cmd/dist: refactor generated cgo-support logic

During bootstrapping, cmd/dist writes a file indicating which
GOOS/GOARCH combinations are valid, and which support cgo-enabled
builds. That information previously went into the go/build package,
but today it fits in more naturally in the internal/platform package
(which already has a number of functions indicating feature support
for GOOS/GOARCH combinations).

Moreover, as of CL 450739 the cmd/go logic for determining whether to
use cgo is somewhat more nuanced than the go/build logic: cmd/go
checks for the presence of a C compiler, whereas go/build does not
(mostly because it determines its configuration at package-init time,
and checking $PATH for a C compiler is somewhat expensive).

To simplify this situation, this change:

	- consolidates the “cgo supported” check in internal/platform
	  (alongside many other platform-support checks) instead of making
	  it a one-off in go/build,

	- and updates a couple of tests to use testenv.HasCGO instead of
	  build.Default.CgoEnabled to decide whether to test a cgo-specific
	  behavior.

For #58884.
For #59500.

Change-Id: I0bb2502dba4545a3d98c9e915727382ce536a0f3
Reviewed-on: https://go-review.googlesource.com/c/go/+/483695
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>

Author: Bryan C. Mills

.gitignore

@@ -38,6 +38,7 @@ _testmain.go
 /src/go/build/zcgo.go
 /src/go/doc/headscan
 /src/internal/buildcfg/zbootstrap.go
+/src/internal/platform/zosarch.go
 /src/runtime/internal/sys/zversion.go
 /src/unicode/maketables
 /src/time/tzdata/zzipdata.go

src/cmd/dist/build.go

@@ -626,14 +626,16 @@ var deptab = []struct {
 }{
 	{"cmd/go/internal/cfg", []string{
 		"zdefaultcc.go",
+	}},
+	{"go/build", []string{
+		"zcgo.go",
+	}},
+	{"internal/platform", []string{
 		"zosarch.go",
 	}},
 	{"runtime/internal/sys", []string{
 		"zversion.go",
 	}},
-	{"go/build", []string{
-		"zcgo.go",
-	}},
 	{"time/tzdata", []string{
 		"zzipdata.go",
 	}},
@@ -650,10 +652,10 @@ var gentab = []struct {
 	nameprefix string
 	gen        func(string, string)
 }{
+	{"zcgo.go", mkzcgo},
 	{"zdefaultcc.go", mkzdefaultcc},
 	{"zosarch.go", mkzosarch},
 	{"zversion.go", mkzversion},
-	{"zcgo.go", mkzcgo},
 	{"zzipdata.go", mktzdata},
 
 	// not generated anymore, but delete the file if we see it
@@ -1196,6 +1198,7 @@ var cleanlist = []string{
 	"runtime/internal/sys",
 	"cmd/cgo",
 	"cmd/go/internal/cfg",
+	"internal/platform",
 	"go/build",
 }
 

src/cmd/dist/buildgo.go

@@ -91,55 +91,19 @@ func defaultCCFunc(name string, defaultcc map[string]string) string {
 	return buf.String()
 }
 
-// mkzosarch writes zosarch.go for cmd/go.
-func mkzosarch(dir, file string) {
-	// sort for deterministic zosarch.go file
-	var list []string
-	for plat := range cgoEnabled {
-		list = append(list, plat)
-	}
-	sort.Strings(list)
-
-	var buf strings.Builder
-	fmt.Fprintf(&buf, "// Code generated by go tool dist; DO NOT EDIT.\n\n")
-	fmt.Fprintf(&buf, "package cfg\n\n")
-	fmt.Fprintf(&buf, "var OSArchSupportsCgo = map[string]bool{\n")
-	for _, plat := range list {
-		fmt.Fprintf(&buf, "\t%s: %v,\n", quote(plat), cgoEnabled[plat])
-	}
-	fmt.Fprintf(&buf, "}\n")
-
-	writefile(buf.String(), file, writeSkipSame)
-}
-
 // mkzcgo writes zcgo.go for the go/build package:
 //
 //	package build
-//	var cgoEnabled = map[string]bool{}
+//	const defaultCGO_ENABLED = <CGO_ENABLED>
 //
 // It is invoked to write go/build/zcgo.go.
 func mkzcgo(dir, file string) {
-	// sort for deterministic zcgo.go file
-	var list []string
-	for plat, hasCgo := range cgoEnabled {
-		if hasCgo {
-			list = append(list, plat)
-		}
-	}
-	sort.Strings(list)
-
 	var buf strings.Builder
 	fmt.Fprintf(&buf, "// Code generated by go tool dist; DO NOT EDIT.\n")
 	fmt.Fprintln(&buf)
 	fmt.Fprintf(&buf, "package build\n")
 	fmt.Fprintln(&buf)
 	fmt.Fprintf(&buf, "const defaultCGO_ENABLED = %s\n", quote(os.Getenv("CGO_ENABLED")))
-	fmt.Fprintln(&buf)
-	fmt.Fprintf(&buf, "var cgoEnabled = map[string]bool{\n")
-	for _, plat := range list {
-		fmt.Fprintf(&buf, "\t%s: true,\n", quote(plat))
-	}
-	fmt.Fprintf(&buf, "}\n")
 
 	writefile(buf.String(), file, writeSkipSame)
 }

src/cmd/dist/buildruntime.go

@@ -6,6 +6,7 @@ package main
 
 import (
 	"fmt"
+	"sort"
 	"strings"
 )
 
