Add ReusePort config to confighttp

This adds in a new field, `ReusePort` that, if set, sets the
SO_REUSEPORT socket option on the listener port. If we're on non unix,
this errors out instead as AFAIK SO_REUSEPORT isn't available. Cursory
testing says that SO_REUSEADDR _might_ work, but I don't have a windows
machine to test on.

Signed-off-by: sinkingpoint <[email protected]>

Author: sinkingpoint

.chloggen/so-reuse-port.yaml

@@ -0,0 +1,25 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: enhancement
+
+# The name of the component, or a single word describing the area of concern, (e.g. otlpreceiver)
+component: pkg/config/confighttp
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Added ReusePort option to confighttp.ServerConfig to enable SO_REUSEPORT on supported platforms.
+
+# One or more tracking issues or pull requests related to the change
+issues: [14046]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: []

config/confighttp/go.mod

@@ -66,7 +66,7 @@ require (
 	go.uber.org/multierr v1.11.0 // indirect
 	go.yaml.in/yaml/v3 v3.0.4 // indirect
 	golang.org/x/crypto v0.41.0 // indirect
-	golang.org/x/sys v0.35.0 // indirect
+	golang.org/x/sys v0.35.0
 	golang.org/x/text v0.28.0 // indirect
 	google.golang.org/genproto/googleapis/rpc v0.0.0-20250804133106-a7a43d27e69b // indirect
 	google.golang.org/grpc v1.76.0 // indirect

config/confighttp/listen_config_other.go

@@ -0,0 +1,30 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+//go:build !windows
+
+package confighttp
+
+import (
+	"net"
+	"syscall"
+
+	"golang.org/x/sys/unix"
+)
+
+func (sc *ServerConfig) getListenConfig() (net.ListenConfig, error) {
+	cfg := net.ListenConfig{}
+	if sc.ReusePort {
+		cfg.Control = func(network, address string, c syscall.RawConn) error {
+			var controlErr error
+			err := c.Control(func(fd uintptr) {
+				controlErr = unix.SetsockoptInt(int(fd), unix.SOL_SOCKET, unix.SO_REUSEPORT, 1)
+			})
+			if err != nil {
+				return err
+			}
+			return controlErr
+		}
+	}
+
+	return cfg, nil
+}

config/confighttp/listen_config_windows.go

@@ -0,0 +1,18 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+//go:build windows
+
+package confighttp
+
+import (
+	"errors"
+	"net"
+)
+
+func (sc *ServerConfig) getListenConfig() (net.ListenConfig, error) {
+	if sc.ReusePort {
+		return net.ListenConfig{}, errors.New("ReusePort is not supported on this platform")
+	}
+
+	return net.ListenConfig{}, nil
+}

config/confighttp/server.go

@@ -96,6 +96,12 @@ type ServerConfig struct {
 	// KeepAlivesEnabled controls whether HTTP keep-alives are enabled.
 	// By default, keep-alives are always enabled. Only very resource-constrained environments should disable them.
 	KeepAlivesEnabled bool `mapstructure:"keep_alives_enabled,omitempty"`
+
+	// ReusePort enables the SO_REUSEPORT socket option on the listener.
+	// This allows multiple server instances to bind to the same address/port.
+	// This is useful for horizontal scaling and zero-downtime restarts.
+	// Note: This option is not supported on all operating systems.
+	ReusePort bool `mapstructure:"reuse_port,omitempty"`
 }
 
 // NewDefaultServerConfig returns ServerConfig type object with default values.
@@ -123,7 +129,12 @@ type AuthConfig struct {
 
 // ToListener creates a net.Listener.
 func (sc *ServerConfig) ToListener(ctx context.Context) (net.Listener, error) {
-	listener, err := net.Listen("tcp", sc.Endpoint)
+	cfg, err := sc.getListenConfig() // See listen_config_*.go for platform-specific implementations
+	if err != nil {
+		return nil, err
+	}
+
+	listener, err := cfg.Listen(ctx, "tcp", sc.Endpoint)
 	if err != nil {
 		return nil, err
 	}

config/confighttp/server_test.go

@@ -12,6 +12,7 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"path/filepath"
+	"runtime"
 	"strconv"
 	"strings"
 	"testing"
@@ -1169,3 +1170,50 @@ func TestServerMiddlewaresFieldCompatibility(t *testing.T) {
 	assert.Len(t, serverConfig.Middlewares, 1)
 	assert.Equal(t, component.MustNewID("server_middleware"), serverConfig.Middlewares[0].ID)
 }
+
+func TestServerReusePort(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("skipping test: SO_REUSEPORT is not supported on windows")
+	}
+
+	tests := []struct {
+		name          string
+		reusePort     bool
+		expectedError bool
+	}{
+		{
+			name:          "ReusePort enabled",
+			reusePort:     true,
+			expectedError: false,
+		},
+		{
+			name:          "ReusePort disabled",
+			reusePort:     false,
+			expectedError: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			sc := &ServerConfig{
+				Endpoint:  "localhost:4318",
+				ReusePort: tt.reusePort,
+			}
+
+			ln1, err := sc.ToListener(t.Context())
+			require.NoError(t, err)
+			defer ln1.Close()
+
+			ln2, err := sc.ToListener(t.Context())
+			if tt.expectedError {
+				assert.Error(t, err)
+			} else {
+				assert.NoError(t, err)
+			}
+
+			if ln2 != nil {
+				ln2.Close()
+			}
+		})
+	}
+}