support env var to customize Accept header

api/client/client.go

@@ -6,6 +6,7 @@ import (
 	"net"
 	"net/http"
 	"net/url"
+	"os"
 
 	"github.com/pkg/errors"
 )
@@ -14,14 +15,17 @@ const (
 	MaxBodySize      int64 = 1 << 23 // 8MB default, WithMaxBodySize can override
 	MaxBodySizeState int64 = 1 << 29 // 512MB
 	MaxErrBodySize   int64 = 1 << 17 // 128KB
+
+	envNameOverrideAccept = "PRYSM_API_OVERRIDE_ACCEPT"
 )
 
 // Client is a wrapper object around the HTTP client.
 type Client struct {
-	hc          *http.Client
-	baseURL     *url.URL
-	token       string
-	maxBodySize int64
+	hc           *http.Client
+	baseURL      *url.URL
+	token        string
+	maxBodySize  int64
+	reqOverrides []ReqOption
 }
 
 // NewClient constructs a new client with the provided options (ex WithTimeout).
@@ -40,9 +44,21 @@ func NewClient(host string, opts ...ClientOpt) (*Client, error) {
 	for _, o := range opts {
 		o(c)
 	}
+	c.appendAcceptOverride()
 	return c, nil
 }
 
+// appendAcceptOverride enables the Accept header to be customized at runtime via an environment variable.
+// This is specified as an env var because it is a niche option that prysm may use for performance testing or debugging
+// bug which users are unlikely to need. Using an env var keeps the set of user-facing flags cleaner.
+func (c *Client) appendAcceptOverride() {
+	if accept := os.Getenv(envNameOverrideAccept); accept != "" {
+		c.reqOverrides = append(c.reqOverrides, func(req *http.Request) {
+			req.Header.Set("Accept", accept)
+		})
+	}
+}
+
 // Token returns the bearer token used for jwt authentication
 func (c *Client) Token() string {
 	return c.token
@@ -55,6 +71,9 @@ func (c *Client) BaseURL() *url.URL {
 
 // Do execute the request against the http client
 func (c *Client) Do(req *http.Request) (*http.Response, error) {
+	for _, o := range c.reqOverrides {
+		o(req)
+	}
 	return c.hc.Do(req)
 }
 
@@ -87,7 +106,7 @@ func (c *Client) Get(ctx context.Context, path string, opts ...ReqOption) ([]byt
 	for _, o := range opts {
 		o(req)
 	}
-	r, err := c.hc.Do(req)
+	r, err := c.Do(req)
 	if err != nil {
 		return nil, err
 	}

api/client/client_test.go

@@ -1,7 +1,10 @@
 package client
 
 import (
+	"net/http"
+	"net/http/httptest"
 	"net/url"
+	"os"
 	"testing"
 
 	"github.com/OffchainLabs/prysm/v6/testing/require"
@@ -46,3 +49,23 @@ func TestBaseURL(t *testing.T) {
 	require.Equal(t, "www.offchainlabs.com", cl.BaseURL().Hostname())
 	require.Equal(t, "3500", cl.BaseURL().Port())
 }
+
+func TestAcceptOverride(t *testing.T) {
+	name := "TestAcceptOverride"
+	orig := os.Getenv(envNameOverrideAccept)
+	defer func() {
+		os.Setenv(envNameOverrideAccept, orig)
+	}()
+	os.Setenv(envNameOverrideAccept, name)
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		require.Equal(t, name, r.Header.Get("Accept"))
+		w.WriteHeader(200)
+		w.Write([]byte("ok"))
+	}))
+	defer srv.Close()
+	c, err := NewClient(srv.URL)
+	require.NoError(t, err)
+	b, err := c.Get(t.Context(), "/test")
+	require.NoError(t, err)
+	require.Equal(t, "ok", string(b))
+}

changelog/kasey_customize-http-accept-header.md

@@ -0,0 +1,2 @@
+## Added
+- env var to force beacon api client to use a custom accept header (eg force json responses).