Only log fault that configureDefaultLoggingSubsystem has not been called once

Sources/SKLogging/LoggingScope.swift

@@ -16,7 +16,11 @@ import Foundation
 public final class LoggingScope {
 
   /// The name of the default logging subsystem if no task-local value is set.
-  fileprivate static let defaultSubsystem: ThreadSafeBox<String?> = .init(initialValue: nil)
+  private static let defaultSubsystem = ThreadSafeBox<String?>(initialValue: nil)
+
+  /// Whether we have logged a fault that `subsystem` has been accessed without calling
+  /// `configureDefaultLoggingSubsystem` first.
+  private static let hasLoggedNoSubsystemConfiguredFault = AtomicBool(initialValue: false)
 
   /// The name of the current logging subsystem or `nil` if no logging scope is set.
   @TaskLocal fileprivate static var _subsystem: String?
@@ -31,9 +35,16 @@ public final class LoggingScope {
     } else if let defaultSubsystem = defaultSubsystem.value {
       return defaultSubsystem
     } else {
-      Logger(subsystem: "org.swift.sklogging", category: "configuration")
-        .log(level: .fault, "default logging subsystem was not configured before first use")
-      return "org.swift"
+      if !hasLoggedNoSubsystemConfiguredFault.setAndGet(newValue: true) {
+        Logger(subsystem: "default", category: "sklogging")
+          .fault(
+            """
+            Default logging subsystem was not configured before first use of SKLogging. \
+            Ensure that configureDefaultLoggingSubsystem is called before the first log call.
+            """
+          )
+      }
+      return "default"
     }
   }
 
@@ -42,8 +53,16 @@ public final class LoggingScope {
     return _scope ?? "default"
   }
 
+  /// Set the logging subsystem that is used task local subsystem is set using `withLoggingSubsystemAndScope`.
+  ///
+  /// Must be called before the first log call.
   public static func configureDefaultLoggingSubsystem(_ subsystem: String) {
-    LoggingScope.defaultSubsystem.withLock { $0 = subsystem }
+    LoggingScope.defaultSubsystem.withLock { defaultSubsystem in
+      if let defaultSubsystem, defaultSubsystem != subsystem {
+        logger.log("Changing default log subsystem from \(defaultSubsystem) to \(subsystem)")
+      }
+      defaultSubsystem = subsystem
+    }
   }
 }
 

Sources/ToolsProtocolsCAtomics/include/ToolsProtocolsCAtomics.h

@@ -17,6 +17,7 @@
 #include <stdint.h>
 #include <sys/types.h>
 #include <stdlib.h>
+#include <stdatomic.h>
 
 typedef struct {
   _Atomic(uint32_t) value;
@@ -32,6 +33,10 @@ static inline uint32_t atomic_uint32_get(CAtomicUInt32 *_Nonnull atomic) {
   return atomic->value;
 }
 
+static inline uint32_t atomic_uint32_get_and_set(CAtomicUInt32 *_Nonnull atomic, uint32_t newValue) {
+  return __c11_atomic_exchange(&atomic->value, newValue, __ATOMIC_SEQ_CST);
+}
+
 static inline void atomic_uint32_set(CAtomicUInt32 *_Nonnull atomic, uint32_t newValue) {
   atomic->value = newValue;
 }

Sources/ToolsProtocolsSwiftExtensions/Atomics.swift

@@ -32,6 +32,11 @@ import ToolsProtocolsCAtomics
       atomic_uint32_set(atomic, newValue ? 1 : 0)
     }
   }
+
+  /// Sets the boolean to the new value and returns the previous value.
+  @_spi(SourceKitLSP) public func setAndGet(newValue: Bool) -> Bool {
+    return atomic_uint32_get_and_set(atomic, newValue ? 1 : 0) != 0
+  }
 }
 
 @_spi(SourceKitLSP) public final class AtomicUInt8: Sendable {