Add fuzz test for CellUnion (#193)

Add fuzz test and jointly fixing panic that can happen for bad input.

Slice creation with n < 0 panics.

Starting point for #175

Author: panmari

s2/cellunion.go

@@ -539,6 +539,7 @@ func (cu *CellUnion) ExactArea() float64 {
 }
 
 // Encode encodes the CellUnion.
+// Note: The CellUnion is not required to be valid according to IsValid().
 func (cu *CellUnion) Encode(w io.Writer) error {
 	e := &encoder{w: w}
 	cu.encode(e)
@@ -554,6 +555,7 @@ func (cu *CellUnion) encode(e *encoder) {
 }
 
 // Decode decodes the CellUnion.
+// Note: The returned CellUnion is not necessarily valid according to IsValid().
 func (cu *CellUnion) Decode(r io.Reader) error {
 	d := &decoder{r: asByteReader(r)}
 	cu.decode(d)
@@ -569,7 +571,7 @@ func (cu *CellUnion) decode(d *decoder) {
 		d.err = fmt.Errorf("only version %d is supported", encodingVersion)
 		return
 	}
-	n := d.readInt64()
+	n := d.readUint64()
 	if d.err != nil {
 		return
 	}

s2/encode.go

@@ -163,14 +163,6 @@ func (d *decoder) readInt8() (x int8) {
 	return
 }
 
-func (d *decoder) readInt64() (x int64) {
-	if d.err != nil {
-		return
-	}
-	d.err = binary.Read(d.r, binary.LittleEndian, &x)
-	return
-}
-
 func (d *decoder) readUint8() (x uint8) {
 	if d.err != nil {
 		return

s2/encode_fuzz_test.go

@@ -0,0 +1,33 @@
+// Fuzz tests for s2 decoding. Run using
+// go test -fuzz=Fuzz github.com/golang/geo/s2
+package s2
+
+import (
+	"bytes"
+	"testing"
+)
+
+func FuzzDecodeCellUnion(f *testing.F) {
+	cu := CellUnion([]CellID{
+		CellID(0x33),
+		CellID(0x8e3748fab),
+		CellID(0x91230abcdef83427),
+	})
+	buf := new(bytes.Buffer)
+	if err := cu.Encode(buf); err != nil {
+		f.Errorf("error encoding %v: ", err)
+	}
+	f.Add(buf.Bytes())
+
+	f.Fuzz(func(t *testing.T, encoded []byte) {
+		var c CellUnion
+		if err := c.Decode(bytes.NewReader(encoded)); err != nil {
+			// Construction failed, no need to test further.
+			return
+		}
+		if got := c.ApproxArea(); got < 0 {
+			t.Errorf("ApproxArea() = %v, want >= 0. CellUnion: %v", got, c)
+		}
+		// TODO: Test more invariants that should hold for all CellUnion.
+	})
+}