pkg/log: ability for debug logs (#1689)

* pkg/log: ability for debug logs

When the debug plugin is enabled all log.Debug calls will print to
standard; if not there are a noop (almost).

The log package wraps some standard log functions as well, so just
replacing "log" with "plugin/pkg/log" should be enough to use this
package.

* docs

* Add docs

* lint

* Test fallthrough to log pkg as well

* simple package - up test coverage

* add other log levels as well

* update docs

plugin/pkg/log/log.go

@@ -0,0 +1,69 @@
+// Package log implements a small wrapper around the std lib log package.
+// It implements log levels by prefixing the logs with [INFO], [DEBUG],
+// [WARNING] or [ERROR].
+// Debug logging is available and enabled if the *debug* plugin is used.
+//
+// log.Info("this is some logging"), will log on the Info level.
+//
+// log.Debug("this is debug output"), will log in the Debug level.
+package log
+
+import (
+	"fmt"
+	golog "log"
+)
+
+// D controls whether we should ouput debug logs. If true, we do.
+var D bool
+
+// logf calls log.Printf prefixed with level.
+func logf(level, format string, v ...interface{}) {
+	s := level + fmt.Sprintf(format, v...)
+	golog.Print(s)
+}
+
+// log calls log.Print prefixed with level.
+func log(level string, v ...interface{}) { s := level + fmt.Sprint(v...); golog.Print(s) }
+
+// Debug is equivalent to log.Print(), but prefixed with "[DEBUG] ". It only outputs something
+// if D is true.
+func Debug(v ...interface{}) {
+	if !D {
+		return
+	}
+	log(debug, v...)
+}
+
+// Debugf is equivalent to log.Printf(), but prefixed with "[DEBUG] ". It only outputs something
+// if D is true.
+func Debugf(format string, v ...interface{}) {
+	if !D {
+		return
+	}
+	logf(debug, format, v...)
+}
+
+// Info is equivalent to log.Print, but prefixed with "[INFO] ".
+func Info(v ...interface{}) { log(info, v...) }
+
+// Infof is equivalent to log.Printf, but prefixed with "[INFO] ".
+func Infof(format string, v ...interface{}) { logf(info, format, v...) }
+
+// Warning is equivalent to log.Print, but prefixed with "[WARNING] ".
+func Warning(v ...interface{}) { log(warning, v...) }
+
+// Warningf is equivalent to log.Printf, but prefixed with "[WARNING] ".
+func Warningf(format string, v ...interface{}) { logf(warning, format, v...) }
+
+// Error is equivalent to log.Print, but prefixed with "[ERROR] ".
+func Error(v ...interface{}) { log(err, v...) }
+
+// Errorf is equivalent to log.Printf, but prefixed with "[ERROR] ".
+func Errorf(format string, v ...interface{}) { logf(err, format, v...) }
+
+const (
+	debug   = "[DEBUG] "
+	err     = "[ERROR] "
+	warning = "[WARNING] "
+	info    = "[INFO] "
+)