Better handler for --fast-sync-min-peers flag

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

@@ -1929,11 +1929,10 @@ private void issueOptionWarnings() {
             "--Xminer-remote-sealers-limit",
             "--Xminer-remote-sealers-hashrate-ttl"));
 
-    CommandLineUtils.checkOptionDependencies(
-        logger,
+    CommandLineUtils.failIfOptionDoesntMeetRequirement(
         commandLine,
-        "--sync-mode",
-        SyncMode.isFullSync(syncMode),
+        "--fast-sync-min-peers can't be used with FULL sync-mode",
+        !SyncMode.isFullSync(getDefaultSyncModeIfNotSet(syncMode)),
         singletonList("--fast-sync-min-peers"));
 
     if (!securityModuleName.equals(DEFAULT_SECURITY_MODULE)
@@ -1966,14 +1965,7 @@ private void issueOptionWarnings() {
   private void configure() throws Exception {
     checkPortClash();
 
-    syncMode =
-        Optional.ofNullable(syncMode)
-            .orElse(
-                genesisFile == null
-                        && !privacyOptionGroup.isPrivacyEnabled
-                        && Optional.ofNullable(network).map(NetworkName::canFastSync).orElse(false)
-                    ? SyncMode.FAST
-                    : SyncMode.FULL);
+    syncMode = getDefaultSyncModeIfNotSet(syncMode);
 
     ethNetworkConfig = updateNetworkConfig(network);
 
@@ -3291,4 +3283,14 @@ public static List<String> getJDKEnabledProtocols() {
       throw new RuntimeException(e);
     }
   }
+
+  private SyncMode getDefaultSyncModeIfNotSet(final SyncMode syncMode) {
+    return Optional.ofNullable(syncMode)
+        .orElse(
+            genesisFile == null
+                    && !privacyOptionGroup.isPrivacyEnabled
+                    && Optional.ofNullable(network).map(NetworkName::canFastSync).orElse(false)
+                ? SyncMode.FAST
+                : SyncMode.FULL);
+  }
 }

besu/src/main/java/org/hyperledger/besu/cli/util/CommandLineUtils.java

@@ -97,6 +97,20 @@ public static void checkMultiOptionDependencies(
     }
   }
 
+  public static void failIfOptionDoesntMeetRequirement(
+      final CommandLine commandLine,
+      final String errorMessage,
+      final boolean requirement,
+      final List<String> dependentOptionsNames) {
+    if (!requirement) {
+      final String affectedOptions = getAffectedOptions(commandLine, dependentOptionsNames);
+
+      if (!affectedOptions.isEmpty()) {
+        throw new CommandLine.ParameterException(commandLine, errorMessage);
+      }
+    }
+  }
+
   private static String getAffectedOptions(
       final CommandLine commandLine, final List<String> dependentOptionsNames) {
     return commandLine.getCommandSpec().options().stream()

besu/src/test/java/org/hyperledger/besu/cli/BesuCommandTest.java

@@ -2293,28 +2293,6 @@ public void privacyTlsOptionsRequiresPrivacyToBeEnabledToml() throws IOException
     assertThat(commandErrorOutput.toString(UTF_8)).isEmpty();
   }
 
-  @Test
-  public void fastSyncOptionsRequiresFastSyncModeToBeSet() {
-    parseCommand("--fast-sync-min-peers", "5");
-
-    verifyOptionsConstraintLoggerCall("--sync-mode", "--fast-sync-min-peers");
-
-    assertThat(commandOutput.toString(UTF_8)).isEmpty();
-    assertThat(commandErrorOutput.toString(UTF_8)).isEmpty();
-  }
-
-  @Test
-  public void fastSyncOptionsRequiresFastSyncModeToBeSetToml() throws IOException {
-    final Path toml = createTempFile("toml", "fast-sync-min-peers=5\n");
-
-    parseCommand("--config-file", toml.toString());
-
-    verifyOptionsConstraintLoggerCall("--sync-mode", "--fast-sync-min-peers");
-
-    assertThat(commandOutput.toString(UTF_8)).isEmpty();
-    assertThat(commandErrorOutput.toString(UTF_8)).isEmpty();
-  }
-
   @Test
   public void rpcApisPropertyWithInvalidEntryMustDisplayError() {
     parseCommand("--rpc-http-api", "BOB");
@@ -5350,4 +5328,11 @@ public void logsSuggestInstallingJemallocWhenEnvVarNotPresent() {
     verify(mockLogger)
         .info("jemalloc library not found, memory usage may be reduced by installing it");
   }
+
+  @Test
+  public void logWarnIfFastSyncMinPeersUsedWithFullSync() {
+    parseCommand("--sync-mode", "FULL", "--fast-sync-min-peers", "1");
+    assertThat(commandErrorOutput.toString(UTF_8))
+        .contains("--fast-sync-min-peers can't be used with FULL sync-mode");
+  }
 }