Convert to HTTP handlers inside outside of the main package

Previously, the top-level package exported functions with the
signature `func(*util.Context) util.StatusError`, and the `main`
package was responsible for wrapping these and producing normal
HTTP handlers. However, the Google Cloud Functions execution
environment expects to be able to invoke HTTP handlers which are
exported directly from the top-level package.

We move the logic of converting these inner handlers into HTTP
handlers to the `internal/util` package, allowing us to expose
HTTP handlers from the top-level package

functions/challenge.go

@@ -8,7 +8,9 @@ import (
 )
 
 // ChallengeHandler is a handler for the /challenge endpoint.
-func ChallengeHandler(ctx *util.Context) util.StatusError {
+var ChallengeHandler = util.MakeHTTPHandler(challengeHandler)
+
+func challengeHandler(ctx *util.Context) util.StatusError {
 	if err := util.ValidateRequestMethod(ctx, "GET", ""); err != nil {
 		return err
 	}

functions/cmd/main.go

@@ -1,20 +1,17 @@
 package main
 
 import (
-	"encoding/json"
 	"fmt"
 	"log"
-	"net/http"
 	"os"
 
 	"functions"
-	"functions/internal/util"
 
 	"github.com/GoogleCloudPlatform/functions-framework-go/funcframework"
 )
 
 func main() {
-	funcframework.RegisterHTTPFunction("/challenge", makeHTTPHandler(functions.ChallengeHandler))
+	funcframework.RegisterHTTPFunction("/challenge", functions.ChallengeHandler)
 	// Use PORT environment variable, or default to 8080.
 	port := "8080"
 	if envPort := os.Getenv("PORT"); envPort != "" {
@@ -26,30 +23,3 @@ func main() {
 		log.Fatalf("funcframework.Start: %v\n", err)
 	}
 }
-
-func makeHTTPHandler(handler func(ctx *util.Context) util.StatusError) func(http.ResponseWriter, *http.Request) {
-	return func(w http.ResponseWriter, r *http.Request) {
-		ctx, err := util.NewContext(w, r)
-		if err != nil {
-			writeStatusError(w, r, err)
-			return
-		}
-
-		if err := handler(&ctx); err != nil {
-			writeStatusError(w, r, err)
-		}
-	}
-}
-
-func writeStatusError(w http.ResponseWriter, r *http.Request, err util.StatusError) {
-	type response struct {
-		Message string `json:"message"`
-	}
-
-	w.Header().Set("Content-Type", "application/json; charset=utf-8")
-	w.WriteHeader(err.HTTPStatusCode())
-	json.NewEncoder(w).Encode(response{Message: err.Message()})
-
-	log.Printf("[%v %v %v]: responding with error code %v and message \"%v\"",
-		r.RemoteAddr, r.Method, r.URL, err.HTTPStatusCode(), err.Message())
-}

functions/internal/util/handler.go

@@ -0,0 +1,43 @@
+package util
+
+import (
+	"encoding/json"
+	"log"
+	"net/http"
+)
+
+// Handler is a handler for a request to this service. Use MakeHTTPHandler to
+// wrap a Handler with the logic necessary to produce a handler which can be
+// registered with the "net/http" package.
+type Handler = func(ctx *Context) StatusError
+
+// MakeHTTPHandler wraps a Handler, producing a handler which can be registered
+// with the "net/http" package. The returned handler is responsible for:
+//  - Constructing a *Context
+//  - Converting any errors into an HTTP response
+func MakeHTTPHandler(handler func(ctx *Context) StatusError) func(http.ResponseWriter, *http.Request) {
+	return func(w http.ResponseWriter, r *http.Request) {
+		ctx, err := NewContext(w, r)
+		if err != nil {
+			writeStatusError(w, r, err)
+			return
+		}
+
+		if err := handler(&ctx); err != nil {
+			writeStatusError(w, r, err)
+		}
+	}
+}
+
+func writeStatusError(w http.ResponseWriter, r *http.Request, err StatusError) {
+	type response struct {
+		Message string `json:"message"`
+	}
+
+	w.Header().Set("Content-Type", "application/json; charset=utf-8")
+	w.WriteHeader(err.HTTPStatusCode())
+	json.NewEncoder(w).Encode(response{Message: err.Message()})
+
+	log.Printf("[%v %v %v]: responding with error code %v and message \"%v\" (error: %v)",
+		r.RemoteAddr, r.Method, r.URL, err.HTTPStatusCode(), err.Message(), err)
+}