Class: ClaudeAgentSDK::SubprocessCLITransport

Inherits:
Transport
  • Object
show all
Defined in:
lib/claude_agent_sdk/subprocess_cli_transport.rb

Overview

Subprocess transport using Claude Code CLI

Constant Summary collapse

DEFAULT_MAX_BUFFER_SIZE =

1MB buffer limit

1024 * 1024
MINIMUM_CLAUDE_CODE_VERSION = 
'2.0.0'
SKIP_VERSION_CHECK_ENV_VAR = 
'CLAUDE_AGENT_SDK_SKIP_VERSION_CHECK'
VERSION_CHECK_TIMEOUT_SECONDS =

mirrors Python’s anyio.fail_after(2)

2
RECENT_STDERR_LINES_LIMIT = 
20
ACTIVE_PROCESSES =

Track live CLI subprocesses so we can terminate them when the parent Ruby process exits. Mirrors the Python (PR #916, a ‘set`) and TypeScript SDKs’ parent-exit cleanup, preventing orphaned ‘claude` processes from leaking when callers crash or exit before reaching #close. A Set keyed by object identity (like Python’s set) keeps the hot path off ‘#pid` — only #kill_active_processes touches `#pid`/`#alive?`, at exit. Guarded by a mutex because #close can run on a FiberBoundary worker thread while #connect runs on the reactor fiber. Stored in CONSTANTS (not class instance variables) so the registry is a single shared instance across this class and any subclass: constants resolve through the ancestor chain, whereas class ivars are NOT inherited — a `SubprocessCLITransport` subclass instance calling `self.class.register_active_process` would otherwise reach a nil mutex and raise mid-#connect, orphaning the just-spawned child. The base-class at_exit handler must be able to see every subprocess, a subclass’s too.

Set.new
ACTIVE_PROCESSES_MUTEX = 
Mutex.new

Class Method Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(options_or_prompt = nil, options = nil) ⇒ SubprocessCLITransport

Returns a new instance of SubprocessCLITransport.



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# File 'lib/claude_agent_sdk/subprocess_cli_transport.rb', line 90

def initialize(options_or_prompt = nil, options = nil)
  # Support both new single-arg form and legacy two-arg form
  @options = options.nil? ? options_or_prompt : options
  @cli_path = @options.cli_path || find_cli
  @cwd = @options.cwd
  @process = nil
  @stdin = nil
  @stdout = nil
  @stderr = nil
  @ready = false
  @exit_error = nil
  @max_buffer_size = @options.max_buffer_size || DEFAULT_MAX_BUFFER_SIZE
  @stderr_task = nil
  @recent_stderr = []
  @recent_stderr_mutex = Mutex.new
  # Serializes stdin access across the reactor fiber (transport writes
  # from inside Async) and user-callback threads spawned via FiberBoundary
  # (tool handlers / hooks calling Client#query). Without this lock,
  # close can nil @stdin between write's readiness check and the actual
  # @stdin.write call, producing NoMethodError on nil.
  @stdin_mutex = Mutex.new
end

Class Method Details

.active_processes ⇒ Object

Public readers (the test suite uses ‘described_class.active_processes`); they return the shared constants so subclasses observe the same objects.



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# File 'lib/claude_agent_sdk/subprocess_cli_transport.rb', line 42

def active_processes
  ACTIVE_PROCESSES
end

.active_processes_mutex ⇒ Object 



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# File 'lib/claude_agent_sdk/subprocess_cli_transport.rb', line 46

def active_processes_mutex
  ACTIVE_PROCESSES_MUTEX
end

.deregister_active_process(wait_thr) ⇒ Object 



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# File 'lib/claude_agent_sdk/subprocess_cli_transport.rb', line 57

def deregister_active_process(wait_thr)
  return unless wait_thr

  active_processes_mutex.synchronize { active_processes.delete(wait_thr) }
end

.kill_active_processes ⇒ Object

Best-effort SIGTERM to every still-running child. Registered with at_exit at the bottom of this file. Never reaps (a blocking wait could hang interpreter shutdown) — the OS reparents and reaps orphans.

Deliberately does NOT take active_processes_mutex: at interpreter shutdown Ruby runs at_exit handlers before terminating other threads, and Mutex is unfair, so blocking here while a still-live worker churns register/deregister can starve this handler and hang the process. A lock-free read is safe — a torn snapshot at worst misses or repeats a SIGTERM, both harmless. The outer rescue guarantees the handler never raises (e.g. ThreadError if reached from a trap context, or a concurrent-modification error from the unlocked read), honoring the “never interrupt interpreter shutdown” contract.



