Make background tasks lazy

Previously when a background task was defined it would perform work on initialization, this fails if the first task is not actually executed yet:

```term
:::>- pre.erb background.start('heroku run:inside <%= dyno_ps_name %> "bash"', wait: "$", timeout: 120, name: "heroku_run")
:::-- background.stdin_write("ls -lah", name: "heroku_run", wait: "$", timeout: 30)
:::-- background.stdin_write("exit", name: "heroku_run", wait: "exit")
:::-> background.stop(name: "heroku_run")
```

Gives:

```
/Users/rschneeman/.gem/ruby/3.3.1/gems/rundoc-4.1.1/lib/rundoc/code_command/background/process_spawn.rb:41:in `find': Could not find task with name "heroku_run", known task names: [] (RuntimeError)
	from /Users/rschneeman/.gem/ruby/3.3.1/gems/rundoc-4.1.1/lib/rundoc/code_command/background/stdin_write.rb:9:in `initialize'
	from /Users/rschneeman/.gem/ruby/3.3.1/gems/rundoc-4.1.1/lib/rundoc.rb:13:in `new'
	from /Users/rschneeman/.gem/ruby/3.3.1/gems/rundoc-4.1.1/lib/rundoc.rb:13:in `code_command_from_keyword'
	from /Users/rschneeman/.gem/ruby/3.3.1/gems/rundoc-4.1.1/lib/rundoc/peg_parser.rb:210:in `block in <class:PegTransformer>'
	from /Users/rschneeman/.gem/ruby/3.3.1/gems/parslet-2.0.0/lib/parslet/transform.rb:217:in `instance_eval'
	from /Users/rschneeman/.gem/ruby/3.3.1/gems/parslet-2.0.0/lib/parslet/transform.rb:217:in `call_on_match'
	from /Users/rschneeman/.gem/ruby/3.3.1/gems/parslet-2.0.0/lib/parslet/transform.rb:235:in `block in transform_elt'
	from /Users/rschneeman/.gem/ruby/3.3.1/gems/parslet-2.0.0/lib/parslet/transform.rb:232:in `each'
```

Because the `background.stdin_write` call to `new()` happens at parse time, but the `background.start` call to `new()` happens at runtime, (Because `pre.erb` is the command, and it hasn't yet created the start command and pushed it onto the stack).

This change makes the lookup for background tasks lazy, so as long as the task is started by execution (runtime) of the command it will succeed.

The added test fails on main (with the error below) and passes on this branch.

```
  1) Error:
BackgroundTest#test_stdin_with_cat_echo:
RuntimeError: Could not find task with name "cat", known task names: ["script", "tail"]
    lib/rundoc/code_command/background/process_spawn.rb:41:in `find'
    lib/rundoc/code_command/background/stdin_write.rb:9:in `initialize'
    test/rundoc/code_commands/background_test.rb:9:in `new'
    test/rundoc/code_commands/background_test.rb:9:in `block (2 levels) in test_stdin_with_cat_echo'
    test/rundoc/code_commands/background_test.rb:6:in `chdir'
    test/rundoc/code_commands/background_test.rb:6:in `block in test_stdin_with_cat_echo'
    /Users/rschneeman/.rubies/ruby-3.3.1/lib/ruby/3.3.0/tmpdir.rb:99:in `mktmpdir'
    test/rundoc/code_commands/background_test.rb:5:in `test_stdin_with_cat_echo'

87 runs, 249 assertions, 0 failures, 1 errors, 0 skips
```

CHANGELOG.md

@@ -1,5 +1,7 @@
 ## HEAD
 
+- Fix: Background task name lookup is now lazy, this fixes a bug when using `:::>- pre.erb background.start(...)` (https://github.com/zombocom/rundoc/pull/95)
+
 ## 4.1.1
 
 - Fix: Visibility forwarding for `pre.erb` was accidentally reversed, this is now fixed. (https://github.com/zombocom/rundoc/pull/93)

lib/rundoc/code_command/background/log/clear.rb

@@ -1,15 +1,20 @@
 class Rundoc::CodeCommand::Background::Log
   class Clear < Rundoc::CodeCommand
     def initialize(name:)
-      @spawn = Rundoc::CodeCommand::Background::ProcessSpawn.find(name)
+      @name = name
+      @background = nil
+    end
+
+    def background
+      @background ||= Rundoc::CodeCommand::Background::ProcessSpawn.find(@name)
     end
 
     def to_md(env = {})
       ""
     end
 
     def call(env = {})
-      @spawn.log.truncate(0)
+      background.log.truncate(0)
       ""
     end
   end

lib/rundoc/code_command/background/log/read.rb

@@ -1,15 +1,20 @@
 class Rundoc::CodeCommand::Background::Log
   class Read < Rundoc::CodeCommand
     def initialize(name:)
-      @spawn = Rundoc::CodeCommand::Background::ProcessSpawn.find(name)
+      @name = name
+      @background = nil
+    end
+
+    def background
+      @background ||= Rundoc::CodeCommand::Background::ProcessSpawn.find(@name)
     end
 
     def to_md(env = {})
       ""
     end
 
     def call(env = {})
-      @spawn.log.read
+      background.log.read
     end
   end
 end

lib/rundoc/code_command/background/start.rb

@@ -3,35 +3,43 @@
 class Rundoc::CodeCommand::Background
   class Start < Rundoc::CodeCommand
     def initialize(command, name:, wait: nil, timeout: 5, log: Tempfile.new("log"), out: "2>&1", allow_fail: false)
+      @timeout = timeout
       @command = command
       @name = name
       @wait = wait
       @allow_fail = allow_fail
-      FileUtils.touch(log)
+      @log = log
+      @redirect = out
+      FileUtils.touch(@log)
 
