logonce is a package that prints each distinct message only once

diff --git a/logonce/logonce.go b/logonce/logonce.go
new file mode 100644 (file)
index 0000000..2744282
--- /dev/null
+++ b/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{}),
+       }
+}