Fix hang in Linux process termination handler

Calling waitid inside the _childProcessContinuations lock can cause deadlocks in setting up the termination handlers of one process while waiting for another to complete. Instead, only hold the lock when the continuations map is actually being accessed.

Author: jakepetroules

Sources/Subprocess/Platforms/Subprocess+Linux.swift

@@ -281,26 +281,26 @@ private extension siginfo_t {
 
 private let setup: () = {
     signalSource.setEventHandler {
-        _childProcessContinuations.withLock { continuations in
-            while true {
-                var siginfo = siginfo_t()
-                guard waitid(P_ALL, id_t(0), &siginfo, WEXITED) == 0 else {
-                    return
-                }
-                var status: TerminationStatus? = nil
-                switch siginfo.si_code {
-                case .init(CLD_EXITED):
-                    status = .exited(siginfo.si_status)
-                case .init(CLD_KILLED), .init(CLD_DUMPED):
-                    status = .unhandledException(siginfo.si_status)
-                case .init(CLD_TRAPPED), .init(CLD_STOPPED), .init(CLD_CONTINUED):
-                    // Ignore these signals because they are not related to
-                    // process exiting
-                    break
-                default:
-                    fatalError("Unexpected exit status: \(siginfo.si_code)")
-                }
-                if let status = status {
+        while true {
+            var siginfo = siginfo_t()
+            guard waitid(P_ALL, id_t(0), &siginfo, WEXITED) == 0 || errno == EINTR else {
+                return
+            }
+            var status: TerminationStatus? = nil
+            switch siginfo.si_code {
+            case .init(CLD_EXITED):
+                status = .exited(siginfo.si_status)
+            case .init(CLD_KILLED), .init(CLD_DUMPED):
+                status = .unhandledException(siginfo.si_status)
+            case .init(CLD_TRAPPED), .init(CLD_STOPPED), .init(CLD_CONTINUED):
+                // Ignore these signals because they are not related to
+                // process exiting
+                break
+            default:
+                fatalError("Unexpected exit status: \(siginfo.si_code)")
+            }
+            if let status = status {
+                _childProcessContinuations.withLock { continuations in
                     let pid = siginfo.si_pid
                     if let existing = continuations.removeValue(forKey: pid),
                         case .continuation(let c) = existing