@@ -82,3 +83,26 @@ func mkobjabi(file string) {
 
 	writefile(buf.String(), file, writeSkipSame)
 }
+
+// mkzosarch writes zosarch.go for internal/platform.
+func mkzosarch(dir, file string) {
+	// sort for deterministic file contents.
+	var list []string
+	for plat := range cgoEnabled {
+		list = append(list, plat)
+	}
+	sort.Strings(list)
+
+	var buf strings.Builder
+	fmt.Fprintf(&buf, "// Code generated by go tool dist; DO NOT EDIT.\n")
+	fmt.Fprintln(&buf)
+	fmt.Fprintf(&buf, "package platform\n")
+	fmt.Fprintln(&buf)
+	fmt.Fprintf(&buf, "var osArchSupportsCgo = map[string]bool{\n")
+	for _, plat := range list {
+		fmt.Fprintf(&buf, "\t\t%s: %v,\n", quote(plat), cgoEnabled[plat])
+	}
+	fmt.Fprintf(&buf, "}\n")
+
+	writefile(buf.String(), file, writeSkipSame)
+}

src/cmd/dist/buildtool.go

@@ -121,6 +121,7 @@ func bootstrapBuildTools() {
 
 	mkbuildcfg(pathf("%s/src/internal/buildcfg/zbootstrap.go", goroot))
 	mkobjabi(pathf("%s/src/cmd/internal/objabi/zbootstrap.go", goroot))
+	mkzosarch("", pathf("%s/src/internal/platform/zosarch.go", goroot))
 
 	// Use $GOROOT/pkg/bootstrap as the bootstrap workspace root.
 	// We use a subdirectory of $GOROOT/pkg because that's the

src/cmd/go/internal/cfg/cfg.go

@@ -141,6 +141,7 @@ func defaultContext() build.Context {
 		// (1) environment, (2) go/env file, (3) runtime constants,
 		// while go/build.Default.GOOS/GOARCH are derived from the preference list
 		// (1) environment, (2) runtime constants.
+		//
 		// We know ctxt.GOOS/GOARCH == runtime.GOOS/GOARCH;
 		// no matter how that happened, go/build.Default will make the
 		// same decision (either the environment variables are set explicitly

src/cmd/go/internal/work/action.go

@@ -14,6 +14,7 @@ import (
 	"debug/elf"
 	"encoding/json"
 	"fmt"
+	"internal/platform"
 	"os"
 	"path/filepath"
 	"strings"
@@ -355,7 +356,7 @@ func closeBuilders() {
 }
 
 func CheckGOOSARCHPair(goos, goarch string) error {
-	if _, ok := cfg.OSArchSupportsCgo[goos+"/"+goarch]; !ok && cfg.BuildContext.Compiler == "gc" {
+	if !platform.BuildModeSupported(cfg.BuildContext.Compiler, "default", goos, goarch) {
 		return fmt.Errorf("unsupported GOOS/GOARCH pair %s/%s", goos, goarch)
 	}
 	return nil

src/cmd/go/internal/work/init.go

@@ -283,7 +283,7 @@ func buildModeInit() {
 		base.Fatalf("buildmode=%s not supported", cfg.BuildBuildmode)
 	}
 
-	if !platform.BuildModeSupported(cfg.BuildToolchainName, cfg.BuildBuildmode, cfg.Goos, cfg.Goarch) {
+	if cfg.BuildBuildmode != "default" && !platform.BuildModeSupported(cfg.BuildToolchainName, cfg.BuildBuildmode, cfg.Goos, cfg.Goarch) {
 		base.Fatalf("-buildmode=%s not supported on %s/%s\n", cfg.BuildBuildmode, cfg.Goos, cfg.Goarch)
 	}
 

src/cmd/go/note_test.go

@@ -5,7 +5,7 @@
 package main_test
 
 import (
-	"go/build"
+	"internal/testenv"
 	"runtime"
 	"testing"
 
@@ -32,7 +32,7 @@ func TestNoteReading(t *testing.T) {
 	}
 
 	switch {
-	case !build.Default.CgoEnabled:
+	case !testenv.HasCGO():
 		t.Skipf("skipping - no cgo, so assuming external linking not available")
 	case runtime.GOOS == "plan9":
 		t.Skipf("skipping - external linking not supported")

src/cmd/go/testdata/script/list_all_gobuild.txt

@@ -1,5 +1,6 @@
 # go list all should work with GOOS=linux because all packages build on Linux
 env GOOS=linux
+env GOARCH=amd64
 go list all
 
 # go list all should work with GOOS=darwin, but it used to fail because