decode: Fix decoding an actively growing file

4 files changed, 28 insertions, 7 deletions

diff --git a/ReleaseNotes.md b/ReleaseNotes.md
index 5d85254..f71967c 100644
--- a/ReleaseNotes.md
+++ b/ReleaseNotes.md
@@ -1,3 +1,14 @@
+# v0.3.5 (TBD)
+
+  Theme: TBD
+
+  User-facing changes:
+
+   - Decoder: Fixes a bug where if an EOF is encountered, the reader
+     is appended to, then another Decode is attempted, that EOF
+     poisons future Decodes.  This is something that `encoding/json`
+     supports.
+
 # v0.3.4 (2023-02-05)
 
   Theme: Fix compilation with Go 1.20
diff --git a/decode.go b/decode.go
index 60b530f..8638148 100644
--- a/decode.go
+++ b/decode.go
@@ -90,7 +90,6 @@ type Decoder struct {
 	useNumber             bool
 
 	// state
-	err         error
 	posStack    []int64
 	structStack []decodeStackItem
 }
@@ -240,18 +239,13 @@ func (dec *Decoder) Decode(ptr any) (err error) {
 		}
 	}
 
-	if dec.err != nil {
-		return dec.err
-	}
-
 	dec.io.Reset()
 	dec.io.PushReadBarrier()
 	defer func() {
 		if r := recover(); r != nil {
 			if de, ok := r.(decodeError); ok {
 				pub := DecodeError(de)
-				dec.err = &pub
-				err = dec.err
+				err = &pub
 			} else {
 				panic(r)
 			}
diff --git a/decode_scan.go b/decode_scan.go
index e233caf..521c5c4 100644
--- a/decode_scan.go
+++ b/decode_scan.go
@@ -45,6 +45,8 @@ func (sc *runeTypeScanner) Reset() {
 		}
 		// tell it to use that rType and rErr
 		_ = sc.UnreadRune() // we set it up to always succeed
+	} else {
+		sc.initialized = false
 	}
 }
 
diff --git a/decode_test.go b/decode_test.go
index 73ce903..456f363 100644
--- a/decode_test.go
+++ b/decode_test.go
@@ -5,6 +5,7 @@
 package lowmemjson
 
 import (
+	"bytes"
 	"io"
 	"strings"
 	"testing"
@@ -34,3 +35,16 @@ func TestDecodeObject(t *testing.T) {
 		})
 	assert.ErrorContains(t, err, "did not consume entire")
 }
+
+func TestDecodeGrowing(t *testing.T) {
+	t.Parallel()
+	var buf bytes.Buffer
+	dec := NewDecoder(&buf)
+	var x any
+	assert.ErrorIs(t, dec.Decode(&x), io.EOF)
+	buf.WriteString("1\n")
+	assert.NoError(t, dec.Decode(&x))
+	buf.WriteString("1\n")
+	assert.NoError(t, dec.Decode(&x))
+	assert.ErrorIs(t, dec.Decode(&x), io.EOF)
+}