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>

2 files changed, 72 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
 }
diff --git a/src/encoding/gob/encoder_test.go b/src/encoding/gob/encoder_test.go
index 6d50b82573..a358d5bc30 100644
--- a/src/encoding/gob/encoder_test.go
+++ b/src/encoding/gob/encoder_test.go
@@ -1202,3 +1202,66 @@ func TestMarshalFloatMap(t *testing.T) {
 		t.Fatalf("\nEncode: %v\nDecode: %v", want, got)
 	}
 }
+
+func TestDecodePartial(t *testing.T) {
+	type T struct {
+		X []int
+		Y string
+	}
+
+	var buf bytes.Buffer
+	t1 := T{X: []int{1, 2, 3}, Y: "foo"}
+	t2 := T{X: []int{4, 5, 6}, Y: "bar"}
+	enc := NewEncoder(&buf)
+
+	t1start := 0
+	if err := enc.Encode(&t1); err != nil {
+		t.Fatal(err)
+	}
+
+	t2start := buf.Len()
+	if err := enc.Encode(&t2); err != nil {
+		t.Fatal(err)
+	}
+
+	data := buf.Bytes()
+	for i := 0; i <= len(data); i++ {
+		bufr := bytes.NewReader(data[:i])
+
+		// Decode both values, stopping at the first error.
+		var t1b, t2b T
+		dec := NewDecoder(bufr)
+		var err error
+		err = dec.Decode(&t1b)
+		if err == nil {
+			err = dec.Decode(&t2b)
+		}
+
+		switch i {
+		case t1start, t2start:
+			// Either the first or the second Decode calls had zero input.
+			if err != io.EOF {
+				t.Errorf("%d/%d: expected io.EOF: %v", i, len(data), err)
+			}
+		case len(data):
+			// We reached the end of the entire input.
+			if err != nil {
+				t.Errorf("%d/%d: unexpected error: %v", i, len(data), err)
+			}
+			if !reflect.DeepEqual(t1b, t1) {
+				t.Fatalf("t1 value mismatch: got %v, want %v", t1b, t1)
+			}
+			if !reflect.DeepEqual(t2b, t2) {
+				t.Fatalf("t2 value mismatch: got %v, want %v", t2b, t2)
+			}
+		default:
+			// In between, we must see io.ErrUnexpectedEOF.
+			// The decoder used to erroneously return io.EOF in some cases here,
+			// such as if the input was cut off right after some type specs,
+			// but before any value was actually transmitted.
+			if err != io.ErrUnexpectedEOF {
+				t.Errorf("%d/%d: expected io.ErrUnexpectedEOF: %v", i, len(data), err)
+			}
+		}
+	}
+}