From 8327fd2dd7431355d865189a2e43674c8679c49e Mon Sep 17 00:00:00 2001
From: Swarna Sekhar Dhar <bapay.9@gmail.com>
Date: Tue, 6 Oct 2026 13:41:24 +0530
Subject: [PATCH] Add a regression test for trailing skippable frames

A valid zstd stream followed by a trailing skippable frame used to fail
one-shot Decompress with "unexpected EOF": the streaming fallback stopped
after the first frame and treated the skippable frame as truncated input.
The behaviour changed in "Keep decompressing as long as we have compressed
data", but no test covers the trailing-skippable shape specifically, and
issue #129 remains open as a result.

The test uses a highly compressible payload, since a large compressed
input takes the direct path and never hits the fallback. It covers both
the one-shot Decompress path and the streaming reader.

Refs #129

Signed-off-by: Swarna Sekhar Dhar <bapay.9@gmail.com>
---
 zstd_stream_skippable_test.go | 70 +++++++++++++++++++++++++++++++++++
 1 file changed, 70 insertions(+)
 create mode 100644 zstd_stream_skippable_test.go

diff --git a/zstd_stream_skippable_test.go b/zstd_stream_skippable_test.go
new file mode 100644
index 0000000..5d6472b
--- /dev/null
+++ b/zstd_stream_skippable_test.go
@@ -0,0 +1,70 @@
+package zstd
+
+import (
+	"bytes"
+	"encoding/binary"
+	"io"
+	"io/ioutil"
+	"testing"
+)
+
+// skippableFrame returns a valid zstd skippable frame carrying 4 bytes of user data.
+// https://github.com/facebook/zstd/blob/dev/doc/zstd_compression_format.md#skippable-frames
+func skippableFrame(userValue uint32) []byte {
+	skipFrame := make([]byte, 12)
+	binary.LittleEndian.PutUint32(skipFrame, 0x184D2A50) // user-defined skippable magic
+	binary.LittleEndian.PutUint32(skipFrame[4:], 4)      // frame content size
+	binary.LittleEndian.PutUint32(skipFrame[8:], userValue)
+	return skipFrame
+}
+
+// TestDecompressTrailingSkippableFrame verifies that one-shot Decompress accepts a
+// valid zstd stream followed by a trailing skippable frame. The payload is highly
+// compressible, so the compressed input is tiny relative to its decompressed size;
+// that is the shape where the streaming fallback previously stopped after the first
+// frame and reported unexpected EOF on the trailing skippable frame.
+// Regression context: https://github.com/DataDog/zstd/issues/129
+func TestDecompressTrailingSkippableFrame(t *testing.T) {
+	totalSize := 1_000_001
+	rawData := make([]byte, totalSize) // all zeros: highly compressible
+
+	compressed, err := Compress(nil, rawData)
+	if err != nil {
+		t.Fatal(err)
+	}
+	compressed = append(compressed, skippableFrame(12345)...)
+
+	decompressed, err := Decompress(nil, compressed)
+	if err != nil {
+		t.Fatalf("Decompress: %s", err)
+	}
+	if !bytes.Equal(rawData, decompressed) {
+		t.Fatal("decompressed data does not match original")
+	}
+}
+
+// TestReaderTrailingSkippableFrame verifies the same through the streaming reader.
+func TestReaderTrailingSkippableFrame(t *testing.T) {
+	totalSize := 1_000_001
+	rawData := make([]byte, totalSize)
+	r := NewRandBytes()
+	if _, err := io.ReadFull(r, rawData); err != nil && err != io.EOF {
+		t.Fatal(err)
+	}
+
+	compressed, err := Compress(nil, rawData)
+	if err != nil {
+		t.Fatal(err)
+	}
+	compressed = append(compressed, skippableFrame(12345)...)
+
+	reader := NewReader(bytes.NewReader(compressed))
+	defer reader.Close()
+	decompressed, err := ioutil.ReadAll(reader)
+	if err != nil {
+		t.Fatalf("stream reader: %s", err)
+	}
+	if !bytes.Equal(rawData, decompressed) {
+		t.Fatal("decompressed data does not match original")
+	}
+}