From 1f96e090e2c7ff1aec310da306dc4c04fffc6ece Mon Sep 17 00:00:00 2001
From: Chen Kai <281165273grape@gmail.com>
Date: Wed, 29 Apr 2026 22:33:50 +0800
Subject: [PATCH] fix: accept identifier-only stream as empty payload
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decode and decodeFromReader rejected a stream that contained only the
10-byte stream-identifier chunk with FrameError.NotFramed, even though
the Snappy framing spec treats it as a valid representation of an empty
payload. Go's snappy.NewReader and Rust's snap::read::FrameDecoder both
accept the same input and decode it to an empty slice; cross-client
interop fixtures (e.g. leanSpec) emit exactly this 10-byte form for
empty input.

The terminal post-loop check in both decode paths now requires that
both saw_stream_identifier and saw_data_chunk are unset to declare the
input unframed. A stream with the identifier alone — and no data
chunks — returns an empty slice.

Adds two regression tests against the canonical 10-byte
"\xff\x06\x00\x00sNaPpY" input (decode and decodeFromReader). The
existing "frame roundtrip samples" test already covered round-tripping
"" through the lib's own encoder, but the encoder appends an empty
data chunk in finish(), which masked the gap on the decode side.

Closes #7.
---
 src/frames.zig | 37 +++++++++++++++++++++++++++++++++++--
 1 file changed, 35 insertions(+), 2 deletions(-)

diff --git a/src/frames.zig b/src/frames.zig
index 0b758d7..d9d3861 100644
--- a/src/frames.zig
+++ b/src/frames.zig
@@ -212,7 +212,10 @@ pub fn decodeFromReader(allocator: Allocator, reader: anytype, writer: anytype)
         chunk_buf.clearRetainingCapacity();
     }
 
-    if (!saw_data_chunk) return FrameError.NotFramed;
+    // A stream that contained the identifier but no data chunks is a valid
+    // empty payload per the Snappy framing spec. Only treat the input as
+    // unframed when neither was seen.
+    if (!saw_stream_identifier and !saw_data_chunk) return FrameError.NotFramed;
 }
 
 fn decodeFramed(allocator: Allocator, data: []const u8) ![]u8 {
@@ -273,7 +276,10 @@ fn decodeFramed(allocator: Allocator, data: []const u8) ![]u8 {
         return FrameError.UnsupportedUnskippableChunkType;
     }
 
-    if (!saw_data_chunk) return FrameError.NotFramed;
+    // A stream that contained the identifier but no data chunks is a valid
+    // empty payload per the Snappy framing spec. Only treat the input as
+    // unframed when neither was seen.
+    if (!saw_stream_identifier and !saw_data_chunk) return FrameError.NotFramed;
 
     // Some producers may omit the identifier. Only enforce when data present with mismatched chunk.
     return output.toOwnedSlice(allocator);
@@ -526,6 +532,33 @@ test "decode falls back to raw snappy payloads" {
     try std.testing.expectEqualSlices(u8, sample, decoded);
 }
 
+test "decode accepts identifier-only stream as empty payload" {
+    // A frame containing only the stream identifier (no data chunks) is a
+    // valid empty payload per the Snappy framing spec. Go's snappy.NewReader
+    // and Rust's snap::read::FrameDecoder both decode this 10-byte input to
+    // an empty slice; cross-client interop fixtures (e.g. leanSpec) emit
+    // exactly this representation for empty input.
+    const allocator = std.testing.allocator;
+    const identifier_only = "\xff\x06\x00\x00sNaPpY";
+
+    const decoded = try decode(allocator, identifier_only);
+    defer allocator.free(decoded);
+
+    try std.testing.expectEqual(@as(usize, 0), decoded.len);
+}
+
+test "decodeFromReader accepts identifier-only stream as empty payload" {
+    const allocator = std.testing.allocator;
+    const identifier_only = "\xff\x06\x00\x00sNaPpY";
+
+    var input_stream = std.io.fixedBufferStream(identifier_only);
+    var output = std.ArrayListUnmanaged(u8).empty;
+    defer output.deinit(allocator);
+
+    try decodeFromReader(allocator, input_stream.reader(), output.writer(allocator));
+    try std.testing.expectEqual(@as(usize, 0), output.items.len);
+}
+
 test "decode rejects invalid stream identifier" {
     const allocator = std.testing.allocator;
     const sample = "identifier";