Go: Improve precision of `go/unhandled-writable-file-close` by owen-mc · Pull Request #21940 · github/codeql

There are paths to the exit of the function which go through the defer
statement and paths which don't, so we add an optional call to the
deferred function. This causes FPs in the query as it stands.

Correct the doc for unhandledCall (it also matches expression statements where the result is discarded) and remove a stale commented-out line in isWritableFileHandle.

A deferred Close runs at function exit, but the CFG splices it in at the exit node where it can be reached along paths that never execute Sync. The previous dominance check therefore produced a false positive when a statement followed the if-block that registered the defer (e.g. deferredCloseWithSync2). For deferred closes, require instead that a handled Sync post-dominates the point where the defer is registered, which guarantees Sync runs before Close on every path on which Close is registered. Non-deferred closes keep the existing dominance check.