fix: return "Session is closed" instead of NoNodeAvailableException during shutdown

[quantumsoul]

Motivation

CqlSession request handlers currently surface NoNodeAvailableException when a request happens to be in flight while the session is shutting down. This is misleading — no node was even attempted; the load balancing policy simply returned an empty query plan because it had already transitioned to CLOSING. See #846 for the report and the exact stack trace.

The existing DefaultSession.execute() and the request handler's timer path already surface IllegalStateException("Session is closed") for this situation. The shutdown-race path should match.

Modifications

• Added DefaultSession.queryPlanExhaustedError(session, errors) — when the errors list is empty and the session is either closing or already closed, returns IllegalStateException("Session is closed"); otherwise delegates to AllNodesFailedException.fromErrors(errors) as before.
• Added DefaultSession.isClosing(), backed by a volatile boolean closing flipped synchronously at the top of closeAsync() / forceCloseAsync(). isClosed() alone would miss the window between close initiation and closeFuture completion — exactly when the LB policy starts returning empty plans.
• Routed CqlRequestHandler, CqlPrepareHandler, GraphRequestHandler, and ContinuousRequestHandlerBase through the helper at their "query plan exhausted" branches.
• Two new unit tests in CqlRequestHandlerTest pin down each branch of the helper: isClosed() (post-shutdown-complete) and isClosing() (shutdown-in-progress).

Result

• Requests that reach a handler with an empty query plan during shutdown now fail with IllegalStateException("Session is closed"), matching the error already produced elsewhere for the same condition.
• Genuine "no node available" outages on a healthy session remain unchanged (non-empty per-node errors still produce AllNodesFailedException).
• No public API changes on CqlSession / AsyncAutoCloseable; isClosing() is a new method on the internal DefaultSession only.

Fixes #846

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: QUIET

Plan: Pro Plus

Run ID: 8a27cd91-899b-45bd-8d6b-2882c87319ed

📥 Commits

Reviewing files that changed from the base of the PR and between d8f6dd3 and d04f37d.

📒 Files selected for processing (6)

• core/src/main/java/com/datastax/dse/driver/internal/core/cql/continuous/ContinuousRequestHandlerBase.java
• core/src/main/java/com/datastax/dse/driver/internal/core/graph/GraphRequestHandler.java
• core/src/main/java/com/datastax/oss/driver/internal/core/cql/CqlPrepareHandler.java
• core/src/main/java/com/datastax/oss/driver/internal/core/cql/CqlRequestHandler.java
• core/src/main/java/com/datastax/oss/driver/internal/core/session/DefaultSession.java
• core/src/test/java/com/datastax/oss/driver/internal/core/cql/CqlRequestHandlerTest.java

Included review availability: Your plan provides up to 1 included review per hour; 0 remain after this review.

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📝 Walkthrough

Walkthrough

DefaultSession now tracks shutdown before closure completes and provides queryPlanExhaustedError. Request handlers use this helper when query plans are exhausted. Empty query plans during session closure or after closure now produce IllegalStateException("Session is closed"). Existing per-node errors continue through AllNodesFailedException.fromErrors. Tests cover both closed and closing sessions.

Suggested reviewers: dkropachev, nikagra

Merge Risk: ⚪ Minimal · up to d04f3

Requests that race with session shutdown now receive the intended "Session is closed" error, while genuine node-availability failures remain unchanged; no actionable merge-blocking risk remains beyond normal checks and review.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 46.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 15 functions across 6 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the shutdown error-handling change.
Description check	✅ Passed	The description directly explains the shutdown race, implementation, affected handlers, tests, and expected error behavior.
Linked Issues check	✅ Passed	The changes satisfy issue #846 by returning IllegalStateException("Session is closed") for empty query plans during shutdown while preserving AllNodesFailedException for genuine node-availability fail…
Out of Scope Changes check	✅ Passed	The modified handlers, DefaultSession state, helper method, and tests are directly related to the shutdown error-handling objective. No unrelated code changes are identified.

Full details: Linked Issues check

Explanation

The changes satisfy issue #846 by returning IllegalStateException("Session is closed") for empty query plans during shutdown while preserving AllNodesFailedException for genuine node-availability failures.

• Fix all pre-merge checks with AI

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