promhttp: Isolate zstd support and klauspost/compress library use to promhttp/zstd package (#1765)

Signed-off-by: Jordan Liggitt <liggitt@google.com>

Author: liggitt

prometheus/promhttp/http.go

@@ -41,11 +41,11 @@ import (
 	"sync"
 	"time"
 
-	"github.com/klauspost/compress/zstd"
 	"github.com/prometheus/common/expfmt"
 
 	"github.com/prometheus/client_golang/internal/github.com/golang/gddo/httputil"
 	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promhttp/internal"
 )
 
 const (
@@ -65,7 +65,13 @@ const (
 	Zstd     Compression = "zstd"
 )
 
-var defaultCompressionFormats = []Compression{Identity, Gzip, Zstd}
+func defaultCompressionFormats() []Compression {
+	if internal.NewZstdWriter != nil {
+		return []Compression{Identity, Gzip, Zstd}
+	} else {
+		return []Compression{Identity, Gzip}
+	}
+}
 
 var gzipPool = sync.Pool{
 	New: func() interface{} {
@@ -138,7 +144,7 @@ func HandlerForTransactional(reg prometheus.TransactionalGatherer, opts HandlerO
 	// Select compression formats to offer based on default or user choice.
 	var compressions []string
 	if !opts.DisableCompression {
-		offers := defaultCompressionFormats
+		offers := defaultCompressionFormats()
 		if len(opts.OfferedCompressions) > 0 {
 			offers = opts.OfferedCompressions
 		}
@@ -466,14 +472,12 @@ func negotiateEncodingWriter(r *http.Request, rw io.Writer, compressions []strin
 
 	switch selected {
 	case "zstd":
-		// TODO(mrueg): Replace klauspost/compress with stdlib implementation once https://github.com/golang/go/issues/62513 is implemented.
-		z, err := zstd.NewWriter(rw, zstd.WithEncoderLevel(zstd.SpeedFastest))
-		if err != nil {
-			return nil, "", func() {}, err
+		if internal.NewZstdWriter == nil {
+			// The content encoding was not implemented yet.
+			return nil, "", func() {}, fmt.Errorf("content compression format not recognized: %s. Valid formats are: %s", selected, defaultCompressionFormats())
 		}
-
-		z.Reset(rw)
-		return z, selected, func() { _ = z.Close() }, nil
+		writer, closeWriter, err := internal.NewZstdWriter(rw)
+		return writer, selected, closeWriter, err
 	case "gzip":
 		gz := gzipPool.Get().(*gzip.Writer)
 		gz.Reset(rw)
@@ -483,6 +487,6 @@ func negotiateEncodingWriter(r *http.Request, rw io.Writer, compressions []strin
 		return rw, selected, func() {}, nil
 	default:
 		// The content encoding was not implemented yet.
-		return nil, "", func() {}, fmt.Errorf("content compression format not recognized: %s. Valid formats are: %s", selected, defaultCompressionFormats)
+		return nil, "", func() {}, fmt.Errorf("content compression format not recognized: %s. Valid formats are: %s", selected, defaultCompressionFormats())
 	}
 }

prometheus/promhttp/http_test.go

@@ -30,6 +30,7 @@ import (
 	dto "github.com/prometheus/client_model/go"
 
 	"github.com/prometheus/client_golang/prometheus"
+	_ "github.com/prometheus/client_golang/prometheus/promhttp/zstd"
 )
 
 type errorCollector struct{}
@@ -484,7 +485,7 @@ func TestInstrumentMetricHandlerWithCompression(t *testing.T) {
 func TestNegotiateEncodingWriter(t *testing.T) {
 	var defaultCompressions []string
 
-	for _, comp := range defaultCompressionFormats {
+	for _, comp := range defaultCompressionFormats() {
 		defaultCompressions = append(defaultCompressions, string(comp))
 	}
 

prometheus/promhttp/internal/compression.go

@@ -0,0 +1,21 @@
+// 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 internal
+
+import (
+	"io"
+)
+
+// NewZstdWriter enables zstd write support if non-nil.
+var NewZstdWriter func(rw io.Writer) (_ io.Writer, closeWriter func(), _ error)

prometheus/promhttp/zstd/zstd.go

@@ -0,0 +1,47 @@
+// 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 activates support for zstd compression.
+// To enable zstd compression support, import like this:
+//
+//	import (
+//	        _ "github.com/prometheus/client_golang/prometheus/promhttp/zstd"
+//	)
+//
+// This support is currently implemented via the github.com/klauspost/compress library,
+// so importing this package requires linking and building that library.
+// Once stdlib support is added to the Go standard library (https://github.com/golang/go/issues/62513),
+// this package is expected to become a no-op, and zstd support will be enabled by default.
+package zstd
+
+import (
+	"io"
+
+	"github.com/klauspost/compress/zstd"
+
+	"github.com/prometheus/client_golang/prometheus/promhttp/internal"
+)
+
+func init() {
+	// Enable zstd support
+	internal.NewZstdWriter = func(rw io.Writer) (_ io.Writer, closeWriter func(), _ error) {
+		// TODO(mrueg): Replace klauspost/compress with stdlib implementation once https://github.com/golang/go/issues/62513 is implemented, and move this package to be a no-op / backfill for older go versions.
+		z, err := zstd.NewWriter(rw, zstd.WithEncoderLevel(zstd.SpeedFastest))
+		if err != nil {
+			return nil, func() {}, err
+		}
+
+		z.Reset(rw)
+		return z, func() { _ = z.Close() }, nil
+	}
+}