Emulate POSIX_SPAWN_CLOEXEC_DEFAULT in fork/exec (#79)

* Emulate POSIX_SPAWN_CLOEXEC_DEFAULT in fork/exec.

POSIX_SPAWN_CLOEXEC_DEFAULT is only available on Darwin. Emulate POSIX_SPAWN_CLOEXEC_DEFAULT on other platforms by calling close after fork, before exec.

This commit also removes _subprocess_posix_spawn_fallback because we can't emulate POSIX_SPAWN_CLOEXEC_DEFAULT in a thread-safe manner while using posix_spawn.

* Use NSIG_MAX insead of a constant

* Update Linux testSuspendResumeProcess to use waitpid instead of polling

* Use closefrom on OpenBSD instead of looping through fds to close

Author: iCharlesHu

Sources/Subprocess/Platforms/Subprocess+Linux.swift

@@ -389,7 +389,7 @@ internal func monitorProcessTermination(
                         if siginfo.si_pid == 0 && siginfo.si_signo == 0 {
                             // Save this continuation to be called by signal hander
                             var newState = storage
-                            newState.continuations[processIdentifier.processDescriptor] = continuation
+                            newState.continuations[processIdentifier.value] = continuation
                             state = .started(newState)
                             return nil
                         }
@@ -472,7 +472,7 @@ internal extension siginfo_t {
 // Okay to be unlocked global mutable because this value is only set once like dispatch_once
 private nonisolated(unsafe) var _signalPipe: (readEnd: CInt, writeEnd: CInt) = (readEnd: -1, writeEnd: -1)
 // Okay to be unlocked global mutable because this value is only set once like dispatch_once
-private nonisolated(unsafe) var _waitprocessDescriptorSupported = false
+private nonisolated(unsafe) var _waitProcessDescriptorSupported = false
 private let _processMonitorState: Mutex<ProcessMonitorState> = .init(.notStarted)
 
 private func shutdown() {
@@ -568,8 +568,8 @@ private func monitorThreadFunc(context: MonitorThreadContext) {
             }
 
             // P_PIDFD requires Linux Kernel 5.4 and above
-            if _waitprocessDescriptorSupported {
-                _blockAndWaitForprocessDescriptor(targetFileDescriptor, context: context)
+            if _waitProcessDescriptorSupported {
+                _blockAndWaitForProcessDescriptor(targetFileDescriptor, context: context)
             } else {
                 _reapAllKnownChildProcesses(targetFileDescriptor, context: context)
             }
@@ -658,7 +658,7 @@ private let setup: () = {
         }
     } else {
         // Mark waitid(P_PIDFD) as supported
-        _waitprocessDescriptorSupported = true
+        _waitProcessDescriptorSupported = true
     }
     let monitorThreadContext = MonitorThreadContext(
         epollFileDescriptor: epollFileDescriptor,
@@ -705,7 +705,7 @@ internal func _setupMonitorSignalHandler() {
     setup
 }
 
-private func _blockAndWaitForprocessDescriptor(_ pidfd: CInt, context: MonitorThreadContext) {
+private func _blockAndWaitForProcessDescriptor(_ pidfd: CInt, context: MonitorThreadContext) {
     var terminationStatus: Result<TerminationStatus, SubprocessError>
 
     var siginfo = siginfo_t()

Sources/Subprocess/Platforms/Subprocess+Unix.swift

@@ -112,19 +112,30 @@ extension Execution {
 
         #if os(Linux) || os(Android)
         // On linux, use pidfd_send_signal if possible
-        if shouldSendToProcessGroup {
+        if shouldSendToProcessGroup || self.processIdentifier.processDescriptor < 0 {
             // pidfd_send_signal does not support sending signal to process group
             try _kill(pid, signal: signal)
         } else {
-            guard _pidfd_send_signal(
+            let rc = _pidfd_send_signal(
                 processIdentifier.processDescriptor,
                 signal.rawValue
-            ) == 0 else {
-                throw SubprocessError(
-                    code: .init(.failedToSendSignal(signal.rawValue)),
-                    underlyingError: .init(rawValue: errno)
-                )
+            )
+            let capturedErrno = errno
+            if rc == 0 {
+                // _pidfd_send_signal succeeded
+                return
             }
+            if capturedErrno == ENOSYS {
+                // _pidfd_send_signal is not implemented. Fallback to kill
+                try _kill(pid, signal: signal)
+                return
+            }
+
+            // Throw all other errors
+            throw SubprocessError(
+                code: .init(.failedToSendSignal(signal.rawValue)),
+                underlyingError: .init(rawValue: capturedErrno)
+            )
         }
         #else
         try _kill(pid, signal: signal)