gateway: fix CORs headers

fixes #5138 -- always add user-agent to access-control-allow-headers.
fixes #5888 -- same with content-type.
fixes #5892 -- extend user-provided headers instead of overriding them.

License: MIT
Signed-off-by: Steven Allen <steven@stebalien.com>

core/corehttp/gateway.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"net"
 	"net/http"
+	"sort"
 
 	version "github.com/ipfs/go-ipfs"
 	core "github.com/ipfs/go-ipfs/core"
@@ -18,6 +19,26 @@ type GatewayConfig struct {
 	PathPrefixes []string
 }
 
+// A helper function to clean up a set of headers:
+// 1. Canonicalizes.
+// 2. Deduplicates.
+// 3. Sorts.
+func cleanHeaderSet(headers []string) []string {
+	// Deduplicate and canonicalize.
+	m := make(map[string]struct{}, len(headers))
+	for _, h := range headers {
+		m[http.CanonicalHeaderKey(h)] = struct{}{}
+	}
+	result := make([]string, 0, len(m))
+	for k := range m {
+		result = append(result, k)
+	}
+
+	// Sort
+	sort.Strings(result)
+	return result
+}
+
 func GatewayOption(writable bool, paths ...string) ServeOption {
 	return func(n *core.IpfsNode, _ net.Listener, mux *http.ServeMux) (*http.ServeMux, error) {
 		cfg, err := n.Repo.Config()
@@ -30,8 +51,42 @@ func GatewayOption(writable bool, paths ...string) ServeOption {
 			return nil, err
 		}
 
+		headers := make(map[string][]string, len(cfg.Gateway.HTTPHeaders))
+		for h, v := range cfg.Gateway.HTTPHeaders {
+			headers[h] = v
+		}
+
+		// Hard-coded headers.
+		const ACAHeadersName = "Access-Control-Allow-Headers"
+		const ACEHeadersName = "Access-Control-Expose-Headers"
+		const ACAOriginName = "Access-Control-Allow-Origin"
+		const ACAMethodsName = "Access-Control-Allow-Methods"
+
+		if _, ok := headers[ACAOriginName]; !ok {
+			// Default to *all*
+			headers[ACAOriginName] = []string{"*"}
+		}
+		if _, ok := headers[ACAMethodsName]; !ok {
+			// Default to GET
+			headers[ACAMethodsName] = []string{"GET"}
+		}
+
+		headers[ACAHeadersName] = cleanHeaderSet(
+			append([]string{
+				"Content-Range",
+				"Content-Type",
+				"User-Agent",
+				"X-Requested-With",
+			}, headers[ACAHeadersName]...))
+
+		headers[ACEHeadersName] = cleanHeaderSet(
+			append([]string{
+				"X-Chunked-Output",
+				"X-Stream-Output",
+			}, headers[ACEHeadersName]...))
+
 		gateway := newGatewayHandler(n, GatewayConfig{
-			Headers:      cfg.Gateway.HTTPHeaders,
+			Headers:      headers,
 			Writable:     writable,
 			PathPrefixes: cfg.Gateway.PathPrefixes,
 		}, api)

core/corehttp/gateway_handler.go

@@ -184,19 +184,6 @@ func (i *gatewayHandler) getOrHeadHandler(ctx context.Context, w http.ResponseWr
 	w.Header().Set("X-IPFS-Path", urlPath)
 	w.Header().Set("Etag", etag)
 
-	// set 'allowed' headers
-	// & expose those headers
-	var allowedHeadersArr = []string{
-		"Content-Range",
-		"X-Chunked-Output",
-		"X-Stream-Output",
-	}
-
-	var allowedHeaders = strings.Join(allowedHeadersArr, ", ")
-
-	w.Header().Set("Access-Control-Allow-Headers", allowedHeaders)
-	w.Header().Set("Access-Control-Expose-Headers", allowedHeaders)
-
 	// Suborigin header, sandboxes apps from each other in the browser (even
 	// though they are served from the same gateway domain).
 	//