OK, the issue identified by Hirokazu Yamamoto in msg113106 only actually affects the 2.x series, because of the awkwardly multiple-level interaction between file handles. The io rewrite in 3.x seems not to suffer the same way. Nonetheless, the proposed adhok.patch (slightly updated to match the current codebase) seems to cover all the cases, and is rather more elegant. So I'll revert the C module changes and apply that instead to all branches.