feat(udp): handle partial sendmsg_x progress#2748
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cc @larseggert |
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I like it! |
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thomaseizinger
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Jul 22, 2026
| /// contents are empty. | ||
| pub fn datagram_count(&self) -> usize { | ||
| match self.segment_size { | ||
| Some(0) => panic!("segment size must be non-zero"), |
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This is a bit of an annoying case.
Has it been discussed before to change segment size to a NonZeroUsize?
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I've removed this in favor of treating Some(0) like an unsegmented datagram.
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| // these by automatically clamping the MTUD upper bound to the interface MTU. | ||
| Err(e) if e.raw_os_error() == Some(libc::EMSGSIZE) => Ok(()), | ||
| Err(e) if e.raw_os_error() == Some(libc::EMSGSIZE) => { | ||
| Ok(SendCount::from_datagram_count(attempted_datagrams)) |
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This is also somewhat unfortunate. If it were after me, we should remove /deprecate the send functions.
The private `sendmsg_x` API that we use for batching sends on Apple has roughly the same semantics as sendmmsg on Linux. Most importantly, it can make partial progress and it will return the number of successfully sent datagrams as a return value. Right now, we are ignorning this return value, resulting in silent packet drops under load. This has been confirmed in a LAN benchmark with ~ 2Gbit/s throughput. `quinn-udp` doesn't integrate with any runtime and needs to offer an API that makes sense across platforms. The new API tries to navigate both of these constraints. The public API remains largely unmodified except for the additions of two items: 1. `UdpSocketState::send` now returns a `SendCount` that is `#[must_use]` 2. `Transmit` exposes an `advance` function that consumes the `SendCount` Downstream users of `quinn-udp` will therefore receive a compiler warning as soon as they upgrade to this new version. If ignored, existing behaviour is preserved (i.e. packets get dropped).
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This PR offers an alternative to #2672 that doesn't require buffering. For a detailed description, see the commit message.