tests: Remove asserts on the value of local Vsock CID (#2975)

### Motivation:

We have some tests for our Vsock support that only run when we detect
the system supports Vsock. These weren't running in CI but, now we've
migrated CI to GitHub Actions, they can. In prototyping this though, I
discovered that one of our tests was too strict, and will fail for VMs
running on Hyper-V, which is the hypervisor used by GitHub actions.

Specifically, we have support for getting the local context ID by a
channel option. This is implemented by an ioctl, but the semantics of
the return value differ between the various Vsock transport kernel
modules:

- `virtio_transport`: returns the guest CID, which is a number greater
than `VMADDR_CID_HOST`, but less than `VMADDR_CID_ANY`, and returns
`VMADDR_CID_ANY` as an error.
- `vsock_loopback`: returns `VMADDR_CID_LOCAL` always.
- `vmci_transport`: returns the guest CID if the guest and host
transport are active; `VMADDR_CID_HOST` if only the host transport is
active; and `VMCI_INVALID_ID` otherwise, which happens to be the same
value as `VMADDR_CID_ANY`.
- `hyperv_transport`: returns `VMADDR_CID_ANY` always.

For this reason, we should probably remove any attempts to interpret the
validity of the value that comes back from the driver and users will
need to know what to do with it.

### Modifications:

- tests: Only run Vsock echo tests on Linux, when `vsock_loopback` is
loaded.
- tests: Remove asserts on the value of local Vsock CID.
- ci: Add pipeline that runs Vsock tests.

### Result:

- Vsock tests will no longer run unless `vsock_loopback` kernel module
is loaded.
- Vsock tests will no longer fail in Hyper-V VMs.
- Vsock tests will now run in CI.

.github/workflows/pull_request.yml

@@ -41,3 +41,21 @@ jobs:
     with:
       name: "Integration tests"
       matrix_linux_command: "apt-get update -y -q && apt-get install -y -q lsof dnsutils netcat-openbsd net-tools curl jq && ./scripts/integration_tests.sh"
+
+  vsock-tests:
+    name: Vsock tests
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+        with:
+          persist-credentials: false
+      - name: Load vsock_loopback kernel module
+        run: sudo modprobe vsock_loopback
+      - name: Build package tests
+        run: swift build --build-tests
+      - name: Run Vsock tests
+        shell: bash  # explicitly choose bash, which ensures -o pipefail
+        run: swift test --filter "(?i)vsock" | tee test.out
+      - name: Check for skipped tests
+        run: test -r test.out && ! grep -i skipped test.out

Sources/NIOPosix/VsockAddress.swift

@@ -217,7 +217,7 @@ extension VsockAddress.ContextID {
     /// - On Darwin, the `ioctl()` request operates on a socket.
     /// - On Linux, the `ioctl()` request operates on the `/dev/vsock` device.
     ///
-    /// - Note: On Linux, ``local`` may be a better choice.
+    /// - Note: The semantics of this `ioctl` vary between vsock transports on Linux; ``local`` may be more suitable.
     static func getLocalContextID(_ socketFD: NIOBSDSocket.Handle) throws -> Self {
         #if canImport(Darwin)
         let request = CNIODarwin_IOCTL_VM_SOCKETS_GET_LOCAL_CID

Tests/NIOPosixTests/AsyncChannelBootstrapTests.swift

@@ -1141,7 +1141,7 @@ final class AsyncChannelBootstrapTests: XCTestCase {
     // MARK: VSock
 
     func testVSock() async throws {
-        try XCTSkipUnless(System.supportsVsock, "No vsock transport available")
+        try XCTSkipUnless(System.supportsVsockLoopback, "No vsock loopback transport available")
         let eventLoopGroup = MultiThreadedEventLoopGroup(numberOfThreads: 3)
         defer {
             try! eventLoopGroup.syncShutdownGracefully()

Tests/NIOPosixTests/EchoServerClientTest.swift

@@ -234,7 +234,7 @@ class EchoServerClientTest: XCTestCase {
     }
 
     func testEchoVsock() throws {
-        try XCTSkipUnless(System.supportsVsock, "No vsock transport available")
+        try XCTSkipUnless(System.supportsVsockLoopback, "No vsock loopback transport available")
         let group = MultiThreadedEventLoopGroup(numberOfThreads: 1)
         defer {
             XCTAssertNoThrow(try group.syncShutdownGracefully())

Tests/NIOPosixTests/TestUtils.swift

@@ -30,19 +30,10 @@ extension System {
         }
     }
 
-    static var supportsVsock: Bool {
-        #if canImport(Darwin) || os(Linux) || os(Android)
-        guard let socket = try? Socket(protocolFamily: .vsock, type: .stream) else { return false }
-        XCTAssertNoThrow(try socket.close())
-        #if !canImport(Darwin)
-        do {
-            let fd = try Posix.open(file: "/dev/vsock", oFlag: O_RDONLY | O_CLOEXEC)
-            try Posix.close(descriptor: fd)
-        } catch {
-            return false
-        }
-        #endif
-        return true
+    static var supportsVsockLoopback: Bool {
+        #if os(Linux) || os(Android)
+        guard let modules = try? String(contentsOf: URL(fileURLWithPath: "/proc/modules")) else { return false }
+        return modules.split(separator: "\n").compactMap({ $0.split(separator: " ").first }).contains("vsock_loopback")
         #else
         return false
         #endif

Tests/NIOPosixTests/VsockAddressTest.swift

@@ -58,15 +58,15 @@ class VsockAddressTest: XCTestCase {
     }
 
     func testGetLocalCID() throws {
-        try XCTSkipUnless(System.supportsVsock)
+        try XCTSkipUnless(System.supportsVsockLoopback, "No vsock loopback transport available")
 
         let socket = try ServerSocket(protocolFamily: .vsock, setNonBlocking: true)
         defer { try? socket.close() }
 
-        // Check the local CID is valid: higher than reserved values, but not VMADDR_CID_ANY.
+        // Check we can get the local CID using the static property on ContextID.
         let localCID = try socket.withUnsafeHandle(VsockAddress.ContextID.getLocalContextID)
-        XCTAssertNotEqual(localCID, .any)
-        XCTAssertGreaterThan(localCID.rawValue, VsockAddress.ContextID.host.rawValue)
+
+        // Check the local CID from the socket matches.
         XCTAssertEqual(try socket.getLocalVsockContextID(), localCID)
 
         // Check the local CID from the channel option matches.