cmd/go: add import config debugging flag

Change-Id: I11d83c8841f3de4ed4d9d014dec65d6f20464b11
Reviewed-on: https://go-review.googlesource.com/78396Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/cmd/go/internal/load/icfg.go

+// Copyright 2017 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.
+
+package load
+
+import (
+	"bytes"
+	"encoding/json"
+	"errors"
+	"io/ioutil"
+)
+
+// DebugDeprecatedImportcfg is installed as the undocumented -debug-deprecated-importcfg build flag.
+// It is useful for debugging subtle problems in the go command logic but not something
+// we want users to depend on. The hope is that the "deprecated" will make that clear.
+// We intend to remove this flag in Go 1.11.
+var DebugDeprecatedImportcfg debugDeprecatedImportcfgFlag
+
+type debugDeprecatedImportcfgFlag struct {
+	enabled bool
+	pkgs    map[string]*debugDeprecatedImportcfgPkg
+}
+
+type debugDeprecatedImportcfgPkg struct {
+	Dir    string
+	Import map[string]string
+}
+
+var (
+	debugDeprecatedImportcfgMagic = []byte("# debug-deprecated-importcfg\n")
+	errImportcfgSyntax            = errors.New("malformed syntax")
+)
+
+func (f *debugDeprecatedImportcfgFlag) String() string { return "" }
+
+func (f *debugDeprecatedImportcfgFlag) Set(x string) error {
+	if x == "" {
+		*f = debugDeprecatedImportcfgFlag{}
+		return nil
+	}
+	data, err := ioutil.ReadFile(x)
+	if err != nil {
+		return err
+	}
+
+	if !bytes.HasPrefix(data, debugDeprecatedImportcfgMagic) {
+		return errImportcfgSyntax
+	}
+	data = data[len(debugDeprecatedImportcfgMagic):]
+
+	f.pkgs = nil
+	if err := json.Unmarshal(data, &f.pkgs); err != nil {
+		return errImportcfgSyntax
+	}
+	f.enabled = true
+	return nil
+}
+
+func (f *debugDeprecatedImportcfgFlag) lookup(parent *Package, path string) (dir, newPath string) {
+	if parent == nil {
+		if p1 := f.pkgs[path]; p1 != nil {
+			return p1.Dir, path
+		}
+		return "", ""
+	}
+	if p1 := f.pkgs[parent.ImportPath]; p1 != nil {
+		if newPath := p1.Import[path]; newPath != "" {
+			if p2 := f.pkgs[newPath]; p2 != nil {
+				return p2.Dir, newPath
+			}
+		}
+	}
+	return "", ""
+}

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

@@ -383,8 +383,14 @@ func LoadImport(path, srcDir string, parent *Package, stk *ImportStack, importPo
 	importPath := path
 	origPath := path
 	isLocal := build.IsLocalImport(path)
+	var debugDeprecatedImportcfgDir string
 	if isLocal {
 		importPath = dirToImportPath(filepath.Join(srcDir, path))
+	} else if DebugDeprecatedImportcfg.enabled {
+		if d, i := DebugDeprecatedImportcfg.lookup(parent, path); d != "" {
+			debugDeprecatedImportcfgDir = d
+			importPath = i
+		}
 	} else if mode&UseVendor != 0 {
 		// We do our own vendor resolution, because we want to
 		// find out the key to use in packageCache without the
@@ -406,17 +412,23 @@ func LoadImport(path, srcDir string, parent *Package, stk *ImportStack, importPo
 		// Load package.
 		// Import always returns bp != nil, even if an error occurs,
 		// in order to return partial information.
-		buildMode := build.ImportComment
-		if mode&UseVendor == 0 || path != origPath {
-			// Not vendoring, or we already found the vendored path.
-			buildMode |= build.IgnoreVendor
+		var bp *build.Package
+		var err error
+		if debugDeprecatedImportcfgDir != "" {
+			bp, err = cfg.BuildContext.ImportDir(debugDeprecatedImportcfgDir, 0)
+		} else {
+			buildMode := build.ImportComment
+			if mode&UseVendor == 0 || path != origPath {
+				// Not vendoring, or we already found the vendored path.
+				buildMode |= build.IgnoreVendor
+			}
+			bp, err = cfg.BuildContext.Import(path, srcDir, buildMode)
 		}
-		bp, err := cfg.BuildContext.Import(path, srcDir, buildMode)
 		bp.ImportPath = importPath
 		if cfg.GOBIN != "" {
 			bp.BinDir = cfg.GOBIN
 		}
-		if err == nil && !isLocal && bp.ImportComment != "" && bp.ImportComment != path &&
+		if debugDeprecatedImportcfgDir == "" && err == nil && !isLocal && bp.ImportComment != "" && bp.ImportComment != path &&
 			!strings.Contains(path, "/vendor/") && !strings.HasPrefix(path, "vendor/") {
 			err = fmt.Errorf("code in directory %s expects import %q", bp.Dir, bp.ImportComment)
 		}
@@ -425,7 +437,7 @@ func LoadImport(path, srcDir string, parent *Package, stk *ImportStack, importPo
 			p = setErrorPos(p, importPos)
 		}
 
-		if origPath != cleanImport(origPath) {
+		if debugDeprecatedImportcfgDir == "" && origPath != cleanImport(origPath) {
 			p.Error = &PackageError{
 				ImportStack: stk.Copy(),
 				Err:         fmt.Sprintf("non-canonical import path: %q should be %q", origPath, pathpkg.Clean(origPath)),

src/cmd/go/internal/work/build.go

@@ -229,6 +229,7 @@ func AddBuildFlags(cmd *base.Command) {
 
 	// Undocumented, unstable debugging flags.
 	cmd.Flag.StringVar(&cfg.DebugActiongraph, "debug-actiongraph", "", "")
+	cmd.Flag.Var(&load.DebugDeprecatedImportcfg, "debug-deprecated-importcfg", "")
 }
 
 // fileExtSplit expects a filename and returns the name