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# File 'lib/claude_agent_sdk/subprocess_cli_transport.rb', line 76

def kill_active_processes
  active_processes.to_a.each do |wait_thr|
    next unless wait_thr.alive?

    Process.kill('TERM', wait_thr.pid)
  rescue StandardError
    # Process already gone (Errno::ESRCH), not permitted, or invalid pid.
  end
  active_processes.clear
rescue StandardError
  # Never let cleanup interfere with interpreter shutdown.
end

.register_active_process(wait_thr) ⇒ Object

wait_thr is the Process::Waiter returned by Open3.popen3.



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# File 'lib/claude_agent_sdk/subprocess_cli_transport.rb', line 51

def register_active_process(wait_thr)
  return unless wait_thr

  active_processes_mutex.synchronize { active_processes.add(wait_thr) }
end

Instance Method Details

#build_command ⇒ Object 



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# File 'lib/claude_agent_sdk/subprocess_cli_transport.rb', line 179

def build_command
  CommandBuilder.new(@cli_path, @options).build
end

#check_claude_version ⇒ Object 



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# File 'lib/claude_agent_sdk/subprocess_cli_transport.rb', line 583

def check_claude_version
  # Mirrors Python's os.environ.get truthiness: any non-empty value skips,
  # including '0'/'false'/' '; unset or empty string runs the check.
  skip = ENV.fetch(SKIP_VERSION_CHECK_ENV_VAR, nil)
  return if skip && !skip.empty?

  begin
    output = capture_cli_version_output
    # Residual divergence from Python (anchored re.match over the first
    # stdout chunk): this searches anywhere in stdout+stderr, so leading
    # noise (a shim's own version line) could be mistaken for the CLI
    # version. Pre-existing shape; the check is best-effort only.
    if match = output.match(/([0-9]+\.[0-9]+\.[0-9]+)/)
      version = match[1]
      version_parts = version.split('.').map(&:to_i)
      min_parts = MINIMUM_CLAUDE_CODE_VERSION.split('.').map(&:to_i)

      # Array has no #< — the old `version_parts < min_parts` raised
      # NoMethodError into the blanket rescue, so the warning never fired.
      if (version_parts <=> min_parts).negative?
        warning = "Warning: Claude Code version #{version} at #{@cli_path} is unsupported in the Agent SDK. " \
                  "Minimum required version is #{MINIMUM_CLAUDE_CODE_VERSION}. " \
                  "Some features may not work correctly."
        warn warning
      end
    end
  rescue StandardError
    # Ignore version check errors — including Timeout::Error from the
    # probe deadline, mirroring Python's `except Exception: pass`.
  end
end

#close ⇒ Object 



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# File 'lib/claude_agent_sdk/subprocess_cli_transport.rb', line 327

def close
  @ready = false
  return unless @process

  cleanup_errors = []

  # Kill stderr thread
  if @stderr_task&.alive?
    begin
      @stderr_task.kill
      @stderr_task.join(1)
    rescue StandardError => e
      cleanup_errors << "stderr thread: #{e.message}"
    end
  end

  # Close stdin under the same lock that guards write — otherwise a
  # concurrent writer (callbacks running on FiberBoundary threads) can
  # see @stdin nilled mid-write and hit NoMethodError on nil.
  @stdin_mutex.synchronize do
    begin
      @stdin&.close
    rescue IOError
      # Already closed, ignore
    rescue StandardError => e
      cleanup_errors << "stdin: #{e.message}"
    end
    @stdin = nil
  end

  begin
    @stdout&.close
  rescue IOError
    # Already closed, ignore
  rescue StandardError => e
    cleanup_errors << "stdout: #{e.message}"
  end

  begin
    @stderr&.close
  rescue IOError
    # Already closed, ignore
  rescue StandardError => e
    cleanup_errors << "stderr: #{e.message}"
  end