-      @spawn = ProcessSpawn.new(
+      @background = nil
+    end
+
+    def background
+      @background ||= ProcessSpawn.new(
         @command,
-        timeout: timeout,
-        log: log,
-        out: out
-      )
-      puts "Spawning commmand: `#{@spawn.command}`"
-      ProcessSpawn.add(@name, @spawn)
+        timeout: @timeout,
+        log: @log,
+        out: @redirect
+      ).tap do |spawn|
+        puts "Spawning commmand: `#{spawn.command}`"
+        ProcessSpawn.add(@name, spawn)
+      end
     end
 
     def to_md(env = {})
       "$ #{@command}"
     end
 
     def call(env = {})
-      @spawn.wait(@wait)
-      @spawn.check_alive! unless @allow_fail
+      background.wait(@wait)
+      background.check_alive! unless @allow_fail
 
-      @spawn.log.read
+      background.log.read
     end
 
     def alive?
-      !!@spawn.alive?
+      !!background.alive?
     end
   end
 end

lib/rundoc/code_command/background/stdin_write.rb

@@ -6,10 +6,15 @@ class StdinWrite < Rundoc::CodeCommand
     def initialize(contents, name:, wait:, timeout: 5, ending: $/)
       @contents = contents
       @ending = ending
-      @spawn = Rundoc::CodeCommand::Background::ProcessSpawn.find(name)
       @wait = wait
+      @name = name
       @timeout_value = Integer(timeout)
       @contents_written = nil
+      @background = nil
+    end
+
+    def background
+      @background ||= Rundoc::CodeCommand::Background::ProcessSpawn.find(@name)
     end
 
     # The command is rendered (`:::>-`) by the output of the `def call` method.
@@ -20,11 +25,11 @@ def to_md(env = {})
     # The contents produced by the command (`:::->`) are rendered by the `def to_md` method.
     def call(env = {})
       writecontents
-      @spawn.log.read
+      background.log.read
     end
 
     def writecontents
-      @contents_written ||= @spawn.stdin_write(
+      @contents_written ||= background.stdin_write(
         contents,
         wait: @wait,
         ending: @ending,

lib/rundoc/code_command/background/stop.rb

@@ -1,16 +1,21 @@
 class Rundoc::CodeCommand::Background
   class Stop < Rundoc::CodeCommand
     def initialize(name:)
-      @spawn = Rundoc::CodeCommand::Background::ProcessSpawn.find(name)
+      @name = name
+      @background = nil
+    end
+
+    def background
+      @background ||= Rundoc::CodeCommand::Background::ProcessSpawn.find(@name)
     end
 
     def to_md(env = {})
       ""
     end
 
     def call(env = {})
-      @spawn.stop
-      @spawn.log.read
+      background.stop
+      background.log.read
     end
   end
 end

lib/rundoc/code_command/background/wait.rb

@@ -1,17 +1,22 @@
 class Rundoc::CodeCommand::Background
   class Wait < Rundoc::CodeCommand
     def initialize(name:, wait:, timeout: 5)
-      @spawn = Rundoc::CodeCommand::Background::ProcessSpawn.find(name)
+      @name = name
       @wait = wait
       @timeout_value = Integer(timeout)
+      @background = nil
+    end
+
+    def background
+      @background ||= Rundoc::CodeCommand::Background::ProcessSpawn.find(name)
     end
 
     def to_md(env = {})
       ""
     end
 
     def call(env = {})
-      @spawn.wait(@wait, @timeout_value)
+      background.wait(@wait, @timeout_value)
       ""
     end
   end

test/rundoc/code_commands/background_test.rb

@@ -4,15 +4,19 @@ class BackgroundTest < Minitest::Test
   def test_stdin_with_cat_echo
     Dir.mktmpdir do |dir|
       Dir.chdir(dir) do
-        background_start = Rundoc::CodeCommand::Background::Start.new("cat",
-          name: "cat")
-        background_start.call
-
-        output = Rundoc::CodeCommand::Background::StdinWrite.new(
+        # Intentionally out of order, should not raise an error as long as "cat"
+        # command exists at execution time
+        stdin_write = Rundoc::CodeCommand::Background::StdinWrite.new(
           "hello there",
           name: "cat",
           wait: "hello"
-        ).call
+        )
+
+        background_start = Rundoc::CodeCommand::Background::Start.new("cat",
+          name: "cat")
+
+        background_start.call
+        output = stdin_write.call
         assert_equal("hello there" + $/, output)
 
         Rundoc::CodeCommand::Background::Log::Clear.new(