refactor/importgraph: don't ignore imports in packages with errors.

And in gomvpkg, don't stop just because some packages had errors.
This is inevitable in a large GOPATH tree.

Fixes issue golang/go#10907

Change-Id: I9a60b070228d06d44880202eeef54394e914f5d5
Reviewed-on: https://go-review.googlesource.com/10715
Reviewed-by: Sameer Ajmani <sameer@golang.org>

refactor/importgraph/graph.go

@@ -54,7 +54,8 @@ func (g Graph) Search(roots ...string) map[string]bool {
 // Build scans the specified Go workspace and builds the forward and
 // reverse import dependency graphs for all its packages.
 // It also returns a mapping from import paths to errors for packages
-// that could not be loaded.
+// whose loading was not entirely successful.
+// A package may appear in the graph and in the errors mapping.
 func Build(ctxt *build.Context) (forward, reverse Graph, errors map[string]error) {
 	type importEdge struct {
 		from, to string
@@ -75,14 +76,17 @@ func Build(ctxt *build.Context) (forward, reverse Graph, errors map[string]error
 				ch <- pathError{path, err}
 				return
 			}
+
 			bp, err := ctxt.Import(path, "", 0)
-			if _, ok := err.(*build.NoGoError); ok {
-				return // empty directory is not an error
-			}
 			if err != nil {
+				if _, ok := err.(*build.NoGoError); ok {
+					// empty directory is not an error
+				} else {
 					ch <- pathError{path, err}
-				return
 				}
+				// Even in error cases, Import usually returns a package.
+			}
+			if bp != nil {
 				for _, imp := range bp.Imports {
 					ch <- importEdge{path, imp}
 				}
@@ -92,6 +96,7 @@ func Build(ctxt *build.Context) (forward, reverse Graph, errors map[string]error
 				for _, imp := range bp.XTestImports {
 					ch <- importEdge{path, imp}
 				}
+			}
 		}()
 	})
 	go func() {

refactor/rename/mvpkg.go

@@ -61,11 +61,12 @@ func Move(ctxt *build.Context, from, to, moveTmpl string) error {
 	// Build the import graph and figure out which packages to update.
 	fwd, rev, errors := importgraph.Build(ctxt)
 	if len(errors) > 0 {
+		// With a large GOPATH tree, errors are inevitable.
+		// Report them but proceed.
 		fmt.Fprintf(os.Stderr, "While scanning Go workspace:\n")
 		for path, err := range errors {
 			fmt.Fprintf(os.Stderr, "Package %q: %s.\n", path, err)
 		}
-		return fmt.Errorf("failed to construct import graph")
 	}
 
 	// Determine the affected packages---the set of packages whose import

refactor/rename/rename.go

@@ -254,6 +254,8 @@ func Main(ctxt *build.Context, offsetFlag, fromFlag, to string) error {
 		// Scan the workspace and build the import graph.
 		_, rev, errors := importgraph.Build(ctxt)
 		if len(errors) > 0 {
+			// With a large GOPATH tree, errors are inevitable.
+			// Report them but proceed.
 			fmt.Fprintf(os.Stderr, "While scanning Go workspace:\n")
 			for path, err := range errors {
 				fmt.Fprintf(os.Stderr, "Package %q: %s.\n", path, err)