  # Wait for graceful shutdown after stdin EOF, then terminate if needed.
  # The subprocess needs time to flush its session file after receiving
  # EOF on stdin. Without this grace period, SIGTERM can interrupt the
  # write and cause the last assistant message to be lost.
  begin
    wait_process_with_timeout(5) if @process.alive?
  rescue Timeout::Error
    # Graceful shutdown timed out — send SIGTERM
    begin
      Process.kill('TERM', @process.pid)
      wait_process_with_timeout(2)
    rescue Timeout::Error
      # SIGTERM didn't work — force kill
      begin
        Process.kill('KILL', @process.pid)
        @process.value
      rescue StandardError => e
        cleanup_errors << "force kill: #{e.message}"
      end
    rescue Errno::ESRCH
      # Process already dead
    end
  rescue Errno::ESRCH
    # Process already dead, ignore
  rescue StandardError => e
    cleanup_errors << "process termination: #{e.message}"
  end

  # Log any cleanup errors (non-fatal)
  if cleanup_errors.any?
    warn "Claude SDK: Cleanup warnings: #{cleanup_errors.join(', ')}"
  end

  self.class.deregister_active_process(@process)
  @process = nil
  @stdout = nil
  # @stdin already nilled under the mutex above.
  @stderr = nil
  @stderr_task = nil
  @exit_error = nil
end

#connect ⇒ Object 



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# File 'lib/claude_agent_sdk/subprocess_cli_transport.rb', line 183

def connect
  return if @process

  check_claude_version

  cmd = build_command

  # Build environment
  # Convert symbol keys to strings for spawn compatibility
  custom_env = @options.env.transform_keys { |k| k.to_s }
  # Explicitly unset CLAUDECODE to prevent "nested session" detection when the SDK
  # launches Claude Code from within an existing Claude Code terminal.
  # NOTE: Must set to nil (not just omit the key) — Ruby's spawn only overlays
  # the env hash on top of the parent environment; a nil value actively unsets.
  # ENTRYPOINT defaults to sdk-rb regardless of inherited process env
  # (the old ||= let an inherited 'cli' win and mis-attribute telemetry);
  # options.env may still override it. VERSION is merged last: always
  # set by the SDK, never overridable (Python merge-order parity).
  process_env = ENV.to_h
                   .merge('CLAUDECODE' => nil, 'CLAUDE_CODE_ENTRYPOINT' => 'sdk-rb')
                   .merge(custom_env)
                   .merge('CLAUDE_AGENT_SDK_VERSION' => VERSION)
  # Propagate the active OTel trace context to the CLI so its spans parent
  # under the caller's distributed trace (Python SDK #821 parity). No-op
  # when opentelemetry is not loaded or there is no active span.
  inject_otel_trace_context(process_env, custom_env)
  process_env['CLAUDE_CODE_ENABLE_SDK_FILE_CHECKPOINTING'] = 'true' if @options.enable_file_checkpointing
  process_env['PWD'] = @cwd.to_s if @cwd

  # Determine stderr handling
  should_pipe_stderr = @options.stderr || @options.debug_stderr || @options.extra_args.key?('debug-to-stderr')

  begin
    # Start process using Open3
    # :uid mirrors Python's anyio.open_process(user=...): String username
    # or Integer uid (Unix; requires privileges — typically root). The
    # .compact is mandatory: uid: nil raises TypeError on every connect.
    # On Windows spawn raises for :uid, wrapped below into
    # CLIConnectionError — fail-loud instead of the old silent ignore.
    opts = { chdir: @cwd&.to_s, uid: @options.user }.compact

    @stdin, @stdout, @stderr, @process = Open3.popen3(process_env, *cmd, opts)
    # The CLI emits UTF-8 regardless of the parent locale. popen3 pipes
    # default to Encoding.default_external (US-ASCII under LANG=C/LC_ALL=C
    # — minimal Docker images, systemd, CI), which makes String#strip on
    # multibyte CLI output raise Encoding::CompatibilityError and kill the
    # read loop (its rescue only catches IOError). Mirrors the Python
    # SDK's TextReceiveStream(stdout), which always decodes UTF-8.
    @stdout&.set_encoding(Encoding::UTF_8)
    @stderr&.set_encoding(Encoding::UTF_8)
    self.class.register_active_process(@process)

    # Always drain stderr to prevent pipe buffer deadlock.
    # Without this, --verbose output fills the OS pipe buffer (~64KB),
    # the subprocess blocks on write, and all pipes stall → EPIPE.
    if @stderr
      if should_pipe_stderr # rubocop:disable Style/ConditionalAssignment
        @stderr_task = Thread.new do
          handle_stderr
        rescue StandardError
          # Ignore errors during stderr reading
        end
      else
        # Silently drain stderr so the subprocess never blocks,
        # but still accumulate recent lines for error reporting.
        @stderr_task = Thread.new do
          drain_stderr_with_accumulation
        rescue StandardError
          # Ignore — process may have already exited
        end
      end
    end

