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+# Introduction
+
+The [Subplot][] library `daemon` for Python provides scenario steps
+and their implementations for running a background process and
+terminating at the end of the scenario.
+
+[Subplot]: https://subplot.liw.fi/
+
+This document explains the acceptance criteria for the library and how
+they're verified. It uses the steps and functions from the
+`lib/daemon` library. The scenarios all have the same structure: run a
+command, then examine the exit code, verify the process is running.
+
+# Daemon is started and terminated
+
+This scenario starts a background process, verifies it's started, and
+verifies it's terminated after the scenario ends.
+
+~~~scenario
+given there is no "sleep 12765" process
+when I start "sleep 12765" as a background process as sleepyhead
+then a process "sleep 12765" is running
+when I stop background process sleepyhead
+then there is no "sleep 12765" process
+~~~
+
+
+# Daemon takes a while to open its port
+
+This scenario verifies that if the background process doesn't immediately start
+listening on its port, the daemon library handles that correctly. We do this
+with a helper script that waits 2 seconds before opening the port. The
+lib/daemon code will wait for the script by repeatedly trying to connect. Once
+successful, it immediately closes the port, which causes the script to
+terminate.
+
+~~~scenario
+given a daemon helper shell script slow-start-daemon.py
+given there is no "slow-start-daemon.py" process
+when I try to start "./slow-start-daemon.py" as slow-daemon, on port 8888
+then starting the daemon succeeds
+when I stop background process slow-daemon
+then there is no "slow-start-daemon.py" process
+~~~
+
+~~~{#slow-start-daemon.py .file .python .numberLines}
+#!/usr/bin/env python3
+
+import socket
+import time
+
+time.sleep(2)
+
+s = socket.socket()
+s.bind(("127.0.0.1", 8888))
+s.listen()
+
+(conn, _) = s.accept()
+conn.recv(1)
+s.close()
+
+print("OK")
+~~~
+
+# Daemon never opens the intended port
+
+This scenario verifies that if the background process never starts
+listening on its port, the daemon library handles that correctly.
+
+~~~scenario
+given there is no "sleep 12765" process
+when I try to start "sleep 12765" as sleepyhead, on port 8888
+then starting daemon fails with "ConnectionRefusedError"
+then a process "sleep 12765" is running
+when I stop background process sleepyhead
+then there is no "sleep 12765" process
+~~~
+
+
+# Daemon stdout and stderr are retrievable
+
+Sometimes it's useful for the step functions to be able to retrieve
+the stdout or stderr of of the daemon, after it's started, or even
+after it's terminated. This scenario verifies that `lib/daemon` can do
+that.
+
+~~~scenario
+given a daemon helper shell script chatty-daemon.sh
+given there is no "chatty-daemon" process
+when I start "./chatty-daemon.sh" as a background process as chatty-daemon
+when daemon chatty-daemon has produced output
+when I stop background process chatty-daemon
+then there is no "chatty-daemon" process
+then daemon chatty-daemon stdout is "hi there\n"
+then daemon chatty-daemon stderr is "hola\n"
+~~~
+
+We make for the daemon to exit, to work around a race condition: if
+the test program retrieves the daemon's output too fast, it may not
+have had time to produce it yet.
+
+
+~~~{#chatty-daemon.sh .file .sh .numberLines}
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+trap 'exit 0' TERM
+
+echo hola 1>&2
+echo hi there
+~~~
+
+# Can specify additional environment variables for daemon
+
+Some daemons are configured through their environment rather than configuration
+files. This scenario verifies that a step can set arbitrary variables in the
+daemon's environment.
+
+~~~scenario
+when I start "/usr/bin/env" as a background process as env, with environment {"custom_variable": "has a Value"}
+when daemon env has produced output
+when I stop background process env
+then daemon env stdout contains "custom_variable=has a Value"
+~~~
+
+~~~scenario
+given a daemon helper shell script env-with-port.py
+when I try to start "./env-with-port.py 8765" as env-with-port, on port 8765, with environment {"custom_variable": "1337"}
+when I stop background process env-with-port
+then daemon env-with-port stdout contains "custom_variable=1337"
+~~~
+
+~~~scenario
+given a daemon helper shell script env-with-port.py
+when I start "./env-with-port.py 8766" as a background process as another-env-with-port, on port 8766, with environment {"subplot2": "000"}
+when daemon another-env-with-port has produced output
+when I stop background process another-env-with-port
+then daemon another-env-with-port stdout contains "subplot2=000"
+~~~
+
+It's important that these new environment variables are not inherited by the
+steps that follow. To verify that, we run one more scenario which *doesn't* set
+any variables, but checks that none of the variables we mentioned above are
+present.
+
+~~~scenario
+when I start "/usr/bin/env" as a background process as env2
+when daemon env2 has produced output
+when I stop background process env2
+then daemon env2 stdout doesn't contain "custom_variable=has a Value"
+then daemon env2 stdout doesn't contain "custom_variable=1337"
+then daemon env2 stdout doesn't contain "subplot2=000"
+~~~
+
+~~~{#env-with-port.py .file .python .numberLines}
+#!/usr/bin/env python3
+
+import os
+import socket
+import sys
+import time
+
+for (key, value) in os.environ.items():
+    print(f"{key}={value}")
+
+port = int(sys.argv[1])
+print(f"port is {port}")
+
+s = socket.socket()
+s.bind(("127.0.0.1", port))
+s.listen()
+
+(conn, _) = s.accept()
+conn.recv(1)
+s.close()
+~~~
+
+
+---
+title: Acceptance criteria for the lib/daemon Subplot library
+author: The Subplot project
+bindings:
+- lib/daemon.yaml
+impls:
+  python:
+    - lib/daemon.py
+    - lib/runcmd.py
+...