Add withTimeout() to surface hangs as diagnosable failures

Tests/SubprocessTests/ProcessMonitoringTests.swift

@@ -227,42 +227,44 @@ extension SubprocessProcessMonitoringTests {
     }
 
     @Test func testDoesNotReapUnrelatedChildProcess() async throws {
-        // Make sure we don't reap child exit status that we didn't spawn
-        let child1 = self.immediateExitProcess(withExitCode: 0)
-        let child2 = self.immediateExitProcess(withExitCode: 0)
-        try await withSpawnedExecution(config: child1) { child1Execution in
-            try await withSpawnedExecution(config: child2) { child2Execution in
-                // Monitor child2, but make sure we don't reap child1's status
-                let status = try await monitorProcessTermination(
-                    for: child2Execution.processIdentifier
-                )
-                #expect(status.isSuccess)
-                // Make sure we can still fetch child 1
-                #if os(Windows)
-                let rc = WaitForSingleObject(
-                    child1Execution.processIdentifier.processDescriptor,
-                    INFINITE
-                )
-                #expect(rc == WAIT_OBJECT_0)
-                var child1Status: DWORD = 0
-                let rc2 = GetExitCodeProcess(
-                    child1Execution.processIdentifier.processDescriptor,
-                    &child1Status
-                )
-                #expect(rc2 == true)
-                #expect(child1Status == 0)
-                #else
-                var siginfo = siginfo_t()
-                let rc = waitid(
-                    P_PID,
-                    id_t(child1Execution.processIdentifier.value),
-                    &siginfo,
-                    WEXITED
-                )
-                #expect(rc == 0)
-                #expect(siginfo.si_code == CLD_EXITED)
-                #expect(siginfo.si_status == 0)
-                #endif
+        try await withTimeout(.seconds(60), description: #function) {
+            // Make sure we don't reap child exit status that we didn't spawn
+            let child1 = self.immediateExitProcess(withExitCode: 0)
+            let child2 = self.immediateExitProcess(withExitCode: 0)
+            try await withSpawnedExecution(config: child1) { child1Execution in
+                try await withSpawnedExecution(config: child2) { child2Execution in
+                    // Monitor child2, but make sure we don't reap child1's status
+                    let status = try await monitorProcessTermination(
+                        for: child2Execution.processIdentifier
+                    )
+                    #expect(status.isSuccess)
+                    // Make sure we can still fetch child 1
+                    #if os(Windows)
+                    let rc = WaitForSingleObject(
+                        child1Execution.processIdentifier.processDescriptor,
+                        INFINITE
+                    )
+                    #expect(rc == WAIT_OBJECT_0)
+                    var child1Status: DWORD = 0
+                    let rc2 = GetExitCodeProcess(
+                        child1Execution.processIdentifier.processDescriptor,
+                        &child1Status
+                    )
+                    #expect(rc2 == true)
+                    #expect(child1Status == 0)
+                    #else
+                    var siginfo = siginfo_t()
+                    let rc = waitid(
+                        P_PID,
+                        id_t(child1Execution.processIdentifier.value),
+                        &siginfo,
+                        WEXITED
+                    )
+                    #expect(rc == 0)
+                    #expect(siginfo.si_code == CLD_EXITED)
+                    #expect(siginfo.si_status == 0)
+                    #endif
+                }
             }
         }
     }

Tests/SubprocessTests/TestSupport.swift

@@ -96,3 +96,51 @@ extension Trait where Self == ConditionTrait {
         )
     }
 }
+
+// MARK: - Test Timeout
+
+// Adapted from swift-build to surface hangs in tests as diagnosable failures
+// rather than CI job timeouts.
+// https://github.com/swiftlang/swift-build/blob/main/Sources/SWBTestSupport/Timeout.swift
+
+internal struct TimeoutError: Error, CustomStringConvertible {
+    internal var description: String
+
+    internal init(description: String) {
+        self.description = description
+    }
+}
+
+@discardableResult
+internal func withTimeout<T: Sendable>(
+    _ timeout: Duration,
+    description: String,
+    _ body: sending () async throws -> T
+) async throws -> T {
+    try await withoutActuallyEscaping(body) { escapingClosure in
+        // This lets `body` be captured in the `Sendable` function `work`.
+        // since `work` is `Sendable`, we can smuggle it through.
+        nonisolated(unsafe) let smuggled = escapingClosure
+        let work: @Sendable () async throws -> T = {
+            try await smuggled()
+        }
+        return try await withThrowingTaskGroup(of: T.self) { group in
+            group.addTask {
+                try await work()
+            }
+            group.addTask {
+                try await Task.sleep(for: timeout)
+                throw TimeoutError(
+                    description: "\(description) exceeded timeout of \(timeout)"
+                )
+            }
+            defer { group.cancelAll() }
+            guard let result = try await group.next() else {
+                throw TimeoutError(
+                    description: "\(description) exceeded timeout of \(timeout)"
+                )
+            }
+            return result
+        }
+    }
+}