encoding/gob: follow documented io.EOF semantics

The docs say:

	If the input is at EOF, Decode returns io.EOF and does not modify e.

However, the added test fails:

	--- FAIL: TestDecodePartial (0.00s)
		encoder_test.go:1263: 31/81: expected io.ErrUnexpectedEOF: EOF
		encoder_test.go:1263: 51/81: expected io.ErrUnexpectedEOF: EOF

In particular, the decoder would return io.EOF after reading a valid
message for a type specification, and then hit EOF before reading a data
item message.

Fix that by only allowing a Decode call to return io.EOF if the reader
hits EOF immediately, without successfully reading any message.
Otherwise, hitting EOF is an ErrUnexpectedEOF, like in other cases.

Also fix a net/rpc test that, coincidentally, expected an io.EOF
as an error when feeding bad non-zero data to a gob decoder.
An io.ErrUnexpectedEOF is clearly better in that scenario.

Fixes #48905.

Change-Id: Ied6a0d8ac8377f89646319a18c0380c4f2b09b85
Reviewed-on: https://go-review.googlesource.com/c/go/+/354972
Trust: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

1 files changed, 9 insertions, 0 deletions

diff --git a/src/encoding/gob/decoder.go b/src/encoding/gob/decoder.go
index b476aaac93..5e4ed5a7d7 100644
--- a/src/encoding/gob/decoder.go
+++ b/src/encoding/gob/decoder.go
@@ -138,9 +138,17 @@ func (dec *Decoder) nextUint() uint64 {
 // decoded. If this is an interface value, it can be ignored by
 // resetting that buffer.
 func (dec *Decoder) decodeTypeSequence(isInterface bool) typeId {
+	firstMessage := true
 	for dec.err == nil {
 		if dec.buf.Len() == 0 {
 			if !dec.recvMessage() {
+				// We can only return io.EOF if the input was empty.
+				// If we read one or more type spec messages,
+				// require a data item message to follow.
+				// If we hit an EOF before that, then give ErrUnexpectedEOF.
+				if !firstMessage && dec.err == io.EOF {
+					dec.err = io.ErrUnexpectedEOF
+				}
 				break
 			}
 		}
@@ -166,6 +174,7 @@ func (dec *Decoder) decodeTypeSequence(isInterface bool) typeId {
 			}
 			dec.nextUint()
 		}
+		firstMessage = false
 	}
 	return -1
 }