logonce is a package that prints each distinct message only once

logonce/logonce.go

@@ -0,0 +1,47 @@
+// Package logonce implements an io.Writer facade that only performs distinct
+// writes. This can be used by log.Loggers as they're guaranteed to make a
+// single Write method call for each message. This is useful for loggers that
+// print useful information about unexpected conditions that aren't fatal in
+// code.
+package logonce
+
+import (
+	"io"
+	"log"
+	"os"
+)
+
+// A default logger similar to the default logger in the log package.
+var Stderr *log.Logger
+
+func init() {
+	// This should emulate the default logger in the log package where
+	// possible. No time flag so that messages don't differ by time. Code
+	// debug information is useful.
+	Stderr = log.New(Writer(os.Stderr), "", log.Lshortfile)
+}
+
+type writer struct {
+	w      io.Writer
+	writes map[string]struct{}
+}
+
+func (w writer) Write(p []byte) (n int, err error) {
+	s := string(p)
+	if _, ok := w.writes[s]; ok {
+		return
+	}
+	n, err = w.w.Write(p)
+	if n != len(s) {
+		s = string(p[:n])
+	}
+	w.writes[s] = struct{}{}
+	return
+}
+
+func Writer(w io.Writer) io.Writer {
+	return writer{
+		w:      w,
+		writes: make(map[string]struct{}),
+	}
+}