go/build: invert AllowVendor to IgnoreVendor

Looking for vendor directories is a better default.

Fixes #13772

Change-Id: Iabbaea71ccc67b72f14f1f412dc8ab70cb41996d
Reviewed-on: https://go-review.googlesource.com/18450Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/cmd/go/pkg.go

@@ -348,11 +348,9 @@ func loadImport(path, srcDir string, parent *Package, stk *importStack, importPo
 	// TODO: After Go 1, decide when to pass build.AllowBinary here.
 	// See issue 3268 for mistakes to avoid.
 	buildMode := build.ImportComment
-	if go15VendorExperiment && mode&useVendor != 0 && path == origPath {
-		// We've already searched the vendor directories and didn't find anything.
-		// Let Import search them again so that, if the package is not found anywhere,
-		// the error includes the vendor directories in the list of places considered.
-		buildMode |= build.AllowVendor
+	if !go15VendorExperiment || mode&useVendor == 0 || path != origPath {
+		// Not vendoring, or we already found the vendored path.
+		buildMode |= build.IgnoreVendor
 	}
 	bp, err := buildContext.Import(path, srcDir, buildMode)
 	bp.ImportPath = importPath

src/go/build/build.go

@@ -344,18 +344,20 @@ const (
 	// See golang.org/s/go14customimport for more information.
 	ImportComment
 
-	// If AllowVendor is set, Import searches vendor directories
+	// By default, Import searches vendor directories
 	// that apply in the given source directory before searching
 	// the GOROOT and GOPATH roots.
 	// If an Import finds and returns a package using a vendor
 	// directory, the resulting ImportPath is the complete path
 	// to the package, including the path elements leading up
 	// to and including "vendor".
-	// For example, if Import("y", "x/subdir", AllowVendor) finds
+	// For example, if Import("y", "x/subdir", 0) finds
 	// "x/vendor/y", the returned package's ImportPath is "x/vendor/y",
 	// not plain "y".
 	// See golang.org/s/go15vendor for more information.
-	AllowVendor
+	//
+	// Setting IgnoreVendor ignores vendor directories.
+	IgnoreVendor
 )
 
 // A Package describes the Go package found in a directory.
@@ -571,7 +573,7 @@ func (ctxt *Context) Import(path string, srcDir string, mode ImportMode) (*Packa
 		gopath := ctxt.gopath()
 
 		// Vendor directories get first chance to satisfy import.
-		if mode&AllowVendor != 0 && srcDir != "" {
+		if mode&IgnoreVendor == 0 && srcDir != "" {
 			searchVendor := func(root string, isGoroot bool) bool {
 				sub, ok := ctxt.hasSubdir(root, srcDir)
 				if !ok || !strings.HasPrefix(sub, "src/") || strings.Contains(sub, "/testdata/") {

src/go/build/build_test.go

@@ -303,7 +303,7 @@ func TestImportVendor(t *testing.T) {
 	testenv.MustHaveGoBuild(t) // really must just have source
 	ctxt := Default
 	ctxt.GOPATH = ""
-	p, err := ctxt.Import("golang.org/x/net/http2/hpack", filepath.Join(ctxt.GOROOT, "src/net/http"), AllowVendor)
+	p, err := ctxt.Import("golang.org/x/net/http2/hpack", filepath.Join(ctxt.GOROOT, "src/net/http"), 0)
 	if err != nil {
 		t.Fatalf("cannot find vendored golang.org/x/net/http2/hpack from net/http directory: %v", err)
 	}
@@ -317,7 +317,7 @@ func TestImportVendorFailure(t *testing.T) {
 	testenv.MustHaveGoBuild(t) // really must just have source
 	ctxt := Default
 	ctxt.GOPATH = ""
-	p, err := ctxt.Import("x.com/y/z", filepath.Join(ctxt.GOROOT, "src/net/http"), AllowVendor)
+	p, err := ctxt.Import("x.com/y/z", filepath.Join(ctxt.GOROOT, "src/net/http"), 0)
 	if err == nil {
 		t.Fatalf("found made-up package x.com/y/z in %s", p.Dir)
 	}