Export Context Read methods

- Return errors where failable

pkg/vm/stack/context.go

@@ -2,6 +2,7 @@ package stack
 
 import (
 	"encoding/binary"
+	"errors"
 )
 
 // Context represent the current execution context of the VM.
@@ -95,7 +96,8 @@ func (c *Context) String() string {
 	return "execution context"
 }
 
-func (c *Context) readUint32() uint32 {
+// ReadUint32 reads a uint32 from the script
+func (c *Context) ReadUint32() uint32 {
 	start, end := c.IP(), c.IP()+4
 	if end > len(c.prog) {
 		return 0
@@ -105,7 +107,8 @@ func (c *Context) readUint32() uint32 {
 	return val
 }
 
-func (c *Context) readUint16() uint16 {
+// ReadUint16 reads a uint16 from the script
+func (c *Context) ReadUint16() uint16 {
 	start, end := c.IP(), c.IP()+2
 	if end > len(c.prog) {
 		return 0
@@ -115,23 +118,33 @@ func (c *Context) readUint16() uint16 {
 	return val
 }
 
-func (c *Context) readByte() byte {
-	return c.readBytes(1)[0]
+// ReadByte reads one byte from the script
+func (c *Context) ReadByte() (byte, error) {
+	byt, err := c.ReadBytes(1)
+	if err != nil {
+		return 0, err
+	}
+
+	return byt[0], nil
 }
 
-func (c *Context) readBytes(n int) []byte {
+// ReadBytes will read n bytes from the context
+func (c *Context) ReadBytes(n int) ([]byte, error) {
 	start, end := c.IP(), c.IP()+n
 	if end > len(c.prog) {
-		return nil
+		return nil, errors.New("Too many bytes to read, pointer goes past end of program")
 	}
 
 	out := make([]byte, n)
 	copy(out, c.prog[start:end])
 	c.ip += n
-	return out
+	return out, nil
 }
 
-func (c *Context) readVarBytes() []byte {
-	n := c.readByte()
-	return c.readBytes(int(n))
+func (c *Context) readVarBytes() ([]byte, error) {
+	n, err := c.ReadByte()
+	if err != nil {
+		return nil, err
+	}
+	return c.ReadBytes(int(n))
 }