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ee9bfb583c
In mutation_reader_merger and clustering_order_reader_merger, the operator()() is responsible for producing mutation fragments that will be merged and pushed to the combined reader's buffer. Sometimes, it might have to advance existing readers, open new and / or close some existing ones, which requires calling a helper method and then calling operator()() recursively. In some unlucky circumstances, a stack overflow can occur: - Readers have to be opened incrementally, - Most or all readers must not produce any fragments and need to report end of stream without preemption, - There has to be enough readers opened within the lifetime of the combined reader (~500), - All of the above needs to happen within a single task quota. In order to prevent such a situation, the code of both reader merger classes were modified not to perform recursion at all. Most of the code of the operator()() was moved to maybe_produce_batch which does not recur if it is not possible for it to produce a fragment, instead it returns std::nullopt and operator()() calls this method in a loop via seastar::repeat_until_value. A regression test is added. Fixes: scylladb/scylladb#14415 Closes #14452