cmd/go: disable cgo by default if DefaultCC is absolute and doesn't exist

Also fix the autocgo test from CL 450739 when DefaultCC is absolute.

Change-Id: Ie282a42a1334660225e88680b63b18b7c1ecba2c
Reviewed-on: https://go-review.googlesource.com/c/go/+/451219
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

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

@@ -159,7 +159,14 @@ func defaultContext() build.Context {
 		if ctxt.CgoEnabled {
 			if os.Getenv("CC") == "" {
 				cc := DefaultCC(ctxt.GOOS, ctxt.GOARCH)
-				if _, err := exec.LookPath(cc); err != nil {
+				if filepath.IsAbs(cc) {
+					if _, err := os.Stat(cc); os.IsNotExist(err) {
+						// The default CC is an absolute path that doesn't exist.
+						// (Perhaps make.bash was run on a system with a C compiler
+						// installed, and the current system doesn't have it there.)
+						ctxt.CgoEnabled = false
+					}
+				} else if _, err := exec.LookPath(cc); err != nil {
 					ctxt.CgoEnabled = false
 				}
 			}

src/cmd/go/scriptconds_test.go

@@ -5,6 +5,7 @@
 package main_test
 
 import (
+	"cmd/go/internal/cfg"
 	"cmd/go/internal/script"
 	"cmd/go/internal/script/scripttest"
 	"errors"
@@ -34,6 +35,7 @@ func scriptConditions() map[string]script.Cond {
 		return script.OnceCondition(summary, func() (bool, error) { return f(), nil })
 	}
 
+	add("abscc", script.Condition("default $CC path is absolute and exists", defaultCCIsAbsolute))
 	add("asan", sysCondition("-asan", platform.ASanSupported, true))
 	add("buildmode", script.PrefixCondition("go supports -buildmode=<suffix>", hasBuildmode))
 	add("case-sensitive", script.OnceCondition("$WORK filesystem is case-sensitive", isCaseSensitive))
@@ -55,6 +57,18 @@ func scriptConditions() map[string]script.Cond {
 	return conds
 }
 
+func defaultCCIsAbsolute(s *script.State) (bool, error) {
+	GOOS, _ := s.LookupEnv("GOOS")
+	GOARCH, _ := s.LookupEnv("GOARCH")
+	defaultCC := cfg.DefaultCC(GOOS, GOARCH)
+	if filepath.IsAbs(defaultCC) {
+		if _, err := os.Stat(defaultCC); err == nil {
+			return true, nil
+		}
+	}
+	return false, nil
+}
+
 func isMismatchedGoroot(s *script.State) (bool, error) {
 	gorootFinal, _ := s.LookupEnv("GOROOT_FINAL")
 	if gorootFinal == "" {

src/cmd/go/testdata/script/README

@@ -372,6 +372,8 @@ The available conditions are:
 	GOEXPERIMENT <suffix> is enabled
 [GOOS:*]
 	runtime.GOOS == <suffix>
+[abscc]
+	default $CC path is absolute and exists
 [asan]
 	GOOS/GOARCH supports -asan
 [buildmode:*]

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

@@ -8,11 +8,14 @@ env CGO_ENABLED=
 go env CGO_ENABLED
 stdout 1
 
-# Clearing CC and removing everything but Go from the PATH should disable cgo: no C compiler anymore.
+# Clearing CC and removing everything but Go from the PATH should usually
+# disable cgo: no C compiler anymore (unless the baked-in defaultCC is an
+# absolute path and exists.
 env CC=
 env PATH=$GOROOT/bin
 go env CGO_ENABLED
-stdout 0
+[!abscc] stdout 0
+[abscc] stdout 1
 
 # Setting CC should re-enable cgo.
 env CC=cc