DEV: h2: add a script "mkhdr" to build h2 frames from scratch

It's a real pain to try to trigger certain edge cases in h2, so let's
write a simple tool aiming at creating frames.

dev/h2/mkhdr.sh

@@ -0,0 +1,138 @@
+#!/usr/bin/env bash
+
+# Usage: mkhdr -l <len> -t <type> -f <flags> -sid <sid> > hdr.bin
+# All fields are optional. 0 assumed when absent.
+
+USAGE=\
+"Usage: %s [-l <len>] [-t <type>] [-f <flags>] [-i <sid>] > hdr.bin
+        Numbers are decimal or 0xhex. Not set=0.
+
+Supported symbolic types (case insensitive prefix match):
+   DATA        (0x00)      PUSH_PROMISE   (0x05)
+   HEADERS     (0x01)      PING           (0x06)
+   PRIORITY    (0x02)      GOAWAY         (0x07)
+   RST_STREAM  (0x03)      WINDOW_UPDATE  (0x08)
+   SETTINGS    (0x04)      CONTINUATION   (0x09)
+
+Supported symbolic flags (case insensitive prefix match):
+   ES          (0x01)      PAD            (0x08)
+   EH          (0x04)      PRIO           (0x20)
+
+"
+
+LEN=
+TYPE=
+FLAGS=
+ID=
+
+die() {
+	[ "$#" -eq 0 ] || echo "$*" >&2
+	exit 1
+}
+
+quit() {
+	[ "$#" -eq 0 ] || echo "$*"
+	exit 0
+}
+
+# print usage with $1 as the cmd name
+usage() {
+	printf "$USAGE" "$1";
+}
+
+# Send frame made of $1 $2 $3 $4 to stdout.
+# Usage: mkframe <len> <type> <flags> <id>
+mkframe() {
+	local L="${1:-0}"
+	local T="${2:-0}"
+	local F="${3:-0}"
+	local I="${4:-0}"
+	local t f
+
+	# get the first match in this order
+	for t in DATA:0x00 HEADERS:0x01 RST_STREAM:0x03 SETTINGS:0x04 PING:0x06 \
+		 GOAWAY:0x07 WINDOW_UPDATE:0x08 CONTINUATION:0x09 PRIORITY:0x02 \
+		 PUSH_PROMISE:0x05; do
+		if [ -z "${t##${T^^*}*}" ]; then
+			T="${t##*:}"
+			break
+		fi
+	done
+
+	if [ -n "${T##[0-9]*}" ]; then
+		echo "Unknown type '$T'" >&2
+		usage "${0##*}"
+		die
+	fi
+
+	# get the first match in this order
+	for f in ES:0x01 EH:0x04 PAD:0x08 PRIO:0x20; do
+		if [ -z "${f##${F^^*}*}" ]; then
+			F="${f##*:}"
+		fi
+	done
+
+	if [ -n "${F##[0-9]*}" ]; then
+		echo "Unknown type '$T'" >&2
+		usage "${0##*}"
+		die
+	fi
+
+	L=$(( L )); T=$(( T )); F=$(( F )); I=$(( I ))
+
+	L0=$(( (L >> 16) & 255 )); L0=$(printf "%02x" $L0)
+	L1=$(( (L >>  8) & 255 )); L1=$(printf "%02x" $L1)
+	L2=$(( (L >>  0) & 255 )); L2=$(printf "%02x" $L2)
+
+	T0=$(( (T >>  0) & 255 )); T0=$(printf "%02x" $T0)
+	F0=$(( (F >>  0) & 255 )); F0=$(printf "%02x" $F0)
+
+	I0=$(( (I >> 24) & 127 )); I0=$(printf "%02x" $I0)
+	I1=$(( (I >> 16) & 255 )); I1=$(printf "%02x" $I1)
+	I2=$(( (I >>  8) & 255 )); I2=$(printf "%02x" $I2)
+	I3=$(( (I >>  0) & 255 )); I3=$(printf "%02x" $I3)
+
+	printf "\x$L0\x$L1\x$L2\x$T0\x$F0\x$I0\x$I1\x$I2\x$I3"
+}
+
+## main
+
+if [ $# -le 1 ]; then
+	usage "${0##*}"
+	die
+fi
+
+while [ -n "$1" -a -z "${1##-*}" ]; do
+	case "$1" in
+		-l)        LEN="$2"      ; shift 2 ;;
+		-t)        TYPE="$2"     ; shift 2 ;;
+		-f)        FLAGS="$2"    ; shift 2 ;;
+		-i)        ID="$2"       ; shift 2 ;;
+		-h|--help) usage "${0##*}"; quit;;
+		*)         usage "${0##*}"; die ;;
+	esac
+done
+
+if [ $# -gt 0 ]; then
+	usage "${0##*}"
+	die
+fi
+
+# default values for LEN and ID
+LEN=${LEN:-0};
+if [ -n "${LEN##[0-9]*}" ]; then
+	echo "Unparsable length '$LEN'" >&2
+	usage "${0##*}"
+	die
+fi
+
+ID=${ID:-0};
+if [ -n "${ID##[0-9]*}" ]; then
+	echo "Unparsable stream ID '$ID'" >&2
+	usage "${0##*}"
+	die
+fi
+
+mkframe "$LEN" "$TYPE" "$FLAGS" "$ID"
+
+exit 0