cmd/go: prohibit C sources files unless using cgo

Those C files would have been compiled with 6c.
It's close to impossible to use C correctly anymore,
and the C compilers are going away eventually.
Make them unavailable now.

go1.4.txt change in CL 145890046

LGTM=iant
R=iant
CC=golang-codereviews, r
https://golang.org/cl/149720043

misc/cgo/test/backdoor/backdoor.go

@@ -4,5 +4,4 @@
 
 package backdoor
 
-func LockedOSThread() bool // in runtime.c
-func Issue7695(x1, x2, x3, x4, x5, x6, x7, x8 uintptr)
+func LockedOSThread() bool // in thunk.s

misc/cgo/test/backdoor/backdoor_gccgo.go

-// Copyright 2014 The Go Authors.  All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-// This is the gccgo version of the stub in runtime.c.
-
-// +build gccgo
-
-package backdoor
-
-func Issue7695(x1, x2, x3, x4, x5, x6, x7, x8 uintptr) {}

misc/cgo/test/backdoor/runtime.c

-// Copyright 2011 The Go Authors.  All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-// Expose some runtime functions for testing.
-// Must be in a non-cgo-using package so that
-// the go command compiles this file with 6c, not gcc.
-
-// +build gc
-
-typedef char bool;
-
-// This is what a cgo-compiled stub declaration looks like.
-void
-·Issue7695(struct{void *y[8*sizeof(void*)];}p)
-{
-	USED(p);
-}

misc/cgo/test/issue7695_test.go

-// Copyright 2014 The Go Authors.  All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-// +build ignore
-// This test depends on running C code on Go stacks. Not allowed anymore.
-
-// Demo of deferred C function with untrue prototype
-// breaking stack copying. See golang.org/issue/7695.
-
-package cgotest
-
-import (
-	"testing"
-
-	"./backdoor"
-)
-
-func TestIssue7695(t *testing.T) {
-	defer backdoor.Issue7695(1, 0, 2, 0, 0, 3, 0, 4)
-	recurse(100)
-}
-
-func recurse(n int) {
-	var x [128]int
-	n += x[0]
-	if n > 0 {
-		recurse(n - 1)
-	}
-}

src/cmd/go/pkg.go

@@ -614,6 +614,16 @@ func (p *Package) load(stk *importStack, bp *build.Package, err error) *Package
 	}
 	p.Target = p.target
 
+	// Check for C code compiled with Plan 9 C compiler.
+	// No longer allowed except in runtime and runtime/cgo, for now.
+	if len(p.CFiles) > 0 && !p.usesCgo() && (!p.Standard || p.ImportPath != "runtime") {
+		p.Error = &PackageError{
+			ImportStack: stk.copy(),
+			Err:         fmt.Sprintf("C source files not allowed when not using cgo: %s", strings.Join(p.CFiles, " ")),
+		}
+		return p
+	}
+
 	// In the absence of errors lower in the dependency tree,
 	// check for case-insensitive collisions of import paths.
 	if len(p.DepsErrors) == 0 {

src/cmd/go/test.bash

@@ -157,6 +157,20 @@ fi
 rm -f ./testdata/err
 unset GOPATH
 
+export GOPATH=$(pwd)/testdata/src
+TEST disallowed C source files
+export GOPATH=$(pwd)/testdata
+if ./testgo build badc 2>testdata/err; then
+	echo 'go build badc succeeded'
+	ok=false
+elif ! grep 'C source files not allowed' testdata/err >/dev/null; then
+	echo 'go test did not say C source files not allowed:'
+	cat testdata/err
+	ok=false
+fi
+rm -f ./testdata/err
+unset GOPATH
+
 TEST error message for syntax error in test go file says FAIL
 export GOPATH=$(pwd)/testdata
 if ./testgo test syntaxerror 2>testdata/err; then

src/cmd/go/testdata/src/badc/x.c

+// C code!

src/cmd/go/testdata/src/badc/x.go

+package badc