GeezerVison

Summary

Fixes the GCS destination write path so the *storage.Writer is always released, including when WriteHeader, WriteContent or WriteFooter fails, and so a sync that produces no messages no longer touches a nil handle.

Root Cause

WriteTable in plugins/destination/gcs/client/write.go created the object writer lazily on the first message and returned straight from three error paths without closing it:

h, err = c.Client.WriteHeader(w, table)
if err != nil {
    return err // w.Close() never called
}
if err := h.WriteContent(...); err != nil {
    return err // w.Close() never called
}
if err := h.WriteFooter(); err != nil {
    return err // w.Close() never called
}

Without Close, the in-progress upload and its HTTP connection stay alive until garbage collection, so repeated failing syncs accumulate leaked resources.

Separately, when the message channel was empty the loop body never ran, h stayed nil, and h.WriteFooter() panicked on the nil interface value.

Fix

The write loop moved into an internal writeTable function that takes a writer factory, which keeps the production path identical while making the logic testable with stubs:

• Every error path now returns through closeOnError, which closes the writer and joins any close error with the original write error.
• The writer is wrapped in a small onceCloser so Close is idempotent and the underlying *storage.Writer can only be closed once.
• An empty channel returns nil before touching the handle. The successful path is unchanged: header, content per message, footer, then Close.

No public API or configuration changes.

Test Plan

New unit tests in plugins/destination/gcs/client/write_test.go use a stub types.FileType, stub handle and stub writer, so no GCS credentials or network are needed:

• TestWriteTableClosesWriterOnError covers failing WriteHeader, WriteContent and WriteFooter, asserting each returns the write error and closes the writer exactly once.
• TestWriteTableJoinsCloseError asserts a close failure is joined with the write error.
• TestWriteTableEmptyChannel asserts an empty channel returns nil without creating a writer (this panicked before the fix).
• TestWriteTableSuccess asserts multiple messages produce one content write each, one footer and one close.
• TestOnceCloserClosesUnderlyingWriterOnce asserts repeated Close calls only reach the underlying writer once.

Commands run from plugins/destination/gcs with Go 1.27.3:

• go build ./... : pass
• go vet ./client/ : pass
• go test ./client/ -run 'TestWriteTable|TestOnceCloser' -v -count=1 : all 6 tests pass (the three error subtests, close-error join, empty channel, success, and once-closer idempotency)
• gofmt check on both changed files : clean

Fixes #23173