    # Always keep stdin open — streaming mode uses it for the control protocol
    @ready = true
  rescue Errno::ENOENT => e
    # Check if error is from cwd or CLI
    if @cwd && !File.directory?(@cwd.to_s)
      error = CLIConnectionError.new("Working directory does not exist: #{@cwd}")
      @exit_error = error
      raise error
    end
    error = CLINotFoundError.new("Claude Code not found at: #{@cli_path}")
    @exit_error = error
    raise error
  rescue StandardError, NotImplementedError => e
    # NotImplementedError < ScriptError, not StandardError (the trap this
    # repo keeps hitting): spawn raises it for :uid on platforms without
    # setuid (Windows), and it must wrap like every other spawn failure.
    error = CLIConnectionError.new("Failed to start Claude Code: #{e}")
    @exit_error = error
    raise error
  end
end

#drain_stderr_with_accumulation ⇒ Object 



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# File 'lib/claude_agent_sdk/subprocess_cli_transport.rb', line 316

def drain_stderr_with_accumulation
  return unless @stderr

  @stderr.each_line("\n", @max_buffer_size + 1) do |line|
    line_str = line.chomp
    next if line_str.empty?

    record_bounded_stderr(line_str)
  end
end

#end_input ⇒ Object 



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# File 'lib/claude_agent_sdk/subprocess_cli_transport.rb', line 461

def end_input
  # Under @stdin_mutex like write/close (the transport's documented
  # locking protocol; Python's end_input takes _write_lock too). The
  # nil-guard must live INSIDE the critical section or the TOCTOU
  # returns. NOTE: non-reentrant — close() inlines its own stdin
  # handling and must never delegate here.
  @stdin_mutex.synchronize do
    return unless @stdin

    begin
      @stdin.close
    rescue StandardError
      # Ignore
    end
    @stdin = nil
  end
end

#find_cli ⇒ Object

Raises:



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# File 'lib/claude_agent_sdk/subprocess_cli_transport.rb', line 113

def find_cli
  # Try which command first (using Open3 for thread safety)
  cli = nil
  begin
    stdout, _status = Open3.capture2('which', 'claude')
    cli = stdout.strip
  rescue StandardError
    # which command failed, try common locations
  end
  return cli if cli && !cli.empty? && File.executable?(cli)

  # Try common locations
  locations = [
    File.join(Dir.home, '.claude/local/claude'),  # Claude Code default install location
    File.join(Dir.home, '.npm-global/bin/claude'),
    '/usr/local/bin/claude',
    File.join(Dir.home, '.local/bin/claude'),
    File.join(Dir.home, 'node_modules/.bin/claude'),
    File.join(Dir.home, '.yarn/bin/claude')
  ]

  locations.each do |path|
    return path if File.exist?(path) && File.file?(path)
  end

  raise CLINotFoundError.new(
    "Claude Code not found. Install with:\n" \
    "  npm install -g @anthropic-ai/claude-code\n" \
    "\nIf already installed locally, try:\n" \
    '  export PATH="$HOME/node_modules/.bin:$PATH"' \
    "\n\nOr provide the path via ClaudeAgentOptions:\n" \
    "  ClaudeAgentOptions.new(cli_path: '/path/to/claude')"
  )
end

#handle_stderr ⇒ Object 



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# File 'lib/claude_agent_sdk/subprocess_cli_transport.rb', line 278

def handle_stderr
  return unless @stderr

  @stderr.each_line("\n", @max_buffer_size + 1) do |line|
    line_str = line.chomp
    next if line_str.empty?

    record_bounded_stderr(line_str)

    # Per-line isolation: a callback that raises (e.g. user's logger
    # transiently failing) must not poison the rest of the stderr stream.
    # Without this, the first exception terminates the each_line loop and
    # the SDK silently stops capturing stderr for the lifetime of the
    # process. Matches Python SDK v0.2.82 (PR #932).
    begin
      @options.stderr&.call(line_str)
    rescue StandardError
      # Drop the callback error; the line is already in the recent-stderr
      # ring buffer, which is what ProcessError surfaces on non-zero exit.
    end

