go/build: less aggressive failure when GOROOT not found

R=golang-dev, rsc, dsymonds
CC=golang-dev
https://golang.org/cl/4743041

src/pkg/go/build/path.go

@@ -13,7 +13,7 @@ import (
 	"strings"
 )
 
-// Path is a validated list of Trees derived from $GOPATH at init.
+// Path is a validated list of Trees derived from $GOROOT and $GOPATH at init.
 var Path []*Tree
 
 // Tree describes a Go source tree, either $GOROOT or one from $GOPATH.
@@ -79,7 +79,10 @@ func (t *Tree) HasPkg(pkg string) bool {
 	// TODO(adg): check object version is consistent
 }
 
-var ErrNotFound = os.NewError("package could not be found locally")
+var (
+	ErrNotFound     = os.NewError("go/build: package could not be found locally")
+	ErrTreeNotFound = os.NewError("go/build: no valid GOROOT or GOPATH could be found")
+)
 
 // FindTree takes an import or filesystem path and returns the
 // tree where the package source should be and the package import path.
@@ -111,7 +114,11 @@ func FindTree(path string) (tree *Tree, pkg string, err os.Error) {
 			return
 		}
 	}
-	err = ErrNotFound
+	if tree == nil {
+		err = ErrTreeNotFound
+	} else {
+		err = ErrNotFound
+	}
 	return
 }
 
@@ -133,12 +140,13 @@ var (
 // set up Path: parse and validate GOROOT and GOPATH variables
 func init() {
 	root := runtime.GOROOT()
-	p, err := newTree(root)
+	t, err := newTree(root)
 	if err != nil {
-		log.Fatalf("Invalid GOROOT %q: %v", root, err)
+		log.Printf("go/build: invalid GOROOT %q: %v", root, err)
+	} else {
+		t.Goroot = true
+		Path = []*Tree{t}
 	}
-	p.Goroot = true
-	Path = []*Tree{p}
 
 	for _, p := range filepath.SplitList(os.Getenv("GOPATH")) {
 		if p == "" {
@@ -146,7 +154,7 @@ func init() {
 		}
 		t, err := newTree(p)
 		if err != nil {
-			log.Printf("Invalid GOPATH %q: %v", p, err)
+			log.Printf("go/build: invalid GOPATH %q: %v", p, err)
 			continue
 		}
 		Path = append(Path, t)
@@ -160,7 +168,7 @@ func init() {
 	}
 
 	// use GOROOT if no valid GOPATH specified
-	if defaultTree == nil {
+	if defaultTree == nil && len(Path) > 0 {
 		defaultTree = Path[0]
 	}
 }