gob: turn two panics into errors because they can be triggered

by bogus data, or are in any case recoverable.

Fixes #1598.

R=rsc
CC=golang-dev
https://golang.org/cl/4240101

src/pkg/gob/decode.go

@@ -19,7 +19,7 @@ import (
 var (
 	errBadUint = os.ErrorString("gob: encoded unsigned integer out of range")
 	errBadType = os.ErrorString("gob: unknown type id or corrupted data")
-	errRange   = os.ErrorString("gob: internal error: field numbers out of bounds")
+	errRange   = os.ErrorString("gob: bad data: field numbers out of bounds")
 )
 
 // decoderState is the execution state of an instance of the decoder. A new state
@@ -885,7 +885,7 @@ func (dec *Decoder) decIgnoreOpFor(wireId typeId) decOp {
 		wire := dec.wireType[wireId]
 		switch {
 		case wire == nil:
-			panic("internal error: can't find ignore op for type " + wireId.string())
+			errorf("gob: bad data: undefined type %s", wireId.string())
 		case wire.ArrayT != nil:
 			elemId := wire.ArrayT.Elem
 			elemOp := dec.decIgnoreOpFor(elemId)
@@ -927,7 +927,7 @@ func (dec *Decoder) decIgnoreOpFor(wireId typeId) decOp {
 		}
 	}
 	if op == nil {
-		errorf("ignore can't handle type %s", wireId.string())
+		errorf("gob: bad data: ignore can't handle type %s", wireId.string())
 	}
 	return op
 }

src/pkg/gob/encode.go

@@ -579,7 +579,8 @@ func methodIndex(rt reflect.Type, method string) int {
 			return i
 		}
 	}
-	panic("can't find method " + method)
+	errorf("gob: internal error: can't find method %s", method)
+	return 0
 }
 
 // gobEncodeOpFor returns the op for a type that is known to implement
@@ -628,7 +629,7 @@ func (enc *Encoder) compileEnc(ut *userTypeInfo) *encEngine {
 			wireFieldNum++
 		}
 		if srt.NumField() > 0 && len(engine.instr) == 0 {
-			errorf("type %s has no exported fields", rt)
+			errorf("gob: type %s has no exported fields", rt)
 		}
 		engine.instr = append(engine.instr, encInstr{encStructTerminator, 0, 0, 0})
 	} else {