Define required mirror partial-write semantics#16
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MirrorStore writes the authoritative backend before its required mirror. If the mirror rejects that write, the adapter returns a plain mirror_failed error even though the authoritative value and ETag have already changed. A caller retrying with its previous ETag then conflicts, but the error contains no indication that anything committed.\n\nAdd a deterministic integration test that records the original ETag, rejects the mirror write, proves that a retry with the old ETag conflicts, and fails because the ordinary error hid the committed authoritative value. The test is intentionally red pending a decision on the contract.\n\nPossible fixes are to return typed partial success with the authoritative result and new ETag, durably queue and repair the mirror before reporting success, compensate/roll back the authoritative write when possible, or narrow the meaning of required mode so callers do not interpret this as an atomic failure.
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@chrismccord,
MirrorStorewrites the authoritative backend before its required mirror. If the mirror fails, it returns a plain{:error, {:mirror_failed, reason}}even though the authoritative value and ETag have already changed. Retrying with the caller's old ETag then conflicts.This PR adds a failing integration test that demonstrates the authoritative commit and stale-ETag conflict. It intentionally does not choose a fix yet.
Which contract do you prefer?
:requiredsemantics so this is explicitly non-atomic and callers can recover correctly.The focused test currently fails with:
ordinary mirror failure hid committed authoritative value "value-v2".