t/lib-git-daemon: add network-protocol helpers

All of our git-protocol tests rely on invoking the client
and having it make a request of a server. That gives a nice
real-world test of how the two behave together, but it
doesn't leave any room for testing how a server might react
to _other_ clients.

Let's add a few test helper functions which can be used to
manually conduct a git-protocol conversation with a remote
git-daemon:

  1. To connect to a remote git-daemon, we need something
     like "netcat". But not everybody will have netcat. And
     even if they do, the behavior with respect to
     half-duplex shutdowns is not portable (openbsd netcat
     has "-N", with others you must rely on "-q 1", which is
     racy).

     Here we provide a "fake_nc" that is capable of doing
     a client-side netcat, with sane half-duplex semantics.
     It relies on perl's IO::Socket::INET. That's been in
     the base distribution since 5.6.0, so it's probably
     available everywhere. But just to be on the safe side,
     we'll add a prereq.

  2. To help tests speak and read pktline, this patch adds
     packetize() and depacketize() functions.

I've put fake_nc() into lib-git-daemon.sh, since that's
really the only server where we'd need to use a network
socket.  Whereas the pktline helpers may be of more general
use, so I've added them to test-lib-functions.sh. Programs
like upload-pack speak pktline, but can talk directly over
stdio without a network socket.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib-functions.sh

@@ -1020,3 +1020,37 @@ nongit () {
 		"$@"
 	)
 }
+
+# convert stdin to pktline representation; note that empty input becomes an
+# empty packet, not a flush packet (for that you can just print 0000 yourself).
+packetize() {
+	cat >packetize.tmp &&
+	len=$(wc -c <packetize.tmp) &&
+	printf '%04x%s' "$(($len + 4))" &&
+	cat packetize.tmp &&
+	rm -f packetize.tmp
+}
+
+# Parse the input as a series of pktlines, writing the result to stdout.
+# Sideband markers are removed automatically, and the output is routed to
+# stderr if appropriate.
+#
+# NUL bytes are converted to "\\0" for ease of parsing with text tools.
+depacketize () {
+	perl -e '
+		while (read(STDIN, $len, 4) == 4) {
+			if ($len eq "0000") {
+				print "FLUSH\n";
+			} else {
+				read(STDIN, $buf, hex($len) - 4);
+				$buf =~ s/\0/\\0/g;
+				if ($buf =~ s/^[\x2\x3]//) {
+					print STDERR $buf;
+				} else {
+					$buf =~ s/^\x1//;
+					print $buf;
+				}
+			}
+		}
+	'
+}