[tabgab]

Fixes #1433. On master 7ec0d4d, one commit.

The guard band of a port is the MTU divided by the datarate of the interface. An interface that reports datarate 0 gave an infinite value, and its conversion to simtime_t stopped the simulation in GateScheduleConfiguratorBase::addPorts. Such a port now keeps the default guard band of -1, the "not set" value of the field; nothing reads the field. The port stays in the input, because the ports of its neighbours refer to it as their other port.

-                port->guardBand = (port->maxPacketLength / port->datarate).get<s>();
+                // an interface without a bitrate (e.g. cellular or tunnel) has no guard band: it stays -1
+                if (port->datarate > bps(0))
+                    port->guardBand = (port->maxPacketLength / port->datarate).get<s>();

Why not drop the port or stop with an error

Dropping it leaves the neighbour's otherPort lookup (std::find_if a few lines below) without a match. An error would reject every network with a radio or tunnel interface, which is the common case (a 5G segment represented by other means, no stream crossing it).

What this does not change

A stream whose path crosses a port without a datarate still cannot be scheduled. The configurators divide by that datarate along the path (EagerGateScheduleConfigurator.cc 129-130 and 200-201, Z3GateScheduleConfigurator.cc 77 and 120, TSNschedGateScheduleConfigurator.cc 206) and give the same inf. Z3 also divides by the datarate of every port for its interframe gap (Z3GateScheduleConfigurator.cc 161), so Z3 still stops on such a network; TSNsched writes portSpeed 0 into its input file. A clear error for such a flow is a separate change.

Evidence

• New test tests/module/GateScheduleConfigurator_1.test: the network of the issue with Eager; expects Simulation time limit reached. It fails on master with the error of the issue and passes with the fix.
• With the companion PR (null destination interface), both patches apply with git am without conflict, and the three new GateScheduleConfigurator_* tests pass together.
• showcases/tsn/gatescheduling/eager, config Eager, runs to its time limit with the fix.
• Release build, macOS arm64, OMNeT++ 6.5.0.

Not run: debug build; the full tests/module and fingerprint suites; Z3 and TSNsched (not built here); Linux and Windows.