Paaster: TOCTOU race condition defeats the 'burn after reading' guarantee on view-once pastes

Paaster is a pastebin application. Prior to version 3.3.0, Paaster's "view once" (burn-after-reading) paste mode can be bypassed with concurrent requests. The check that's supposed to enforce single-use is a plain read followed by a separate write, with no atomicity, so multiple simultaneous requests can all see the paste as "not yet read" and all successfully retrieve it. This issue has been patched in version 3.3.0.