CGO_ENABLED=0")
 }
+
+// Issue 16120.
+func TestGenerateUsesBuildContext(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("this test won't run under Windows")
+	}
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.tempDir("src/gen")
+	tg.tempFile("src/gen/gen.go", "package gen\n//go:generate echo $GOOS $GOARCH\n")
+	tg.setenv("GOPATH", tg.path("."))
+
+	tg.setenv("GOOS", "linux")
+	tg.setenv("GOARCH", "amd64")
+	tg.run("generate", "gen")
+	tg.grepStdout("linux amd64", "unexpected GOOS/GOARCH combination")
+
+	tg.setenv("GOOS", "darwin")
+	tg.setenv("GOARCH", "386")
+	tg.run("generate", "gen")
+	tg.grepStdout("darwin 386", "unexpected GOOS/GOARCH combination")
+}