    # Write to debug_stderr file/IO if provided, also isolated.
    begin
      if @options.debug_stderr
        if @options.debug_stderr.respond_to?(:puts)
          @options.debug_stderr.puts(line_str)
        elsif @options.debug_stderr.is_a?(String)
          File.open(@options.debug_stderr, 'a') { |f| f.puts(line_str) }
        end
      end
    rescue StandardError
      # Drop debug_stderr write errors so they never interrupt the loop.
    end
  end
rescue StandardError
  # Stream-level error (pipe closed mid-read); the loop naturally ends here.
end

#inject_otel_trace_context(process_env, custom_env) ⇒ Object

Inject W3C trace context (TRACEPARENT/TRACESTATE, plus BAGGAGE) into the subprocess env when an OTel span is active. Guard via defined? + respond_to?, not require: an active span implies the constant is loaded, and requiring here would break against the test mock / optional gem group. Gate on the carrier’s traceparent key (the W3C propagator writes it only for a valid span context) so a baggage-only carrier or a noop propagator preserves inherited env.



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# File 'lib/claude_agent_sdk/subprocess_cli_transport.rb', line 155

def inject_otel_trace_context(process_env, custom_env)
  return unless defined?(OpenTelemetry) && OpenTelemetry.respond_to?(:propagation)

  carrier = {}
  OpenTelemetry.propagation.inject(carrier)
  return unless carrier.key?('traceparent')

  # Active span: scrub stale inherited W3C context (CI/k8s ambient env)
  # before writing fresh values, so an inherited TRACESTATE is never
  # paired with a new TRACEPARENT. nil actively unsets (spawn overlay
  # semantics — see the CLAUDECODE note in #connect; Python pops from a
  # complete env dict instead). Explicit options.env keys always win.
  %w[TRACEPARENT TRACESTATE].each do |key|
    process_env[key] = nil unless custom_env.key?(key)
  end
  carrier.each do |key, value|
    env_key = key.upcase
    process_env[env_key] = value unless custom_env.key?(env_key)
  end
rescue StandardError, ScriptError
  # Best-effort tracing must never break connect() (Python: except
  # Exception). ScriptError too: NotImplementedError < ScriptError.
end

#read_messages(&block) ⇒ Object

Raises:



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# File 'lib/claude_agent_sdk/subprocess_cli_transport.rb', line 479

def read_messages(&block)
  return enum_for(:read_messages) unless block_given?

  raise CLIConnectionError, 'Not connected' unless @process && @stdout

  json_buffer = ''

  begin
    # The limit bounds per-read allocation: a line longer than
    # max_buffer_size+1 arrives as bounded chunks that the existing
    # accumulation + cap machinery below handles (mirrors Python, where
    # TextReceiveStream yields <=64KB chunks and the cap fires
    # incrementally). +1 so an exactly-max line plus "\n" arrives whole.
    # With UTF-8 external encoding Ruby extends a few bytes past the
    # limit rather than splitting a multibyte char. Without the limit,
    # an oversized line was fully allocated BEFORE the 1MB cap could
    # fire — unbounded memory on hostile/buggy stdout.
    @stdout.each_line("\n", @max_buffer_size + 1) do |line|
      # Position-aware whitespace handling: a chunk of an over-limit line
      # must keep its interior whitespace — a blanket per-chunk strip
      # deleted spaces inside JSON strings straddling the chunk boundary
      # and could let a just-over-cap line PARSE with bytes silently
      # missing instead of raising. Only safe edges are trimmed: full
      # single-chunk lines strip both ends (the common path, original
      # behavior); a truncated line-initial chunk keeps its tail; a
      # continuation chunk keeps its head and only drops the newline.
      ends_line = line.end_with?("\n")
      if json_buffer.empty?
        chunk = ends_line ? line.strip : line.lstrip
        next if chunk.empty?

