Assertion failure with vsock: assert(readResult == .some) in BaseSocketChannel.swift

[aduermael]

Description

When using SwiftNIO with vsock (virtual sockets) through Apple's Containerization framework, the assertion assert(readResult == .some) in BaseSocketChannel.swift (around line 1203) fails intermittently during container startup.

The crash occurs in NIO Event Loop Threads (NIO-ELT-*) with a stack trace pointing to:

NIOPosix/BaseSocketChannel.swift:1203: Assertion failed

Root Cause Analysis

The assertion assumes that kqueue (macOS) and epoll (Linux) provide implicit synchronization - that a POLLIN notification will only be delivered when data is actually available to read. However, vsock (virtio-vsock) does not appear to have the same guarantees.

vsock communication goes through a hypervisor layer (in this case, Apple's Virtualization framework via VZVirtualMachineManager), which may introduce timing differences that cause spurious read notifications - similar to io_uring behavior.

Precedent: io_uring

This assertion is already conditionally disabled for io_uring:

#if !SWIFTNIO_USE_IO_URING
assert(readResult == .some)
#endif

The comment in the code explains that io_uring "lacks implicit synchronization between poll mask modifications and returned events." vsock appears to have the same characteristic when used through a hypervisor.

Reference: 8ea0afb

Proposed Fix

Handle .none gracefully instead of asserting, similar to how io_uring handles this case:

// Instead of:
#if !SWIFTNIO_USE_IO_URING
assert(readResult == .some)
#endif

// Consider:
if readResult == .none {
    // Spurious wakeup - no data available despite POLLIN
    // This can happen with vsock or other non-standard socket types
    self.readIfNeeded0()
    return .none
}

Environment

• macOS 26 (Tahoe)
• Apple Containerization framework (https://github.com/apple/containerization)
• SwiftNIO (latest via Containerization's dependencies)
• ARM64 (Apple Silicon)

Reproduction

The issue occurs when:

1. Using Apple's Containerization framework to create Linux containers
2. The framework establishes vsock connections for host-guest communication
3. During container startup, spurious POLLIN notifications trigger the assertion

I'll also file a related issue on apple/containerization to cross-reference.