Prevent LibGpiodV2 native abort on null edge event

[Copilot]

Fixed #2600

A service watching an input pin via LibGpiodV2Driver could be killed by an uncatchable native abort() (gpiod_edge_event_copy: Assertion 'event' failed) — even with no real edge activity. EdgeEventBuffer.GetEvent forwarded the pointer from gpiod_edge_event_buffer_get_event straight into gpiod_edge_event_copy; that function returns NULL for an unpopulated index and the copy call starts with assert(event), so the whole process aborts with no managed exception to catch.

Changes

• EdgeEventBuffer.GetEvent — null-check the handle before copying; throw a handleable GpiodException instead of passing null into the native call.
• LibGpiodV2EventObserver — bound the observer loop by min(ReadEdgeEvents(...), GetNumEvents()) so unpopulated buffer slots are never read. gpiod_line_request_read_edge_events can report a count larger than what's actually retrievable from the buffer; this avoids hitting the null path during normal operation.

using EdgeEventNotFreeable edgeEventHandle = LibgpiodV2.gpiod_edge_event_buffer_get_event(Handle, index);
if (edgeEventHandle.IsInvalid)
{
    throw new GpiodException($"Edge event at index {index} is not populated in the buffer.");
}

Together these turn a process-killing abort into, at worst, a recoverable managed error, while the loop bound prevents the error path from being reached in the common case.

Notes

• Full build/test could not be validated in this environment (Azure DevOps feeds for Microsoft.DotNet.Arcade.Sdk are unreachable). The affected proxies are internal native-interop wrappers exercised only by hardware integration tests, so no isolated unit test was added.

• Fixes LibGpiodV2: unchecked null from gpiod_edge_event_buffer_get_event aborts the process (gpiod_edge_event_copy: Assertion 'event' failed) #2600

[Copilot]

Pull request overview

This PR addresses a reliability issue in the System.Device.Gpio libgpiod v2 interop path where a NULL edge event pointer could be forwarded into gpiod_edge_event_copy, triggering an uncatchable native abort() and terminating the host process.

Changes:

• Add a null/invalid-handle guard in EdgeEventBuffer.GetEvent to throw a managed GpiodException instead of passing a null pointer into native code.
• Bound the edge-event handling loop in LibGpiodV2EventObserver to the number of events actually stored in the buffer (min(ReadEdgeEvents, GetNumEvents)), avoiding reads of unpopulated buffer slots.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/System.Device.Gpio/System/Device/Gpio/Drivers/LibGpiodV2EventObserver.cs	Bounds event processing by buffered event count to avoid unpopulated reads.
src/System.Device.Gpio/Interop/Unix/libgpiod/V2/Proxies/EdgeEventBuffer.cs	Adds invalid-handle check before copying an edge event to prevent native abort.

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