fix(grpc): avoid nil conn panic when close races asyncInvoke

[cursor]

What?

Capture the gRPC connection before spawning the AsyncInvoke goroutine so client.close() cannot nil c.conn out from under Invoke.

Why?

AsyncInvoke checked c.conn != nil on the VU event loop, then launched a goroutine that called c.conn.Invoke(...). As soon as asyncInvoke() returned, JS could call client.close(), which sets c.conn = nil. When the goroutine then ran, it paniced:

panic: runtime error: invalid memory address or nil pointer dereference
go.k6.io/k6/v2/internal/lib/netext/grpcext.(*Conn).Invoke(0x0, ...)
go.k6.io/k6/v2/internal/js/modules/k6/grpc.(*Client).AsyncInvoke.func1

Trigger:

client.connect(addr);
const p = client.asyncInvoke('service/Method', {});
client.close(); // before await
await p; // process crash

Impact: process crash (SIGSEGV) on a documented API combination. Independent of the grpc-go v1.83.0 bump (#6240); the nil is k6's Client.conn field.

Using a closed-but-non-nil connection returns a gRPC error instead of panicking.

Checklist

• I have performed a self-review of my code.
• I have commented on my code, particularly in hard-to-understand areas.
• I have added tests for my changes.
• I have run linter and tests locally (make check) and all pass. (Focused: go test -race on ./internal/js/modules/k6/grpc/, go vet.)

Checklist: Documentation (only for k6 maintainers and if relevant)

• I have added the correct milestone and labels to the PR.
• I have updated the release notes: link
• I have updated or added an issue to the k6-documentation: grafana/k6-docs#NUMBER if applicable
• I have updated or added an issue to the TypeScript definitions: grafana/k6-DefinitelyTyped#NUMBER if applicable

Related PR(s)/Issue(s)

• Same nil-deref crash class as fix(ws): avoid panic on dial failure when ip system tag is enabled #6241 (k6/ws)
• grpc-go bump fix(deps): update module google.golang.org/grpc to v1.83.0 (master) #6240 audited: no interaction with this race

Validation

• Forced-sleep repro panicked at grpcext.(*Conn).Invoke(0x0) before the fix
• TestAsyncInvokeCloseDoesNotPanic passes under -race -count=5 after the fix
• go test -race ./internal/js/modules/k6/grpc/ -run 'TestClient$|TestAsyncInvoke' passes

  

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