In bug 1755660 we added some tests for cancelling proxied channels, and it's triggered this assertion:

0  XUL!mozilla::psm::TransportSecurityInfo::SetCanceled(int) [TransportSecurityInfo.cpp:ac2c4ef5bde1e7290d0392eb6dcfb40d755ed3d9 : 115 + 0x0]
1  XUL!(anonymous namespace)::checkHandshake(int, bool, PRFileDesc*, nsNSSSocketInfo*) [nsNSSIOLayer.cpp:ac2c4ef5bde1e7290d0392eb6dcfb40d755ed3d9 : 1318 + 0x7]
2  XUL!mozilla::net::nsSocketTransport::IsAlive(bool*) [nsSocketTransport2.cpp:ac2c4ef5bde1e7290d0392eb6dcfb40d755ed3d9 : 2473 + 0x11]
3  XUL!{virtual override thunk({offset(-24)}, mozilla::net::nsSocketTransport::IsAlive(bool*))} [nsSocketTransport2.cpp:ac2c4ef5bde1e7290d0392eb6dcfb40d755ed3d9 : 0 + 0xc]
4  XUL!mozilla::net::nsHttpConnection::IsAlive() [nsHttpConnection.cpp:ac2c4ef5bde1e7290d0392eb6dcfb40d755ed3d9 : 804 + 0x5]
5  XUL!mozilla::net::nsHttpConnection::CanReuse() [nsHttpConnection.cpp:ac2c4ef5bde1e7290d0392eb6dcfb40d755ed3d9 : 742 + 0x14]
6  XUL!mozilla::net::ConnectionEntry::PruneDeadConnections() [ConnectionEntry.cpp:ac2c4ef5bde1e7290d0392eb6dcfb40d755ed3d9 : 437 + 0x7]
7  XUL!mozilla::net::nsHttpConnectionMgr::OnMsgPruneDeadConnections(int, mozilla::net::ARefBase*) [nsHttpConnectionMgr.cpp:ac2c4ef5bde1e7290d0392eb6dcfb40d755ed3d9 : 2223 + 0x4]
8  XUL!RefPtr<mozilla::net::nsHttpConnectionMgr>::Proxy<void, int, mozilla::net::ARefBase*>::operator()<int&, RefPtr<mozilla::net::ARefBase>&>(int&, RefPtr<mozilla::net::ARefBase>&) [RefPtr.h:ac2c4ef5bde1e7290d0392eb6dcfb40d755ed3d9 : 330]
9  XUL!mozilla::net::ConnEvent::Run() [nsHttpConnectionMgr.cpp:ac2c4ef5bde1e7290d0392eb6dcfb40d755ed3d9 : 176 + 0x19]

The line triggering the assertion is here.
I'm not sure if this is pointing to an actual bug or not.

Adding 1789458 to See Also. It might turn out not to be directly related, but suggests there is a deeper issue where we some invalid state during checkHandshake.

The severity field is not set for this bug.
:keeler, could you have a look please?

For more information, please visit auto_nag documentation.

This also restructures checkHandshake to be more clear and understandable, to
simplify maintenance going forward.

Backed out for causing build bustages at nsNSSIOLayer.cpp.

Backout link: https://hg.mozilla.org/integration/autoland/rev/5b95ea71ec704cfae6926f2ee8045e7b8b9a9df4

Push with failures: https://treeherder.mozilla.org/jobs?repo=autoland&resultStatus=testfailed%2Cbusted%2Cexception%2Cretry%2Cusercancel&revision=99ed42b2349bd67e07b07d167efac919eb4181b7&selectedTaskRun=K520m4btSUacjNywu6fHEg.0

Failure log: https://treeherder.mozilla.org/logviewer?job_id=397982990&repo=autoland&lineNumber=34895

https://hg.mozilla.org/mozilla-central/rev/b1a2405349af