Crash in `HTTP2ClientRequestHandler`

Despite what the comment says, this can crash on the client if the server sends a GOAWAY or RST_STREAM at the right time:

https://github.com/swift-server/async-http-client/blob/f95c908967e98c68c5ce3fd61a7974e7e869e303/Sources/AsyncHTTPClient/ConnectionPool/HTTP2/HTTP2ClientRequestHandler.swift#L247-L249

AI Disclosure: yes I used Gemini.

I asked Gemini to write the repro and the following comments, but I reviewed them myself. I found the initial problem while working on the SDK for Google Cloud Storage. That is artisanal, lovingly hand-crafted code, not AI slop. Reducing the problem to a smaller repro did involve LLMs.

HTTP2ClientCrashRepro.tar.gz

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A reproducer for a crash in swift-server/async-http-client (HTTP2ClientRequestHandler.swift:249: Fatal error: Unexpectedly found nil while unwrapping an Optional value).

This package runs an in-process mock HTTP/2 server on 127.0.0.1 and reproduces the crash without external network requests.

---

How to Run

cd HTTP2ClientCrashRepro
swift run

Or run with an explicit trigger action:

# Test stream reset (RST_STREAM) after early response headers (default)
swift run HTTP2ClientCrashRepro rst

# Test connection termination (GOAWAY) after early response headers
swift run HTTP2ClientCrashRepro goaway

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Crash Trace

=== Starting HTTP2ClientCrashRepro ===
Testing isolated HTTP/2 repro without external network calls...
Configured server action: respondThenRstStream(errorCode: HTTP2ErrorCode<0x8 Cancel>)
Mock HTTP/2 Server listening at https://127.0.0.1:39483
Sending streaming request to mock server...
AsyncHTTPClient/HTTP2ClientRequestHandler.swift:249: Fatal error: Unexpectedly found nil while unwrapping an Optional value
Response received: 200 OK

💣 Program crashed: Signal 4: Backtracing from 0x7f...

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Root Cause Analysis

The crash occurs when the server returns a complete response while the client's request body is still streaming, followed by a stream reset (RST_STREAM) or connection closure (GOAWAY):

1. Streaming Request Initiated:
   The client begins an HTTP/2 request with a streaming body (HTTPClientRequest.Body.stream).
2. Server Responds Early:
   The server sends response headers (200 OK, 412 Precondition Failed, etc.) with endStream: true.
3. Response Delivered & Request Cleared:
   HTTP2ClientRequestHandler.run(.forwardResponseEnd) delivers the response to the caller and clears its stored request reference:

   // Sources/AsyncHTTPClient/ConnectionPool/HTTP2/HTTP2ClientRequestHandler.swift:234
   self.request = nil
   

4. Server Resets Stream:
   While the client's request body is still streaming, the server issues a RST_STREAM frame.
5. Stream Error Forwarded:
   SwiftNIO delivers the stream reset to errorCaught.
6. State Machine Returns .failRequest:
   Because the request state machine was in .running(.streaming(...), .endReceived), it transitions to .failed and returns the action .failRequest(error, ...).
7. Force-Unwrap Crash:
   HTTP2ClientRequestHandler.run(.failRequest) force-unwraps self.request!:

   // Sources/AsyncHTTPClient/ConnectionPool/HTTP2/HTTP2ClientRequestHandler.swift:249
   case .failRequest(let error, let finalAction):
       self.request!.fail(error) // CRASH: self.request was set to nil in step 3
   

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Why TLS / HTTPS Is Required

AsyncHTTPClient does not implement cleartext HTTP/2 (h2c / prior knowledge). In HTTPConnectionPool+Factory.swift (lines 256 and 386), all plain http:// schemes use HTTP/1.1:

private func makePlainChannel(...) {
    ...
    bootstrap.connect(target: self.key.connectionTarget).map {
        .http1_1($0)
    }.cascade(to: promise)
}

When connecting to http://, AsyncHTTPClient sends HTTP/1.1 text (POST /test HTTP/1.1\r\n...). An HTTP/2 server expecting the HTTP/2 client connection preface (PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n) rejects this with invalid constant string.

In AsyncHTTPClient, HTTP2ClientRequestHandler is only added when ALPN negotiates "h2" during TLS (HTTPConnectionPool+Factory.swift:428). Therefore, an in-process TLS server with ALPN is required to exercise HTTP/2 and reproduce this bug.

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Test Certificates

The certificate and private key in Sources/main.swift are test credentials:

• They are generated for loopback testing on 127.0.0.1 and not used in production.
• The client sets config.tlsConfiguration?.certificateVerification = .none to accept the local test certificate.
• They are safe to check into source control.

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Why This Matters for Google Cloud & HTTP/2 SDKs

In Google Cloud SDKs (such as Cloud Storage):

• Streaming Uploads: Resumable uploads and multipart object uploads stream request bodies via HTTPClientRequest.Body.stream.
• Early Server Responses: If an upload fails early (e.g. 412 Precondition Failed on ifGenerationMatch, quota exhaustion, or authorization failure), Cloud Storage closes or resets the stream before the client completes writing the request body.
• Process Termination: Rather than delivering an error to the caller (so that retry or recovery logic can execute), AsyncHTTPClient triggers a fatal error (SIGILL), crashing the process.

---

System & Environment Information

The crash was captured and reproduced in the following environment:

Property	Value
Swift Version	Swift version 6.3.3 (swift-6.3.3-RELEASE) / Target: x86_64-unknown-linux-gnu
Operating System	Debian GNU/Linux rodete (VERSION_CODENAME=rodete)
Linux Kernel	6.18.14-1rodete4-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.18.14-1rodete4 (2026-06-15) x86_64
C Library (glibc)	ldd (Debian GLIBC 2.43-4+gl0) 2.43
Architecture	x86_64
Available CPUs (nproc)	64 (AMD EPYC 7B13, 1 socket, 32 cores, 64 threads)
System Memory	117 GiB RAM (Total) / ~102 GiB (Available)

Tested Dependency Versions

Package	Pinned Version	Revision
swift-server/async-http-client	1.36.0	9544287b9416c0bc71e58b9f3aead8dd14b16103
apple/swift-nio	2.101.3	0b18836bd8b0162e7e17a995a3fbee20ed8f3b2b
apple/swift-nio-http2	1.45.0	45bdf670248be5f16ec0340e125dca285536f0fb
apple/swift-nio-ssl	2.37.2	d930168b86f46ca51a4bc09c5ca45c1833db8067

Run the attached HTTP2ClientCrashRepro with the rst and goaway actions to confirm the failure. Inspect Sources/AsyncHTTPClient/ConnectionPool/HTTP2/HTTP2ClientRequestHandler.swift around lines 234–249 and follow the state-machine path into failRequest. Done means both HTTP/2 scenarios report an error without a fatal crash.