tighter constraints on allowed paths.

Change-Id: Ie6b121e4733e45288a36dbc2426f556fd5e2832d

storage.go

@@ -12,7 +12,7 @@ import (
 // Valid checks that p will not confuse the go tool if added to packages.
 func Valid(p string, packages []Package) bool {
 	for _, pkg := range packages {
-		if strings.HasPrefix(pkg.path, p) {
+		if strings.HasPrefix(pkg.path, p) || strings.HasPrefix(p, pkg.path) {
 			return false
 		}
 	}

vain_test.go

@@ -53,13 +53,6 @@ func TestValid(t *testing.T) {
 			in:   "bobo",
 			want: true,
 		},
-		{
-			pkgs: []Package{
-				{path: ""},
-			},
-			in:   "bobo",
-			want: true,
-		},
 		{
 			pkgs: []Package{
 				{path: "bobo"},
@@ -74,6 +67,35 @@ func TestValid(t *testing.T) {
 			in:   "a/b/c",
 			want: false,
 		},
+		{
+			pkgs: []Package{
+				{path: "a/b/c"},
+			},
+			in:   "a/b",
+			want: false,
+		},
+		{
+			pkgs: []Package{
+				{path: "name/db"},
+				{path: "name/lib"},
+			},
+			in:   "name/foo",
+			want: true,
+		},
+		{
+			pkgs: []Package{
+				{path: "a"},
+			},
+			in:   "a/b",
+			want: false,
+		},
+		{
+			pkgs: []Package{
+				{path: "foo"},
+			},
+			in:   "foo/bar",
+			want: false,
+		},
 		{
 			pkgs: []Package{
 				{path: "foo/bar"},