CBL-5780: Fix missing WebSocket close echo on remote-initiated close

[pasin]

• Current coreRequestsClose (the kC4NoFraming requestClose callback) skipped closeRemote whenever state was already CLOSING. On a remote-initiated close, remoteRequestsClose had already made that transition, so the close echo was never sent and HTTPListener::stopTasks() hung.

• Add StateMachine.enterState() which will simply return true if already in the state or if it can legally transition to the state.

• Use ensureState in coreRequestsClose so the close frame is sent both when we initiate the close (local) and when echoing a close the remote initiated.

[pasin]

With the fix, when running URLEndpointListenerTest.testCloseDbWithActiveListeners, we can see the following now:

  WebSocket#63 (C4IncomingRepl#64/PsvRepl#65):
  - 14:00:23.005 line 1823 — Requesting close with status=1000
  - 14:00:23.021 line 2052 — Close confirmed by peer; disconnecting socket now
  - 14:00:23.022 line 2056 — closeSocket
  - 14:00:23.022 line 2065 — Socket disconnected cleanly

  WebSocket #73 (C4IncomingRepl#74/PsvRepl#75):
  - 14:00:23.009 line 1882 — Requesting close with status=1000
  - 14:00:23.048 line 2402 — Close confirmed by peer; disconnecting socket now
  - 14:00:23.049 line 2404 — closeSocket

Log : https://gist.github.com/pasin/c6240fd253381e8bde315da3e725d19f

[Copilot]

Pull request overview

Fixes a WebSocket close-handshake edge case where a remote-initiated close could prevent sending the required close-echo frame, potentially causing HTTPListener::stopTasks() to hang. The PR introduces an idempotent state transition helper and uses it in the close request path so coreRequestsClose still sends the close frame when the socket is already CLOSING.

Changes:

• Added StateMachine.enterState(...) to support idempotent transitions (no-op if already in target state).
• Updated AbstractCBLWebSocket.coreRequestsClose(...) to use an idempotent transition (ensureState(CLOSING)) so remote-initiated closes still echo a close frame.
• Expanded coreRequestsClose documentation to describe local vs remote close flows.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
common/main/java/com/couchbase/lite/internal/utils/StateMachine.java	Adds an idempotent enterState transition helper used to avoid “already in state” failures/logging.
common/main/java/com/couchbase/lite/internal/replicator/AbstractCBLWebSocket.java	Ensures coreRequestsClose proceeds when already CLOSING, enabling close-echo on remote-initiated close.

Comments suppressed due to low confidence (1)

common/main/java/com/couchbase/lite/internal/utils/StateMachine.java:165

• The Javadoc for setState() describes returning the previous state / null, but the method actually returns a boolean. This mismatch is misleading (and now that enterState() references setState(), it’s more likely to be consulted). Update the Javadoc to reflect the boolean return semantics (success/failure).

     * Set the new state.
     * If it is legal to transition to the new state, from the current state, do so,
     * returning the now-previous state.  If the transition is illegal do nothing and return null.
     *
     * @param nextState the requested new state

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