StartExecution returns a Terminal for exec sessions but never wires stdin

[shaaravraghu]

Type
Bug

Severity
High

Modules

• spawn-docker-jdk

Evidence

• spawn-docker-jdk/src/main/java/build/spawn/docker/jdk/command/StartExecution.java:111-128
• The code contains TODO: establish PipedWriter for stdin.
• getInputWriter() returns NullWriter.get() even when terminalRequired is true.

Why this matters

• Docker exec sessions are a common place where callers expect genuine interactivity.
• Returning a terminal with a no-op writer breaks that expectation and can lead to confusing hangs.

Expected behavior

• If terminalRequired is true, stdin should be usable.

Actual behavior

• Callers may receive Optional.of(terminal), but that terminal is effectively output-only.

Suggested reproduction

1. Start an exec session that waits for input.
2. Request terminalRequired=true.
3. Write to the terminal input writer.
4. Observe that the exec session never receives the input.

Suggested fix

• Wire stdin to the Docker exec stream.
• If the implementation cannot support stdin yet, return Optional.empty() or document the limitation explicitly.

Suggested tests

• Add an integration test for interactive docker exec behavior.

Issue body ready to paste
StartExecution can return Optional.of(terminal) for exec sessions, but the terminal input side is not implemented. The code still returns NullWriter, so terminalRequired=true currently produces a read-only terminal rather than a truly interactive exec session.`