Fix #12912: assign FastTerminal before starting the build thread

[gnodet]

Summary

Fixes a race condition introduced in #12921: the FastTerminal constructor started its build thread before returning, so MessageUtils.terminal was still null when the build thread ran the builder callback — a race between the constructor returning and the thread scheduling. This caused a NullPointerException in CI (example):

NullPointerException: Cannot invoke "org.jline.terminal.Terminal.getSize()" because "t" is null

The fix splits construction from start: MessageUtils now assigns the field before calling FastTerminal.start(), and Thread.start() provides the happens-before edge that makes the assignment visible to the build thread without additional synchronization.

• FastTerminal: extract start() method from constructor, document the publish-before-start contract
• MessageUtils.systemInstall(): assign terminal field, then call start()
• New test terminalAssignmentIsVisibleFromBuilderCallback that asserts MessageUtils.getTerminal() is non-null from the builder callback

Test plan

• FastTerminalReentrancyTest — all 5 tests pass (4 existing + 1 new)
• Full maven-jline module test suite passes
• CI validates on Java 17/21/25

🤖 Generated with Claude Code

[gnodet]

Correct fix for a real race condition. The constructor-starts-thread-before-field-assignment pattern was a textbook publish-before-init bug, and separating construction from start() is the right approach.

Highlights:

• The JMM analysis is sound: Thread.start() establishes a happens-before edge (JLS 17.4.4), so the prior assignment to MessageUtils.terminal is guaranteed visible to the build thread without volatile.
• The test terminalAssignmentIsVisibleFromBuilderCallback correctly exercises the invariant.
• The diff is identical to the forward port #12962 targeting master — the two PRs are in sync.

Minor suggestion (non-blocking):

• The new start() method is public, but its only caller is MessageUtils.systemInstall() in the same package. Package-private visibility would narrow the API surface. (LookupInvoker only calls getTerminal(), not start().)

This review was generated by an AI agent and may contain inaccuracies. Please verify all suggestions before applying.

Claude Code on behalf of Guillaume Nodet