Big changes

server/internal/api/update.go

@@ -3,6 +3,7 @@ package api
 import (
 	"context"
 	"net/http"
+	"strconv"
 	"strings"
 	"sync"
 	"time"
@@ -11,6 +12,11 @@ import (
 	"github.com/ponzischeme89/memby/server/internal/store"
 )
 
+// membyProtocolVersion changes only when the client/server wire contract is no longer
+// mutually compatible. App release versions remain independent and are handled by the
+// update policy.
+const membyProtocolVersion = 1
+
 // updatePolicyCache keeps the policy in memory. It is read on every home request, and a
 // database round trip per home load to answer "nothing to say" would be wasteful.
 type updatePolicyCache struct {
@@ -63,6 +69,22 @@ func clientVersion(r *http.Request) string {
 	return strings.TrimSpace(r.Header.Get("X-Memby-Version"))
 }
 
+func clientProtocol(r *http.Request) string {
+	return strings.TrimSpace(r.Header.Get("X-Memby-Protocol"))
+}
+
+func compatibilityFor(r *http.Request) (bool, string) {
+	reported, err := strconv.Atoi(clientProtocol(r))
+	if err != nil || reported != membyProtocolVersion {
+		if clientProtocol(r) == "" {
+			return false, "This Memby app is too old to verify compatibility with the server. Update the app."
+		}
+		return false, "Memby app/server mismatch: app protocol " + clientProtocol(r) +
+			", server protocol " + strconv.Itoa(membyProtocolVersion) + ". Update the app or server."
+	}
+	return true, ""
+}
+
 // handleUpdate answers the client's version check.
 //
 // Its own endpoint rather than a field on /v1/home: the home payload is cached per user,