Have Task choose free ports for dev servers (#6145)

scripts/find-free-port.sh

@@ -0,0 +1,43 @@
+#!/usr/bin/env bash
+# Prints one free TCP port per preferred port given as an argument.
+#
+# For each argument, emits that port if it's free; otherwise emits a random
+# free port in 20000-49999. Probes via bash's /dev/tcp pseudo-device (connect
+# failure = nobody listening). Tracks picks within this run so outputs are
+# guaranteed distinct from each other.
+set -euo pipefail
+
+declare -a picked=()
+
+is_free() {
+  local port=$1
+  for p in ${picked[@]+"${picked[@]}"}; do
+    if [ "$p" = "$port" ]; then return 1; fi
+  done
+  if (exec 3<>"/dev/tcp/127.0.0.1/$port") >/dev/null 2>&1; then
+    return 1
+  fi
+  return 0
+}
+
+random_free_port() {
+  while true; do
+    local port=$((RANDOM % 30000 + 20000))
+    if is_free "$port"; then
+      echo "$port"
+      return
+    fi
+  done
+}
+
+for preferred in "$@"; do
+  if is_free "$preferred"; then
+    picked+=("$preferred")
+  else
+    picked+=("$(random_free_port)")
+  fi
+done
+
+for p in "${picked[@]}"; do
+  echo "$p"
+done