Fix include paths for CGo deps in external repositories

A CGo-generated header file from any repository, external or main, must
always be includeable via its path relative to its repository root.
Previously, this was not true for CGo targets in external repositories,
which required an additional `external/<repo name>/` prefix, thus
breaking references within the external repository.

Fixes #3109

go/private/rules/binary.bzl

@@ -46,6 +46,19 @@ load(
 
 _EMPTY_DEPSET = depset([])
 
+def _include_path(hdr):
+    if not hdr.root.path:
+        fail("Expected hdr to be a generated file, got source file: " + hdr.path)
+
+    root_relative_path = hdr.path[len(hdr.root.path + "/"):]
+    if not root_relative_path.startswith("external/"):
+        return hdr.root.path
+
+    # All headers should be includeable via a path relative to their repository
+    # root, regardless of whether the repository is external or not. If it is,
+    # we thus need to append "external/<external repo name>" to the path.
+    return "/".join([hdr.root.path] + root_relative_path.split("/")[0:2])
+
 def new_cc_import(
         go,
         hdrs = _EMPTY_DEPSET,
@@ -62,7 +75,7 @@ def new_cc_import(
             defines = defines,
             local_defines = local_defines,
             headers = hdrs,
-            includes = depset([hdr.root.path for hdr in hdrs.to_list()]),
+            includes = depset([_include_path(hdr) for hdr in hdrs.to_list()]),
         ),
         linking_context = cc_common.create_linking_context(
             linker_inputs = depset([

tests/core/cgo/BUILD.bazel

@@ -1,4 +1,5 @@
 load("@io_bazel_rules_go//go:def.bzl", "go_binary", "go_library", "go_test")
+load("@io_bazel_rules_go//go/tools/bazel_testing:def.bzl", "go_bazel_test")
 
 go_test(
     name = "opts_test",
@@ -408,3 +409,8 @@ cc_library(
     srcs = ["split_import_c.c"],
     hdrs = ["split_import_c.h"],
 )
+
+go_bazel_test(
+    name = "external_includes_test",
+    srcs = ["external_includes_test.go"],
+)

tests/core/cgo/external_includes_test.go

@@ -0,0 +1,85 @@
+// Copyright 2020 The Bazel Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package external_includes_test
+
+import (
+	"testing"
+
+	"github.com/bazelbuild/rules_go/go/tools/bazel_testing"
+)
+
+func TestMain(m *testing.M) {
+	bazel_testing.TestMain(m, bazel_testing.Args{
+		Main: `
+-- other_repo/WORKSPACE --
+-- other_repo/cc/BUILD.bazel --
+cc_binary(
+    name = "main",
+    srcs = ["main.c"],
+    deps = ["//cgo"],
+)
+-- other_repo/cc/main.c --
+#include "cgo/cgo.h"
+
+int main() {}
+-- other_repo/cgo/BUILD.bazel --
+load("@io_bazel_rules_go//go:def.bzl", "go_binary", "go_library")
+
+go_binary(
+    name = "cgo",
+    embed = [":cgo_lib"],
+    importpath = "example.com/rules_go/cgo",
+    linkmode = "c-archive",
+    visibility = ["//visibility:public"],
+)
+
+go_library(
+    name = "cgo_lib",
+    srcs = ["cgo.go"],
+    cgo = True,
+    importpath = "example.com/rules_go/cgo",
+    visibility = ["//visibility:private"],
+)
+-- other_repo/cgo/cgo.go --
+package main
+
+import "C"
+
+//export HelloCgo
+func HelloCgo() {}
+
+func main() {}
+`,
+	WorkspaceSuffix: `
+local_repository(
+    name = "other_repo",
+    path = "other_repo",
+)
+`,
+	})
+}
+
+func TestExternalIncludes(t *testing.T) {
+	t.Run("default", func(t *testing.T) {
+		if err := bazel_testing.RunBazel("build", "@other_repo//cc:main"); err != nil {
+			t.Fatalf("Did not expect error:\n%+v", err)
+		}
+	})
+	t.Run("experimental_sibling_repository_layout", func(t *testing.T) {
+		if err := bazel_testing.RunBazel("build", "--experimental_sibling_repository_layout", "@other_repo//cc:main"); err != nil {
+			t.Fatalf("Did not expect error:\n%+v", err)
+		}
+	})
+}