cmd/go: allow internal imports based on module paths

Updates #23970.

Change-Id: I2e69ad15b9d1097bfeef9947f03cfa6834a6a049
Reviewed-on: https://go-review.googlesource.com/125676
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/internal/load/pkg.go

@@ -961,16 +961,29 @@ func disallowInternal(srcDir string, p *Package, stk *ImportStack) *Package {
 	if i > 0 {
 		i-- // rewind over slash in ".../internal"
 	}
-	parent := p.Dir[:i+len(p.Dir)-len(p.ImportPath)]
-	if str.HasFilePathPrefix(filepath.Clean(srcDir), filepath.Clean(parent)) {
-		return p
-	}
+	if p.Module == nil {
+		parent := p.Dir[:i+len(p.Dir)-len(p.ImportPath)]
 
-	// Look for symlinks before reporting error.
-	srcDir = expandPath(srcDir)
-	parent = expandPath(parent)
-	if str.HasFilePathPrefix(filepath.Clean(srcDir), filepath.Clean(parent)) {
-		return p
+		if str.HasFilePathPrefix(filepath.Clean(srcDir), filepath.Clean(parent)) {
+			return p
+		}
+
+		// Look for symlinks before reporting error.
+		srcDir = expandPath(srcDir)
+		parent = expandPath(parent)
+		if str.HasFilePathPrefix(filepath.Clean(srcDir), filepath.Clean(parent)) {
+			return p
+		}
+	} else {
+		// p is in a module, so make it available based on the import path instead
+		// of the file path (https://golang.org/issue/23970).
+		parent := p.ImportPath[:i]
+		// TODO(bcmills): In case of replacements, use the module path declared by
+		// the replacement module, not the path seen by the user.
+		importerPath := (*stk)[len(*stk)-2]
+		if strings.HasPrefix(importerPath, parent) {
+			return p
+		}
 	}
 
 	// Internal is present, and srcDir is outside parent's tree. Not allowed.

src/cmd/go/testdata/mod/golang.org_notx_useinternal_v0.1.0.txt

+written by hand — attempts to use a prohibited internal package
+(https://golang.org/s/go14internal)
+
+-- .mod --
+module golang.org/notx/useinternal
+-- .info --
+{"Version":"v0.1.0","Name":"","Short":"","Time":"2018-07-25T17:24:00Z"}
+-- go.mod --
+module golang.org/notx/useinternal
+-- useinternal.go --
+package useinternal
+
+import _ "golang.org/x/internal/subtle"

src/cmd/go/testdata/mod/golang.org_x_internal_v0.1.0.txt

+written by hand — loosely derived from golang.org/x/crypto/internal/subtle,
+but splitting the internal package across a module boundary
+
+-- .mod --
+module golang.org/x/internal
+-- .info --
+{"Version":"v0.1.0","Name":"","Short":"","Time":"2018-07-25T17:24:00Z"}
+-- go.mod --
+module golang.org/x/internal
+-- subtle/aliasing.go --
+// Copyright 2018 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 !appengine
+
+// This is a tiny version of golang.org/x/crypto/internal/subtle.
+
+package subtle
+
+import "unsafe"
+
+func AnyOverlap(x, y []byte) bool {
+	return len(x) > 0 && len(y) > 0 &&
+		uintptr(unsafe.Pointer(&x[0])) <= uintptr(unsafe.Pointer(&y[len(y)-1])) &&
+		uintptr(unsafe.Pointer(&y[0])) <= uintptr(unsafe.Pointer(&x[len(x)-1]))
+}
+-- subtle/aliasing_appengine.go --
+// Copyright 2018 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 appengine
+
+package subtle
+
+import "reflect"
+
+func AnyOverlap(x, y []byte) bool {
+	return len(x) > 0 && len(y) > 0 &&
+		reflect.ValueOf(&x[0]).Pointer() <= reflect.ValueOf(&y[len(y)-1]).Pointer() &&
+		reflect.ValueOf(&y[0]).Pointer() <= reflect.ValueOf(&x[len(x)-1]).Pointer()
+}

src/cmd/go/testdata/mod/golang.org_x_useinternal_v0.1.0.txt

+written by hand — uses an internal package from another module
+(https://golang.org/s/go14internal)
+
+-- .mod --
+module golang.org/x/useinternal
+-- .info --
+{"Version":"v0.1.0","Name":"","Short":"","Time":"2018-07-25T17:24:00Z"}
+-- go.mod --
+module golang.org/x/useinternal
+-- useinternal.go --
+package useinternal
+
+import _ "golang.org/x/internal/subtle"

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

+env GO111MODULE=on
+
+# golang.org/x/internal should be importable from other golang.org/x modules.
+go mod -init -module golang.org/x/anything
+go build .
+
+# ...but that should not leak into other modules.
+! go build ./baddep
+stderr 'use of internal package'
+
+# Internal packages in the standard library should not leak into modules.
+! go build ./fromstd
+stderr 'use of internal package'
+
+
+# Dependencies should be able to use their own internal modules...
+rm go.mod
+go mod -init -module golang.org/notx
+go build ./throughdep
+
+# ... but other modules should not, even if they have transitive dependencies.
+! go build .
+stderr 'use of internal package'
+
+# And transitive dependencies still should not leak.
+! go build ./baddep
+stderr 'use of internal package'
+
+
+-- useinternal.go --
+package useinternal
+import _ "golang.org/x/internal/subtle"
+
+-- throughdep/useinternal.go --
+package throughdep
+import _ "golang.org/x/useinternal"
+
+-- baddep/useinternal.go --
+package baddep
+import _ "golang.org/notx/useinternal"
+
+-- fromstd/useinternal.go --
+package fromstd
+import _ "internal/testenv"