promhttp/zstd: add unit tests for zstd writer registration

Signed-off-by: Ashish Jaiswal <[email protected]>

Author: Ashish Jaiswal

prometheus/promhttp/zstd/zstd_test.go

@@ -0,0 +1,65 @@
+// Copyright 2025 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package zstd
+
+import (
+	"bytes"
+	"io"
+	"testing"
+
+	kpzstd "github.com/klauspost/compress/zstd"
+
+	prominternal "github.com/prometheus/client_golang/prometheus/promhttp/internal"
+)
+
+func TestZstdRegistersWriter(t *testing.T) {
+	if prominternal.NewZstdWriter == nil {
+		t.Fatal("internal.NewZstdWriter is nil; zstd support not registered")
+	}
+}
+
+func TestZstdWriterRoundTrip(t *testing.T) {
+	if prominternal.NewZstdWriter == nil {
+		t.Fatal("internal.NewZstdWriter is nil; zstd support not registered")
+	}
+
+	var buf bytes.Buffer
+	w, closeFn, err := prominternal.NewZstdWriter(&buf)
+	if err != nil {
+		t.Fatalf("NewZstdWriter returned error: %v", err)
+	}
+	if closeFn == nil {
+		t.Fatal("closeFn is nil")
+	}
+
+	in := []byte("hello zstd")
+	if _, err := w.Write(in); err != nil {
+		t.Fatalf("write failed: %v", err)
+	}
+	closeFn()
+
+	r, err := kpzstd.NewReader(bytes.NewReader(buf.Bytes()))
+	if err != nil {
+		t.Fatalf("zstd NewReader failed: %v", err)
+	}
+	defer r.Close()
+
+	out, err := io.ReadAll(r)
+	if err != nil {
+		t.Fatalf("read failed: %v", err)
+	}
+	if !bytes.Equal(out, in) {
+		t.Fatalf("round-trip mismatch: got %q, want %q", out, in)
+	}
+}