sstable: Make sstable Writer.Close idempotent

[bananabrick]

Fixes: cockroachdb/cockroach#75717

[cockroach-teamcity]

This change is 

[bananabrick]

Reviewable status: 0 of 2 files reviewed, 1 unresolved discussion

---

sstable/write_queue.go, line 109 at r1 (raw file):

// finish will return any error which was encountered while tasks were processed.
func (w *writeQueue) finish() error {
	if w.closed {

I make an assumption here that finish won't be called concurrently by two goroutines, which means that Close won't be called concurrently by two goroutines, which seems like a decent assumption.

[itsbilal]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @bananabrick)

---

sstable/write_queue.go, line 109 at r1 (raw file):

Previously, bananabrick (Arjun Nair) wrote…

I make an assumption here that finish won't be called concurrently by two goroutines, which means that Close won't be called concurrently by two goroutines, which seems like a decent assumption.

It's a fair assumption as the writer isn't really safe for concurrent use in general so it should be fine.