Option to set logging verbosity and override handler

vips/govips.go

@@ -77,6 +77,9 @@ func Startup(config *Config) {
 	cName := C.CString("govips")
 	defer freeCString(cName)
 
+	// Initialize govips logging handler and verbosity filter to historical default
+	LoggingSettings(nil, logLevelInfo)
+
 	// Override default glib logging handler to intercept logging messages
 	C.vips_set_logging_handler()
 

vips/logging.go

@@ -16,12 +16,46 @@ const (
 	logLevelDebug    logLevel = C.G_LOG_LEVEL_DEBUG
 )
 
+// Two global variables which keep state of the current logging handler
+// function and desired verbosity for logging. Set by LoggingSettings()
+var (
+	currentLoggingHandlerFunction LoggingHandlerFunction
+	currentLoggingVerbosity       logLevel
+)
+
+// govipsLoggingHandler is the private bridge function exported to the C library
+// and called for each logging message. It will call govipsLog which in turn will
+// filter based on verbosity and direct the messages to the currently chosen
+// LoggingHandlerFunction.
 //export govipsLoggingHandler
 func govipsLoggingHandler(messageDomain *C.char, messageLevel C.int, message *C.char) {
 	govipsLog(C.GoString(messageDomain), logLevel(messageLevel), C.GoString(message))
 }
 
-func govipsLog(messageDomain string, messageLevel logLevel, message string) {
+// LoggingHandlerFunction is a function which will be called for each log message.
+// By default, govips sends logging messages to os.Stderr. If you want to log elsewhere
+// such as to a file or to a state variable which you inspect yourself, define a new
+// logging handler function and set it via LoggingSettings().
+type LoggingHandlerFunction func(messageDomain string, messageLevel logLevel, message string)
+
+// LoggingSettings sets the logging handler and logging verbosity for govips.
+// The handler function is the function which will be called for each log message.
+// You can define one yourself to log somewhere else besides the default (stderr).
+// Define nil to use standard logging handler.
+// Verbosity is the minimum logLevel you want to pass forward. Default is logLevelInfo
+// due to backwards compatibility but it's very verbose for a library.
+// Suggest setting it to at least logLevelWarning. Use logLevelDebug for debugging.
+func LoggingSettings(handler LoggingHandlerFunction, verbosity logLevel) {
+	if handler == nil {
+		currentLoggingHandlerFunction = defaultLoggingHandlerFunction
+	} else {
+		currentLoggingHandlerFunction = handler
+	}
+
+	currentLoggingVerbosity = verbosity
+}
+
+func defaultLoggingHandlerFunction(messageDomain string, messageLevel logLevel, message string) {
 	var messageLevelDescription string
 	switch logLevel(messageLevel) {
 	case logLevelError:
@@ -38,5 +72,13 @@ func govipsLog(messageDomain string, messageLevel logLevel, message string) {
 		messageLevelDescription = "debug"
 	}
 
-	log.Println(messageDomain, "[", messageLevelDescription, "]", message)
+	log.Printf("[%v.%v] %v", messageDomain, messageLevelDescription, message)
+}
+
+// govipsLog is the default function used to log debug or error messages internally in govips.
+// It's used by all govips functionality directly, as well as by glib and libvips via the c bridge.
+func govipsLog(messageDomain string, messageLevel logLevel, message string) {
+	if messageLevel <= currentLoggingVerbosity {
+		currentLoggingHandlerFunction(messageDomain, messageLevel, message)
+	}
 }