container: remove global msg

This frees all container instances of side effects.

Signed-off-by: Ophestra <cat@gensokyo.uk>

container/msg.go

@@ -23,45 +23,88 @@ func GetErrorMessage(err error) (string, bool) {
 	return e.Message(), true
 }
 
+// Msg is used for package-wide verbose logging.
 type Msg interface {
+	// GetLogger returns the address of the underlying [log.Logger].
+	GetLogger() *log.Logger
+
+	// IsVerbose atomically loads and returns whether [Msg] has verbose logging enabled.
 	IsVerbose() bool
+	// SwapVerbose atomically stores a new verbose state and returns the previous value held by [Msg].
+	SwapVerbose(verbose bool) bool
+	// Verbose passes its argument to the Println method of the underlying [log.Logger] if IsVerbose returns true.
 	Verbose(v ...any)
+	// Verbosef passes its argument to the Printf method of the underlying [log.Logger] if IsVerbose returns true.
 	Verbosef(format string, v ...any)
 
-	Suspend()
+	// Suspend causes the embedded [Suspendable] to withhold writes to its downstream [io.Writer].
+	// Suspend returns false and is a noop if called between calls to Suspend and Resume.
+	Suspend() bool
+
+	// Resume dumps the entire buffer held by the embedded [Suspendable] and stops withholding future writes.
+	// Resume returns false and is a noop if a call to Suspend does not precede it.
 	Resume() bool
+
+	// BeforeExit runs implementation-specific cleanup code, and optionally prints warnings.
+	// BeforeExit is called before [os.Exit].
 	BeforeExit()
 }
 
-type DefaultMsg struct{ inactive atomic.Bool }
+// defaultMsg is the default implementation of the [Msg] interface.
+// The zero value is not safe for use. Callers should use the [NewMsg] function instead.
+type defaultMsg struct {
+	verbose atomic.Bool
 
-func (msg *DefaultMsg) IsVerbose() bool { return true }
-func (msg *DefaultMsg) Verbose(v ...any) {
-	if !msg.inactive.Load() {
-		log.Println(v...)
+	logger *log.Logger
+	Suspendable
+}
+
+// NewMsg initialises a downstream [log.Logger] for a new [Msg].
+// The [log.Logger] should no longer be configured after NewMsg returns.
+// If downstream is nil, a new logger is initialised in its place.
+func NewMsg(downstream *log.Logger) Msg {
+	if downstream == nil {
+		downstream = log.New(log.Writer(), "container: ", 0)
+	}
+
+	m := defaultMsg{logger: downstream}
+	m.Suspendable.Downstream = downstream.Writer()
+	downstream.SetOutput(&m.Suspendable)
+	return &m
+}
+
+func (msg *defaultMsg) GetLogger() *log.Logger { return msg.logger }
+
+func (msg *defaultMsg) IsVerbose() bool               { return msg.verbose.Load() }
+func (msg *defaultMsg) SwapVerbose(verbose bool) bool { return msg.verbose.Swap(verbose) }
+func (msg *defaultMsg) Verbose(v ...any) {
+	if msg.verbose.Load() {
+		msg.logger.Println(v...)
 	}
 }
-func (msg *DefaultMsg) Verbosef(format string, v ...any) {
-	if !msg.inactive.Load() {
-		log.Printf(format, v...)
+func (msg *defaultMsg) Verbosef(format string, v ...any) {
+	if msg.verbose.Load() {
+		msg.logger.Printf(format, v...)
 	}
 }
 
-func (msg *DefaultMsg) Suspend()     { msg.inactive.Store(true) }
-func (msg *DefaultMsg) Resume() bool { return msg.inactive.CompareAndSwap(true, false) }
-func (msg *DefaultMsg) BeforeExit()  {}
+// Resume calls [Suspendable.Resume] and prints a message if buffer was filled
+// between calls to [Suspendable.Suspend] and Resume.
+func (msg *defaultMsg) Resume() bool {
+	resumed, dropped, _, err := msg.Suspendable.Resume()
+	if err != nil {
+		// probably going to result in an error as well, so this message is as good as unreachable
+		msg.logger.Printf("cannot dump buffer on resume: %v", err)
+	}
+	if resumed && dropped > 0 {
+		msg.logger.Printf("dropped %d bytes while output is suspended", dropped)
+	}
+	return resumed
+}
 
-// msg is the [Msg] implemented used by all exported [container] functions.
-var msg Msg = new(DefaultMsg)
-
-// GetOutput returns the current active [Msg] implementation.
-func GetOutput() Msg { return msg }
-
-// SetOutput replaces the current active [Msg] implementation.
-func SetOutput(v Msg) {
-	if v == nil {
-		msg = new(DefaultMsg)
-	} else {
-		msg = v
+// BeforeExit prints a message if called between calls to [Suspendable.Suspend] and Resume.
+func (msg *defaultMsg) BeforeExit() {
+	if msg.Resume() {
+		msg.logger.Printf("beforeExit reached on suspended output")
 	}
 }