multiplexer: Improve WebSocket stability

[PrakharJain345]

Summary

Fixes #5224 by making the WebSocket multiplexer tolerate per-request failures without dropping the whole client session. Malformed client messages, token lookup failures, and cluster dial errors are isolated to the affected request/cluster while the multiplexer keeps serving other active watches.

Changes

• Refactor the client read loop to distinguish fatal socket read errors from non-fatal message parsing errors.
• Send routable backend ERROR frames for token and cluster connection failures.
• Route frontend multiplexer ERROR frames through useWebSocket's onError callback instead of fabricating Kubernetes watch update events.
• Avoid resourceVersion parsing failures for non-watch/non-JSON cluster messages from tearing down the session.
• Synchronize connection close-state reads so reconnect checks do not race with safeClose.
• Add backend and frontend regression coverage for multiplexer error handling.

Steps to Test

1. Open Headlamp with multiple resource watches active.
2. Trigger a malformed multiplexer request or a token/cluster connection error for one subscription.
3. Verify the affected subscription receives an error while the multiplexer connection and other watches remain active.

Local Validation

• go test ./cmd - passed
• npm test -- src/lib/k8s/api/v2/multiplexer.test.ts --run - passed
• npm run frontend:tsc - passed
• npm run frontend:lint - passed
• npm run backend:test - passed

Screenshots

N/A - backend/frontend protocol stability fix.

Notes for the Reviewer

Rebased onto latest main and squashed the PR to one guideline-compliant commit: multiplexer: Improve WebSocket stability.

[k8s-ci-robot]

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[Copilot]

Pull request overview

Improves session robustness for the Kubernetes WebSocket multiplexer (so one failing/malformed stream doesn’t drop all watches) and adjusts the Terminal component to more reliably fall back to alternate shells when initialization fails.

Changes:

• Backend: differentiate fatal vs non-fatal client WS read errors; avoid failing on non-JSON payloads when checking resourceVersion.
• Backend tests: update readClientMessage tests for the new (msg, isFatal, err) signature.
• Frontend: treat early ServerError channel output as an initialization failure to trigger shell fallback.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
frontend/src/components/common/Terminal.tsx	Adjusts terminal init failure detection to trigger shell fallback earlier.
backend/cmd/multiplexer.go	Refactors the client WS loop to isolate non-fatal errors; makes resourceVersion detection tolerant of non-JSON messages.
backend/cmd/multiplexer_test.go	Updates tests for the new readClientMessage return signature.

[illume]

Thanks for the contribution.

There are some open Copilot review comments — could you take a look at them?

[illume]

Thanks for this PR.

The CI backend test job has failures. Running cd backend && go test ./... locally should show what's going wrong.

How to run the backend tests

Run cd backend && go test ./... to see all failures. Fix the failing tests and commit the result.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

[illume]

Thanks for these changes.

Can you please address the open review comments?

[illume]

Thanks for these changes.

Can you please have a look at the git commits to see if they meet the contribution guidelines? We use a Linux kernel style of git commits. See the contributing guide for general context, and please see previous git commits with git log for examples.

Commits that need attention

• multiplexer: improve WebSocket stability and terminal shell fallback — Description must start with a capital letter — e.g. frontend: HomeButton: Fix the button not frontend: HomeButton: fix the button.

Commit guidelines

• Use atomic commits focused on a single change.
• Use the title format <area>: <Description of changes> — description must start with a capital letter.
• Keep the title under 72 characters (soft requirement).
• Explain the intention and why the change is needed.
• Make commit titles meaningful and describe what changed.
• Do not add code that a later commit rewrites; squash or reorder commits instead.
• Do not include Fixes #NN in commit messages.

Good examples:

• frontend: HomeButton: Fix so it navigates to home
• backend: config: Add enable-dynamic-clusters flag

[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated 6 comments.

[illume]

Thanks for these changes.

Can you please address the open review comments? Once you've resolved each one, please mark it as resolved.

[illume]

Thanks for the contribution.

it looks like there's a merge-main commit in this PR — could you rebase onto main instead?

Why this matters

Merge commits from main make the PR history harder to review. Please rebase your branch on top of the latest main instead, then update the PR with the rebased commits.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

[illume]

Thanks for the contribution.

it looks like there's a merge-main commit in this PR — could you rebase onto main instead?

Why this matters

Merge commits from main make the PR history harder to review. Please rebase your branch on top of the latest main instead, then update the PR with the rebased commits.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

Comments suppressed due to low confidence (1)

backend/cmd/multiplexer.go:297

• conn.closed is written under c.mu here, but there are unsynchronized reads elsewhere (e.g., reconnect() checks conn.closed without taking conn.mu). That creates a potential data race once safeClose is called concurrently with monitor/reconnect logic. Suggest making closed an atomic (atomic.Bool) or enforcing that all reads/writes go through the same mutex-protected accessor.

		c.mu.Lock()
		c.closed = true
		c.mu.Unlock()

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

[illume]

Thanks for the contribution.

There are some open Copilot review comments — could you take a look at them? Please mark each one as resolved once you've addressed it.