Function is_servable_narinfo 

pub fn is_servable_narinfo(narinfo: &str) -> bool

Is this narinfo text usable by a Nix client at all?

StorePath: is what makes a narinfo a narinfo — nix’s own reader rejects text without one as corrupt: StorePath missing — so this is the minimum bar for both accepting an upload and serving a stored entry.

── WHY THIS IS A SHARED PREDICATE, not an is_empty() at one call site ──── Measured on camelot-eks 2026-08-05: two rows in the durable tier held a ZERO-LENGTH value, and the read path served them as 200 with an empty body. Nix aborted the whole operation on the first one it met while asking the destination which paths it already had, so two poisoned rows out of 6898 broke EVERY nix copy --to against the cache.

An unusable hit is worse than a miss: a miss makes the client build, a malformed hit makes it fail — and the error names nix, not the cache. Both boundaries therefore ask the same question, because fixing only the write leaves existing poison fatal, and fixing only the read leaves the tier accumulating garbage.