add two node local net script (#1781)

* initial work running a two node local net

* use the right incantations so the nodes produce blocks together

* improve internal documentation

polkadot/scripts/two-node-local-net.sh

+#!/usr/bin/env bash
+
+# Run a two node local net.
+# Unlike the docker-compose script in the /docker folder, this version builds the nodes based
+# on the current state of the code, instead of depending on a published version.
+
+set -e
+
+PROJECT_ROOT=$(git rev-parse --show-toplevel)
+# shellcheck disable=SC1090
+source "$(dirname "$0")"/common.sh
+
+cd "$PROJECT_ROOT"
+
+last_modified_rust_file=$(
+  find . -path ./target -prune -o -type f -name '*.rs' -printf '%T@ %p\n' |
+  sort -nr |
+  head -1 |
+  cut -d' ' -f2-
+)
+
+polkadot="target/release/polkadot"
+
+# ensure the polkadot binary exists and is up to date
+if [ ! -x "$polkadot" ] || [ "$polkadot" -ot "$last_modified_rust_file" ]; then
+  cargo build --release
+fi
+
+# setup variables
+node_offset=0
+declare -a node_pids
+declare -a node_pipes
+
+# create a sed expression which injects the node name and stream type into each line
+function make_sed_expr() {
+  name="$1"
+  type="$2"
+
+  printf "s/^/%8s %s: /" "$name" "$type"
+}
+
+# turn a string into a flag
+function flagify() {
+  printf -- '--%s' "$(tr '[:upper:]' '[:lower:]' <<< "$1")"
+}
+
+# start a node and label its output
+#
+# This function takes a single argument, the node name.
+# The name must be one of those which can be passed to the polkadot binary, in un-flagged form,
+# one of:
+#   alice, bob, charlie, dave, eve, ferdie, one, two
+function run_node() {
+  name="$1"
+  # create a named pipe so we can get the node's PID while also sedding its output
+  local stdout
+  local stderr
+  stdout=$(mktemp --dry-run --tmpdir)
+  stderr=$(mktemp --dry-run --tmpdir)
+  mkfifo "$stdout"
+  mkfifo "$stderr"
+  node_pipes+=("$stdout")
+  node_pipes+=("$stderr")
+
+  # compute ports from offset
+  local port=$((30333+node_offset))
+  local rpc_port=$((9933+node_offset))
+  local ws_port=$((9944+node_offset))
+  node_offset=$((node_offset+1))
+
+  # start the node
+  "$polkadot" \
+    --chain polkadot-local \
+    --tmp \
+    --port "$port" \
+    --rpc-port "$rpc_port" \
+    --ws-port "$ws_port" \
+    --rpc-cors all \
+    "$(flagify "$name")" \
+  > "$stdout" \
+  2> "$stderr" \
+  &
+  local pid=$!
+  node_pids+=("$pid")
+
+  # send output from the stdout pipe to stdout, prepending the node name
+  sed -e "$(make_sed_expr "$name" "OUT")" "$stdout" >&1 &
+  # send output from the stderr pipe to stderr, prepending the node name
+  sed -e "$(make_sed_expr "$name" "ERR")" "$stderr" >&2 &
+}
+
+
+# clean up the nodes when this script exits
+function finish {
+  for node_pid in "${node_pids[@]}"; do
+    kill -9 "$node_pid"
+  done
+  for node_pipe in "${node_pipes[@]}"; do
+    rm "$node_pipe"
+  done
+}
+trap finish EXIT
+
+# start the nodes
+run_node Alice
+run_node Bob
+
+# now wait; this will exit on its own only if both subprocesses exit
+# the practical implication, as both subprocesses are supposed to run forever, is that
+# this script will also run forever, until killed, at which point the exit trap should kill
+# the subprocesses
+wait