refactor(oohelperd): better distinguish different helpers (#434)

Part of https://github.com/ooni/probe/issues/1733

internal/cmd/oohelperd/internal/websteps/websteps.go

@@ -0,0 +1,67 @@
+// Package websteps implements the websteps test helper.
+//
+// See https://github.com/ooni/spec/blob/master/backends/th-007-nwcth.md
+package websteps
+
+import (
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/netx/archival"
+	"github.com/ooni/probe-cli/v3/internal/iox"
+	"github.com/ooni/probe-cli/v3/internal/runtimex"
+	"github.com/ooni/probe-cli/v3/internal/version"
+)
+
+// newfailure is a convenience shortcut to save typing
+var newfailure = archival.NewFailure
+
+// maxAcceptableBody is _at the same time_ the maximum acceptable body for incoming
+// API requests and the maximum acceptable body when fetching arbitrary URLs. See
+// https://github.com/ooni/probe/issues/1727 for statistics regarding the test lists
+// including the empirical CDF of the body size for test lists URLs.
+const maxAcceptableBody = 1 << 24
+
+// Handler implements the Web Connectivity test helper HTTP API.
+type Handler struct {
+	Config *Config
+}
+
+// ServeHTTP implements http.Handler.ServeHTTP.
+func (h *Handler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
+	w.Header().Add("Server", fmt.Sprintf(
+		"oohelperd/%s ooniprobe-engine/%s", version.Version, version.Version,
+	))
+	if req.Method != "POST" {
+		w.WriteHeader(400)
+		return
+	}
+	reader := &io.LimitedReader{R: req.Body, N: maxAcceptableBody}
+	data, err := iox.ReadAllContext(req.Context(), reader)
+	if err != nil {
+		w.WriteHeader(400)
+		return
+	}
+	var creq CtrlRequest
+	if err := json.Unmarshal(data, &creq); err != nil {
+		w.WriteHeader(400)
+		return
+	}
+	cresp, err := Measure(req.Context(), &creq, h.Config)
+	if err != nil {
+		if err == ErrInternalServer {
+			w.WriteHeader(500)
+			return
+		}
+		w.WriteHeader(400)
+		return
+	}
+	// We assume that the following call cannot fail because it's a
+	// clearly serializable data structure.
+	data, err = json.Marshal(cresp)
+	runtimex.PanicOnError(err, "json.Marshal failed")
+	w.Header().Add("Content-Type", "application/json")
+	w.Write(data)
+}