http: replace host in chromium WSURL with own host

We are making chromium listen in 0.0.0.0, it is reasonable to think that
however a client reached us, that's how it will reach chromium.

http/http.go

@@ -3,7 +3,9 @@ package http
 import (
 	"encoding/json"
 	"log/slog"
+	"net"
 	"net/http"
+	"net/url"
 
 	"github.com/grafana/crocochrome"
 )
@@ -49,6 +51,18 @@ func (s *Server) Create(rw http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Chromium is guaranteed to run on the same host as this server, and we make it listen in 0.0.0.0. However,
+	// chromium always returns `localhost` as the URL host. Here we replace `localhost` in that URL with the host being
+	// used to reach this service.
+	newURL, err := replaceHost(session.ChromiumVersion.WebSocketDebuggerURL, r.Host)
+	if err != nil {
+		s.logger.Error("replacing chromium url with host header", "err", err)
+		rw.WriteHeader(http.StatusInternalServerError)
+		return
+	}
+
+	session.ChromiumVersion.WebSocketDebuggerURL = newURL
+
 	rw.Header().Add("content-type", "application/json")
 	_ = json.NewEncoder(rw).Encode(session)
 }
@@ -66,3 +80,26 @@ func (s *Server) Delete(rw http.ResponseWriter, r *http.Request) {
 		return
 	}
 }
+
+// replaceHost returns a new url with its hostname replaced with host. The port is kept as it is.
+func replaceHost(urlStr, host string) (string, error) {
+	parsedURL, err := url.Parse(urlStr)
+	if err != nil {
+		return "", err
+	}
+
+	_, port, err := net.SplitHostPort(parsedURL.Host)
+	if err != nil {
+		return "", err
+	}
+
+	// Get rid of the port if a port is present in host.
+	host, _, err = net.SplitHostPort(host)
+	if err != nil {
+		return "", err
+	}
+
+	parsedURL.Host = net.JoinHostPort(host, port)
+
+	return parsedURL.String(), nil
+}

http/http_test.go

@@ -5,6 +5,7 @@ import (
 	"log/slog"
 	"net/http"
 	"net/http/httptest"
+	"net/url"
 	"os"
 	"testing"
 
@@ -49,4 +50,51 @@ func TestHTTP(t *testing.T) {
 			t.Fatalf("webSocketDebuggerUrl is unexpectedly empty")
 		}
 	})
+
+	t.Run("replaces chromium url with host header", func(t *testing.T) {
+		hb := testutil.NewHeartbeat(t)
+		port := testutil.HTTPInfo(t, testutil.ChromiumVersion)
+		cc := crocochrome.New(logger, crocochrome.Options{ChromiumPath: hb.Path, ChromiumPort: port})
+		api := crocohttp.New(logger, cc)
+
+		server := httptest.NewServer(api)
+		t.Cleanup(server.Close)
+
+		req, err := http.NewRequest(http.MethodPost, server.URL+"/sessions", nil)
+		if err != nil {
+			t.Fatalf("building http request: %v", err)
+		}
+
+		resp, err := http.DefaultClient.Do(req)
+		if err != nil {
+			t.Fatalf("making request: %v", err)
+		}
+
+		defer resp.Body.Close()
+
+		var response struct {
+			ID              string `json:"ID"`
+			ChromiumVersion struct {
+				WebSocketDebuggerURL string `json:"webSocketDebuggerUrl"`
+			} `json:"chromiumVersion"`
+		}
+
+		err = json.NewDecoder(resp.Body).Decode(&response)
+		if err != nil {
+			t.Fatalf("decoding response: %v", err)
+		}
+
+		parsedURL, err := url.Parse(response.ChromiumVersion.WebSocketDebuggerURL)
+		if err != nil {
+			t.Fatalf("parsing response url: %v", err)
+		}
+
+		if parsedURL.Hostname() != "127.0.0.1" {
+			t.Fatalf("expected returned url to have 127.0.0.1 as host, got %q", response.ChromiumVersion.WebSocketDebuggerURL)
+		}
+
+		if parsedURL.Port() != "9222" {
+			t.Fatalf("expected returned url to have 9222 as port, got %q", response.ChromiumVersion.WebSocketDebuggerURL)
+		}
+	})
 }