Detect and prevent 'skip_dir' / 'skip_file' rules shadowing 'sync_list' inclusions

This change adds validation to client-side filtering configuration to detect when 'sync_list' inclusion rules are rendered non-viable by 'skip_dir' or 'skip_file'.

The client now checks whether any 'sync_list' include paths would be excluded by the active 'skip_dir' or 'skip_file' rules, using the same runtime exclusion logic as the sync engine. If such a conflict is detected, the client reports a clear configuration error and exits early.

This prevents contradictory filtering configurations where explicitly included files or folders can never be synced, reducing confusion and avoiding unintended or unsafe sync behaviour.

src/clientSideFiltering.d

@@ -99,8 +99,19 @@ class ClientSideFiltering {
 		if (appConfig.getValueBool("skip_dotfiles")) {
 			setSkipDotfiles();
 		}
+
+		// Validate 'sync_list' include rules are not shadowed by 'skip_file' entries
+		if (!validateSyncListNotShadowedBySkipFile()) {
+			return false;
+		}
 		
-		// All configured OK
+		// Validate 'sync_list' include rules are not shadowed by 'skip_dir' entries
+		if (!validateSyncListNotShadowedBySkipDir()) {
+			// The application configuration is invalid .. 'skip_dir' is shadowing paths included by 'sync_list'
+			return false;
+		}
+		
+		// Client Side Filtering has been configured correctly
 		return true;
 	}
 	
@@ -877,4 +888,86 @@ class ClientSideFiltering {
 			return false;
 		}
 	}
+	
+	// Validate that 'sync_list' *include* rules are not rendered non-viable by 'skip_dir' entries.
+	// If an include rule would be excluded by 'skip_dir' evaluation, it is "shadowed" by that entry.
+	bool validateSyncListNotShadowedBySkipDir() {
+		// No sync_list include rules loaded => nothing to validate
+		if (syncListIncludePathsOnly is null || syncListIncludePathsOnly.empty) return true;
+
+		// No skip_dir configured => nothing to validate
+		if (appConfig.getValueString("skip_dir").empty) return true;
+
+		string[] shadowedRules;
+
+		foreach (rule; syncListIncludePathsOnly) {
+			// syncListIncludePathsOnly should only contain include rules, but be defensive.
+			if (rule.empty) continue;
+			if (rule[0] == '!' || rule[0] == '-') continue;
+
+			// Use the *actual* runtime skip_dir evaluation logic (strict/non-strict)
+			// so the check matches real behaviour.
+			if (isDirNameExcluded(rule)) {
+				shadowedRules ~= rule;
+			}
+		}
+
+		if (!shadowedRules.empty) {
+			addLogEntry();
+			addLogEntry("ERROR: Invalid Client Side Filtering configuration detected.", ["info", "notify"]);
+			addLogEntry("       One or more 'sync_list' inclusion rules are shadowed by 'skip_dir' and will never be viable.", ["info", "notify"]);
+			foreach (r; shadowedRules) {
+				addLogEntry("       Shadowed 'sync_list' rule: " ~ r, ["info", "notify"]);
+			}
+			addLogEntry("       Fix: remove or narrow the conflicting 'skip_dir' entry/entries, or adjust your 'sync_list' rules.", ["info", "notify"]);
+			addLogEntry("       See the 'skip_dir' documentation for correct usage and examples.", ["info", "notify"]);
+			addLogEntry();
+			return false;
+		}
+
+		return true;
+	}
+	
+	// Validate that 'sync_list' *include* rules are not rendered non-viable by 'skip_file' entries.
+	// If an include rule would be excluded by 'skip_file' evaluation, it is "shadowed" by that entry.
+	bool validateSyncListNotShadowedBySkipFile() {
+		// No sync_list include rules loaded => nothing to validate
+		if (syncListIncludePathsOnly is null || syncListIncludePathsOnly.empty) return true;
+
+		// No skip_file configured => nothing to validate
+		if (appConfig.getValueString("skip_file").empty) return true;
+
+		string[] shadowedRules;
+
+		foreach (rule; syncListIncludePathsOnly) {
+			// Defensive: ignore empty or explicitly negative rules
+			if (rule.empty) continue;
+			if (rule[0] == '!' || rule[0] == '-') continue;
+
+			// Only validate file-intent rules:
+			// - If it ends with '/', treat as a directory include and do not apply skip_file shadow validation.
+			//   (Users commonly include folders; skip_file patterns like '*.tmp' should not invalidate that.)
+			if (rule.length > 1 && rule[$ - 1] == '/') continue;
+
+			// Use the *actual* runtime skip_file evaluation logic so this check matches real behaviour.
+			if (isFileNameExcluded(rule)) {
+				shadowedRules ~= rule;
+			}
+		}
+
+		if (!shadowedRules.empty) {
+			addLogEntry();
+			addLogEntry("ERROR: Invalid Client Side Filtering configuration detected.", ["info", "notify"]);
+			addLogEntry("       One or more 'sync_list' inclusion rules are shadowed by 'skip_file' and will never be viable.", ["info", "notify"]);
+			foreach (r; shadowedRules) {
+				addLogEntry("       Shadowed 'sync_list' rule: " ~ r, ["info", "notify"]);
+			}
+			addLogEntry("       Fix: remove or narrow the conflicting 'skip_file' entry/entries, or adjust your 'sync_list' rules.", ["info", "notify"]);
+			addLogEntry("       See the 'skip_file' documentation for correct usage and examples.", ["info", "notify"]);
+			addLogEntry();
+			return false;
+		}
+
+		return true;
+	}
 }