Linux: reap child process if fork succeed but exec fails

Author: iCharlesHu

Package.swift

@@ -6,6 +6,8 @@ import PackageDescription
 var dep: [Package.Dependency] = [
     .package(
         url: "https://github.com/apple/swift-system",
+        // Temporarily pin to 1.5.0 because 1.6.0 has a breaking change for Ubuntu Focal
+        // https://github.com/apple/swift-system/issues/237
         exact: "1.5.0"
     )
 ]

Sources/Subprocess/Configuration.swift

@@ -870,10 +870,10 @@ internal struct CreatedPipe: ~Copyable {
             guard let parentEnd, parentEnd != INVALID_HANDLE_VALUE else {
                 // Since we created the pipe with `FILE_FLAG_FIRST_PIPE_INSTANCE`,
                 // if there's already a pipe with the same name, GetLastError()
-                // will be set to FILE_FLAG_FIRST_PIPE_INSTANCE. In this case,
+                // will be set to ERROR_ACCESS_DENIED. In this case,
                 // try again with a different name.
                 let errorCode = GetLastError()
-                guard errorCode != FILE_FLAG_FIRST_PIPE_INSTANCE else {
+                guard errorCode != ERROR_ACCESS_DENIED else {
                     continue
                 }
                 // Throw all other errors

Sources/Subprocess/IO/AsyncIO+Linux.swift

@@ -214,8 +214,16 @@ final class AsyncIO: Sendable {
         _ = _SubprocessCShims.write(currentState.shutdownFileDescriptor, &one, MemoryLayout<UInt64>.stride)
         // Cleanup the monitor thread
         pthread_join(currentState.monitorThread, nil)
-        close(currentState.epollFileDescriptor)
-        close(currentState.shutdownFileDescriptor)
+        var closeError: CInt = 0
+        if _SubprocessCShims.close(currentState.epollFileDescriptor) != 0 {
+            closeError = errno
+        }
+        if _SubprocessCShims.close(currentState.shutdownFileDescriptor) != 0 {
+            closeError = errno
+        }
+        if closeError != 0 {
+            fatalError("Failed to close epollfd: \(String(cString: strerror(closeError)))")
+        }
     }
 
 

Sources/Subprocess/Platforms/Subprocess+Linux.swift

@@ -131,14 +131,6 @@ extension Configuration {
                         underlyingError: .init(rawValue: spawnError)
                     )
                 }
-                func captureError(_ work: () throws -> Void) -> (any Swift.Error)? {
-                    do {
-                        try work()
-                        return nil
-                    } catch {
-                        return error
-                    }
-                }
                 // After spawn finishes, close all child side fds
                 try self.safelyCloseMultiple(
                     inputRead: inputReadFileDescriptor,
@@ -267,7 +259,7 @@ extension String {
 internal func monitorProcessTermination(
     forExecution execution: Execution
 ) async throws -> TerminationStatus {
-    try await withCheckedThrowingContinuation { continuation in
+    return try await withCheckedThrowingContinuation { continuation in
         _childProcessContinuations.withLock { continuations in
             // We don't need to worry about a race condition here because waitid()
             // does not clear the wait/zombie state of the child process. If it sees

Sources/_SubprocessCShims/process_shims.c

@@ -689,7 +689,14 @@ int _subprocess_fork_exec(
                 // exec worked!
                 close(pipefd[0]);
                 return 0;
-            } else if (read_rc > 0) {
+            }
+            // if we reach this point, exec failed.
+            // Since we already have the child pid (fork succeed), reap the child
+            // This mimic posix_spawn behavior
+            siginfo_t info;
+            waitid(P_PID, childPid, &info, WEXITED);
+
+            if (read_rc > 0) {
                 // Child exec failed and reported back
                 close(pipefd[0]);
                 return childError;

Tests/SubprocessTests/SubprocessTests+Unix.swift

@@ -668,7 +668,7 @@ extension SubprocessUnixTests {
         var platformOptions = PlatformOptions()
         platformOptions.supplementaryGroups = Array(expectedGroups)
         let idResult = try await Subprocess.run(
-            .path("/usr/bin/swift"),
+            .name("swift"),
             arguments: [getgroupsSwift.string],
             platformOptions: platformOptions,
             output: .string