prevent startup if PoA and snap/checkpoint

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 ### Breaking Changes
 - RocksDB database metadata format has changed to be more expressive, the migration of an existing metadata file to the new format is automatic at startup. Before performing a downgrade to a previous version it is mandatory to revert to the original format using the subcommand `besu --data-path=/path/to/besu/datadir storage revert-metadata v2-to-v1`.
+- PoA networks won't start with SNAP or CHECKPOINT sync (previously Besu would start with this config but quietly fail to sync, so it's now more obvious that it won't work) [#6625](https://github.com/hyperledger/besu/pull/6625)
 
 ### Upcoming Breaking Changes
 

besu/src/main/java/org/hyperledger/besu/cli/BesuCommand.java

@@ -1606,6 +1606,16 @@ private GenesisConfigOptions readGenesisConfigOptions() {
       throw new ParameterException(
           this.commandLine, "Unable to load genesis file. " + e.getCause());
     }
+    if (genesisConfigOptions.isPoa()) {
+      final String errorSuffix = "can't be used with PoA networks";
+      if (SyncMode.CHECKPOINT.equals(syncMode) || SyncMode.X_CHECKPOINT.equals(syncMode)) {
+        throw new ParameterException(
+            commandLine, String.format("%s %s", "Checkpoint sync", errorSuffix));
+      }
+      if (syncMode == SyncMode.SNAP || syncMode == SyncMode.X_SNAP) {
+        throw new ParameterException(commandLine, String.format("%s %s", "Snap sync", errorSuffix));
+      }
+    }
     return genesisConfigOptions;
   }
 
@@ -1950,10 +1960,10 @@ private PrivacyParameters privacyParameters() {
       if (syncMode == SyncMode.FAST) {
         throw new ParameterException(commandLine, String.format("%s %s", "Fast sync", errorSuffix));
       }
-      if (syncMode == SyncMode.SNAP) {
+      if (syncMode == SyncMode.SNAP || syncMode == SyncMode.X_SNAP) {
         throw new ParameterException(commandLine, String.format("%s %s", "Snap sync", errorSuffix));
       }
-      if (syncMode == SyncMode.CHECKPOINT) {
+      if (syncMode == SyncMode.CHECKPOINT || syncMode == SyncMode.X_CHECKPOINT) {
         throw new ParameterException(
             commandLine, String.format("%s %s", "Checkpoint sync", errorSuffix));
       }