Add streaming command support.

Add options
- `stream-stdout-in-response`
- `stream-stdout-in-response-on-error`
- `stream-command-kill-grace-period-seconds`

to allow defining webhooks which dynamically stream large content back to the
requestor. This allows the creation of download endpoints from scripts, i.e.
running a `git archive` command or a database dump from a docker container,
without needing to buffer up the original.

test/hookstream/hookstream.go

@@ -0,0 +1,107 @@
+// Hook Stream is a simple utility for testing Webhook streaming capability. It spawns a TCP server on execution
+// which echos all connections to its stdout until it receives the string EOF.
+
+package main
+
+import (
+	"fmt"
+	"os"
+	"strings"
+	"strconv"
+	"io"
+	"net"
+	"bufio"
+)
+
+func checkPrefix(prefixMap map[string]struct{}, prefix string, arg string) bool {
+	if _, found := prefixMap[prefix]; found {
+		fmt.Printf("prefix specified more then once: %s", arg)
+		os.Exit(-1)
+	}
+
+	if strings.HasPrefix(arg, prefix) {
+		prefixMap[prefix] = struct{}{}
+		return true
+	}
+
+	return false
+}
+
+func main() {
+	var outputStream io.Writer
+	outputStream = os.Stdout
+	seenPrefixes := make(map[string]struct{})
+	exit_code := 0
+
+	for _, arg := range os.Args[1:] {
+		if checkPrefix(seenPrefixes, "stream=", arg) {
+			switch arg {
+			case "stream=stdout":
+				outputStream = os.Stdout
+			case "stream=stderr":
+				outputStream = os.Stderr
+			case "stream=both":
+				outputStream = io.MultiWriter(os.Stdout, os.Stderr)
+			default:
+				fmt.Printf("unrecognized stream specification: %s", arg)
+				os.Exit(-1)
+			}
+		} else if checkPrefix(seenPrefixes, "exit=", arg) {
+			exit_code_str := arg[5:]
+			var err error
+			exit_code_conv, err := strconv.Atoi(exit_code_str)
+			exit_code = exit_code_conv
+			if err != nil {
+				fmt.Printf("Exit code %s not an int!", exit_code_str)
+				os.Exit(-1)
+			}
+		}
+	}
+
+	l, err := net.Listen("tcp", "localhost:0")
+	if err != nil {
+		fmt.Printf("Error starting tcp server: %v\n", err)
+		os.Exit(-1)
+	}
+	defer l.Close()
+
+	// Emit the address of the server
+	fmt.Printf("%v\n",l.Addr())
+
+	manageCh := make(chan struct{})
+
+	go func() {
+		for {
+			conn, err := l.Accept()
+			if err != nil {
+				fmt.Printf("Error accepting connection: %v\n", err)
+				os.Exit(-1)
+			}
+			go handleRequest(manageCh, outputStream, conn)
+		}
+	}()
+
+	<- manageCh
+	l.Close()
+
+	os.Exit(exit_code)
+}
+
+// Handles incoming requests.
+func handleRequest(manageCh chan<- struct{}, w io.Writer, conn net.Conn) {
+	defer conn.Close()
+	bio := bufio.NewScanner(conn)
+	for bio.Scan() {
+		if line := strings.TrimSuffix(bio.Text(), "\n"); line == "EOF" {
+			// Request program close
+			select {
+				case manageCh <- struct{}{}:
+					// Request sent.
+				default:
+					// Already closing
+			}
+			break
+		}
+		fmt.Fprintf(w, "%s\n", bio.Text())
+	}
+}