Fix/http encoder stream flush - #4227
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Stream compressors buffer small writes internally: after writing a body chunk, Encoder::poll_next's take() returns nothing and the compressed bytes stay inside the encoder until finish() runs at stream end. A streaming response body (SSE, for example) wrapped in the Compress middleware is therefore delivered as a single burst when the stream completes, not incrementally as chunks are produced — the first chunk only ever carries the bare gzip header. Flush the encoder whenever an in-place or blocking write produces no output so each body chunk reaches the client as it is produced. Large writes that produce output immediately are unaffected; full-body compression is unchanged apart from the added sync markers. Regression test gates the body on a channel and asserts compressed bytes for the first events arrive before the gate is released; without the flush it times out because nothing but the gzip header is sent before the stream ends.
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The background of this change is that I deploy my services behind cloud flare. Cloud flare has a timeout on stale connections. Usually this is solved with streaming. However once compression is enabled this no longer works hence this patch.
It was created by AI.
AI Generated:
Stream compressors buffer small writes internally: after writing a body chunk, Encoder::poll_next's take() returns nothing and the compressed bytes stay inside the encoder until finish() runs at stream end. A streaming response body (SSE, for example) wrapped in the Compress middleware is therefore delivered as a single burst when the stream completes, not incrementally as chunks are produced — the first chunk only ever carries the bare gzip header.
Flush the encoder whenever an in-place or blocking write produces no output so each body chunk reaches the client as it is produced. Large writes that produce output immediately are unaffected; full-body compression is unchanged apart from the added sync markers.
Regression test gates the body on a channel and asserts compressed bytes for the first events arrive before the gate is released; without the flush it times out because nothing but the gzip header is sent before the stream ends.
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