commit 65d39ee9f37e45e1fe53e8e19aee15eaf3fa7927
Author: Jordan Orelli <jorelli@etsy.com>
Date:   Wed Nov 26 12:00:45 2014 +0000

    huppend

diff --git a/main.go b/main.go
new file mode 100644
index 0000000..82415ba
--- /dev/null
+++ b/main.go
@@ -0,0 +1,143 @@
+package main
+
+import (
+	"bufio"
+	"flag"
+	"fmt"
+	"io"
+	"os"
+	"os/signal"
+	"syscall"
+)
+
+var options struct {
+	pidfile string
+}
+
+func bail(status int, template string, args ...interface{}) {
+	if status == 0 {
+		fmt.Fprintf(os.Stdout, template, args...)
+	} else {
+		fmt.Fprintf(os.Stderr, template, args...)
+	}
+	os.Exit(status)
+}
+
+func writePid() error {
+	f, err := os.OpenFile(options.pidfile, os.O_WRONLY|os.O_TRUNC|os.O_CREATE, 0644)
+	if err != nil {
+		return err
+	}
+	if _, err := fmt.Fprintln(f, os.Getpid()); err != nil {
+		return err
+	}
+	f.Close()
+	return nil
+}
+
+func writelines(f *fileOpener, c chan []byte) {
+	hup := make(chan os.Signal, 1)
+	signal.Notify(hup, syscall.SIGHUP)
+
+	kill := make(chan os.Signal, 1)
+	signal.Notify(kill, syscall.SIGKILL)
+
+	defer f.Close()
+
+	for {
+		select {
+		case line, ok := <-c:
+			if !ok { // end of file
+				return
+			}
+			if _, err := f.Write(line); err != nil {
+				f.Close()
+				bail(1, "failed to write line: %v", err)
+			}
+		case <-hup:
+			if err := f.Reopen(); err != nil {
+				f.Close()
+				bail(1, "unable to reopen file in writelines loop: %v", err)
+			}
+		case <-kill:
+			return
+		}
+	}
+}
+
+type fileOpener struct {
+	*os.File
+	fname string
+	flag  int
+	mode  os.FileMode
+}
+
+func (f *fileOpener) Open() error {
+	handle, err := os.OpenFile(f.fname, f.flag, f.mode)
+	if err != nil {
+		return fmt.Errorf("fileOpener unable to open file: %v", err)
+	}
+	f.File = handle
+	return nil
+}
+
+func (f *fileOpener) Reopen() error {
+	if err := f.Close(); err != nil {
+		return fmt.Errorf("fileOpener unable to reopen file: %v", err)
+	}
+	if err := f.Open(); err != nil {
+		return fmt.Errorf("fileOpener unable to reopen file: %v", err)
+	}
+	return nil
+}
+
+func (f *fileOpener) Close() error {
+	if f.File == nil {
+		return nil
+	}
+	return f.File.Close()
+}
+
+func main() {
+	flag.Parse()
+
+	if flag.NArg() < 1 {
+		bail(1, "filename required")
+	}
+	fname := flag.Arg(0)
+
+	f := &fileOpener{
+		fname: fname,
+		flag:  os.O_APPEND | os.O_CREATE | os.O_WRONLY,
+		mode:  0644,
+	}
+	if err := f.Open(); err != nil {
+		bail(1, "unable to open output file: %v", err)
+	}
+	defer f.Close()
+
+	if err := writePid(); err != nil {
+		bail(1, "unable to write pidfile: %v", err)
+	}
+
+	c := make(chan []byte)
+	go writelines(f, c)
+
+	r := bufio.NewReader(os.Stdin)
+	for {
+		line, err := r.ReadBytes('\n')
+		switch err {
+		case nil:
+			c <- line
+		case io.EOF:
+			return
+		default:
+			f.Close()
+			bail(2, "something is fucked: %v", err)
+		}
+	}
+}
+
+func init() {
+	flag.StringVar(&options.pidfile, "pidfile", "./huppend.pid", "pidfile")
+}