cmd/go: don't add C compiler ID to hash for standard library

No test because a real test requires installing two different compilers.

For #40042
For #47251

Change-Id: Iefddd67830d242a119378b7ce20be481904806e4
Reviewed-on: https://go-review.googlesource.com/c/go/+/335409
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/go_test.go

@@ -2848,3 +2848,35 @@ func TestExecInDeletedDir(t *testing.T) {
 	// `go version` should not fail
 	tg.run("version")
 }
+
+// A missing C compiler should not force the net package to be stale.
+// Issue 47215.
+func TestMissingCC(t *testing.T) {
+	if !canCgo {
+		t.Skip("test is only meaningful on systems with cgo")
+	}
+	cc := os.Getenv("CC")
+	if cc == "" {
+		cc = "gcc"
+	}
+	if filepath.IsAbs(cc) {
+		t.Skipf(`"CC" (%s) is an absolute path`, cc)
+	}
+	_, err := exec.LookPath(cc)
+	if err != nil {
+		t.Skipf(`"CC" (%s) not on PATH`, cc)
+	}
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	netStale, _ := tg.isStale("net")
+	if netStale {
+		t.Skip(`skipping test because "net" package is currently stale`)
+	}
+
+	tg.setenv("PATH", "") // No C compiler on PATH.
+	netStale, _ = tg.isStale("net")
+	if netStale {
+		t.Error(`clearing "PATH" causes "net" to be stale`)
+	}
+}

src/cmd/go/internal/work/exec.go

@@ -252,8 +252,15 @@ func (b *Builder) buildActionID(a *Action) cache.ActionID {
 
 		ccExe := b.ccExe()
 		fmt.Fprintf(h, "CC=%q %q %q %q\n", ccExe, cppflags, cflags, ldflags)
-		if ccID, err := b.gccToolID(ccExe[0], "c"); err == nil {
+		// Include the C compiler tool ID so that if the C
-			fmt.Fprintf(h, "CC ID=%q\n", ccID)
+		// compiler changes we rebuild the package.
+		// But don't do that for standard library packages like net,
+		// so that the prebuilt .a files from a Go binary install
+		// don't need to be rebuilt with the local compiler.
+		if !p.Standard {
+			if ccID, err := b.gccToolID(ccExe[0], "c"); err == nil {
+				fmt.Fprintf(h, "CC ID=%q\n", ccID)
+			}
 		}
 		if len(p.CXXFiles)+len(p.SwigCXXFiles) > 0 {
 			cxxExe := b.cxxExe()