go/pointer: fix crash: valueNode(*FreeVar) was allocating 1 node, even for multi-word types.

+ regression test.

Fixes golang/go#8172

Also: return error (not panic) when called with empty input.

LGTM=gri
R=crawshaw, gri
CC=golang-codereviews, jon
https://golang.org/cl/104270043

go/pointer/analysis.go

@@ -212,6 +212,9 @@ func (a *analysis) computeTrackBits() {
 // always succeed.  An error can occur only due to an internal bug.
 //
 func Analyze(config *Config) (result *Result, err error) {
+	if config.Mains == nil {
+		return nil, fmt.Errorf("no main/test packages to analyze (check $GOROOT/$GOPATH)")
+	}
 	defer func() {
 		if p := recover(); p != nil {
 			err = fmt.Errorf("internal error in pointer analysis: %v (please report this bug)", p)

go/pointer/gen.go

@@ -226,7 +226,7 @@ func (a *analysis) valueNode(v ssa.Value) nodeid {
 		if a.log != nil {
 			comment = v.String()
 		}
-		id = a.addOneNode(v.Type(), comment, nil)
+		id = a.addNodes(v.Type(), comment)
 		if obj := a.objectNode(nil, v); obj != 0 {
 			a.addressOf(v.Type(), id, obj)
 		}

go/pointer/testdata/func.go

@@ -156,6 +156,39 @@ func func8(x ...int) {
 	print(&x[0]) // @pointsto varargs[*]@varargs:15
 }
 
+type E struct {
+	x1, x2, x3, x4, x5 *int
+}
+
+func (e E) f() {}
+
+func func9() {
+	// Regression test for bug reported by Jon Valdes on golang-dev, Jun 19 2014.
+	// The receiver of a bound method closure may be of a multi-node type, E.
+	// valueNode was reserving only a single node for it, so the
+	// nodes used by the immediately following constraints
+	// (e.g. param 'i') would get clobbered.
+
+	var e E
+	e.x1 = &a
+	e.x2 = &a
+	e.x3 = &a
+	e.x4 = &a
+	e.x5 = &a
+
+	_ = e.f // form a closure---must reserve sizeof(E) nodes
+
+	func(i I) {
+		i.f() // must not crash the solver
+	}(new(D))
+
+	print(e.x1) // @pointsto main.a
+	print(e.x2) // @pointsto main.a
+	print(e.x3) // @pointsto main.a
+	print(e.x4) // @pointsto main.a
+	print(e.x5) // @pointsto main.a
+}
+
 func main() {
 	func1()
 	func2()
@@ -165,6 +198,7 @@ func main() {
 	func6()
 	func7()
 	func8(1, 2, 3) // @line varargs
+	func9()
 }
 
 // @calls <root> -> main.main