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Evan JarrettandClaude Fable 5.1
831b7757bf
Since56bde61a large writer keeps one part in flight while the next buffer fills, and it charged that second buffer to the process-wide upload budget as the buffer grew. Nothing is released before Commit, so enough writers mid-growth could hold the whole budget between them while none of them had reached its peak: no writer could get the bytes it needed to fill a buffer, so none could reach the Commit that would have released any. Every Write then sat out the five minute wait cap and failed. That is a wedge, not backpressure. The invariant now is that a writer only ever waits for its first buffer. One full buffer is all a blob needs to finish: the writer can upload each part where it stands and refill the same buffer. So the second buffer is taken only when the budget can spare it without waiting, through a TryAcquire that also leaves a buffer's worth free for a writer that has not got its first one yet. A writer that cannot have one falls back to the serial upload it did before56bde61, and tries again at the next hand-off, so pipelining comes back as soon as memory does. The second buffer is charged and allocated whole at hand-off rather than grown into, which keeps w.charged exactly equal to the arrays the writer holds and means no later write in the upload asks the budget for anything. Repro, now a regression test: three writers sharing a budget sized for two, each filling its whole first buffer before any of them goes on. Before, 0 of 3 made progress with 64MB of 64MB held; now all three stream 32MB and commit, and the budget comes back whole. The other two new tests pin both sides of the opportunistic charge: a writer with a free budget carries on into a second buffer while its part is held on its way to S3, and a writer given a budget of exactly one buffer still pushes a 48MB blob as three parts without ever holding more than 16MB. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01EvFJr4Dwz8p2NDAeXmgmBt