Fix stale pull diagnostics after document edits

[Kelpy2004]

Addresses microsoft/vscode#317522

Summary

This prevents stale textDocument/diagnostic responses from being applied after a document has changed.

Diagnostic requests now keep the request-time snapshot, compute diagnostics against it, and then verify that the snapshot still matches the session's current document/default-project state before sending the response. If the document or project has moved on, the server returns ContentModified so the client can retry with fresh state instead of showing outdated errors.

Changes

• Added a diagnostic-specific request handler that preserves the request snapshot.
• Added a session freshness check for request-time document snapshots.
• Added regression coverage for a diagnostic request invalidated by a later edit.

Validation

• go test ./internal/lsp -run TestDocumentDiagnosticCancelsStaleSnapshot -v -count=1 -timeout=45s
• go test ./internal/lsp -count=1 -timeout=5m
• go test ./internal/project -run TestSession -count=1 -timeout=2m
• npx hereby check:format
• npx hereby lint
• npx hereby build
• npx hereby test

Note: hereby test passed with the repo warning that the TypeScript submodule is not cloned, so submodule-dependent tests may be skipped.

Disclosure

AI assistance was used to prepare this patch; I reviewed, tested, and take responsibility for the changes.

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds a stale-snapshot guard for document diagnostics requests so that responses computed against an out-of-date snapshot are rejected with ContentModified, allowing the client to re-request fresh diagnostics.

Changes:

• Introduces Session.IsSnapshotCurrentForDocument to compare a request-time snapshot with the session's current snapshot for a document.
• Adds a dedicated registerDocumentDiagnosticHandler that captures the snapshot at request time and returns ContentModified if it becomes stale.
• Adds a test verifying that stale diagnostic responses are rejected after the underlying file changes.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
internal/project/session.go	Adds IsSnapshotCurrentForDocument and sameFileHandle helpers to detect snapshot staleness.
internal/lsp/server.go	Replaces generic diagnostic handler registration with a custom one that checks snapshot freshness before sending the result.
internal/lsp/server_diagnostics_test.go	New test verifying diagnostic requests against stale snapshots return ContentModified.

[Kelpy2004]

Hi @jakebailey, would you be the right person to take a look at this? This follows up on microsoft/vscode#317522 and touches LSP pull diagnostics / snapshot freshness. Happy to adjust the approach if there’s a better place to handle it.

[jakebailey]

I'm not sure why this is required? Pull diags are requested at a specific point in time, and the answer at that point is valid.

If there's a stale bug here, surely it's an LSP client bug.

[jakebailey]

The comments on the linked issue also mentions the client so I'm not sure where this comes from