        # When no partial JSON is buffered, the line must start with `{`
        # to be a valid stream-json message. Stray stderr-like text
        # (e.g., debug warnings the CLI occasionally writes to stdout)
        # would otherwise be appended into json_buffer, poisoning every
        # subsequent parse until the buffer overflows. Matches the Python
        # SDK's `if not json_buffer and not json_line.startswith("{")`.
        next unless chunk.start_with?('{')
      else
        chunk = ends_line ? line.chomp : line
      end

      json_buffer += chunk

      if json_buffer.bytesize > @max_buffer_size
        buffer_length = json_buffer.bytesize
        json_buffer = ''
        raise CLIJSONDecodeError.new(
          "JSON message exceeded maximum buffer size",
          StandardError.new("Buffer size #{buffer_length} exceeds limit #{@max_buffer_size}")
        )
      end

      begin
        data = JSON.parse(json_buffer, symbolize_names: true)
        json_buffer = ''
        yield data
      rescue JSON::ParserError
        # Continue accumulating (multi-line JSON, or a truncated chunk
        # awaiting the rest of its line)
        next
      end
    end
  rescue IOError
    # Stream closed
  rescue StopIteration
    # Client disconnected
  end

  # Check process completion. @process may already be nil (close() ran
  # concurrently and reset it) or already waited on (Errno::ECHILD on
  # double-wait). Both are non-fatal — the message loop just exits.
  returncode = nil
  begin
    status = @process&.value
    returncode = status&.exitstatus
  rescue Errno::ECHILD
    # Process was already reaped (e.g., by close()); no exit status to surface.
    returncode = nil
  end

  # The child has exited and been reaped; drop it from the parent-exit
  # registry now rather than waiting for #close, which a caller may never
  # reach (e.g. a Client abandoned without #disconnect, or direct transport
  # use). Idempotent — #close's own deregister becomes a harmless no-op, and
  # #close still sees @process (left set here) for its termination logic.
  self.class.deregister_active_process(@process)

  if returncode && returncode != 0
    # Wait briefly for stderr thread to finish draining
    @stderr_task&.join(1)

    stderr_text = @recent_stderr_mutex.synchronize { @recent_stderr.last(10).join("\n") }
    stderr_text = 'No stderr output captured' if stderr_text.empty?

    @exit_error = ProcessError.new(
      "Command failed with exit code #{returncode}",
      exit_code: returncode,
      stderr: stderr_text
    )
    raise @exit_error
  end
end

#ready? ⇒ Boolean

Returns:

  • (Boolean) 


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# File 'lib/claude_agent_sdk/subprocess_cli_transport.rb', line 615

def ready?
  @ready
end

#wait_process_with_timeout(timeout_seconds) ⇒ Object

Wait for the spawned process to exit, up to timeout_seconds. Polls @process.alive? rather than using stdlib Timeout.timeout, which raises across threads via Thread#raise and corrupts Async fiber-scheduler state (close is always called inside an Async task). Yields to the current Async task when one is active so the reactor keeps running.



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# File 'lib/claude_agent_sdk/subprocess_cli_transport.rb', line 420

def wait_process_with_timeout(timeout_seconds)
  deadline = Process.clock_gettime(Process::CLOCK_MONOTONIC) + timeout_seconds
  task = defined?(Async::Task) ? Async::Task.current? : nil
  while @process.alive?
    raise Timeout::Error if Process.clock_gettime(Process::CLOCK_MONOTONIC) >= deadline

    task ? task.sleep(0.05) : sleep(0.05)
  end
  @process.value
end

#write(data) ⇒ Object

Raises:



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# File 'lib/claude_agent_sdk/subprocess_cli_transport.rb', line 431

def write(data)
  raise CLIConnectionError, "Cannot write to terminated process" if @process && !@process.alive?
  raise CLIConnectionError, "Cannot write to process that exited with error: #{@exit_error}" if @exit_error

  # Snapshot @stdin under the lock so close() nilling it concurrently is
  # safe, but do the actual blocking IO *outside* the lock. Holding the
  # mutex across @stdin.write would let a full pipe buffer block the
  # writer indefinitely and block close() (which also needs the lock)
  # from killing the subprocess — a hang on disconnect.
  #
  # If close() runs while we are inside the IO call, it will close the
  # underlying stream and Ruby raises IOError("stream closed in another
  # thread") inside @stdin.write — the rescue below converts that into a
  # standard CLIConnectionError so callers see a clean shutdown error.
  stdin = @stdin_mutex.synchronize do
    raise CLIConnectionError, 'ProcessTransport is not ready for writing' unless @ready && @stdin

    @stdin
  end

  begin
    stdin.write(data)
    stdin.flush
  rescue StandardError => e
    @ready = false
    @exit_error = CLIConnectionError.new("Failed to write to process stdin: #{e}")
    raise @exit_error
  end
end