Fix flakiness in asyncReadFileDescriptor test

It's not safe to "modify" a file descriptor while it's in use by DispatchIO, per the documentation, which is the likely cause of flakiness here.

Author: jakepetroules

Sources/SWBUtil/FSProxy.swift

@@ -529,13 +529,9 @@ class LocalFS: FSProxy, @unchecked Sendable {
 
     func write(_ path: Path, contents: (FileDescriptor) async throws -> Void) async throws {
         let fd = try FileDescriptor.open(FilePath(path.str), .writeOnly, options: [.create, .truncate], permissions: [.ownerReadWrite, .groupRead, .otherRead])
-        do {
+        return try await fd.closeAfter {
             try await contents(fd)
-        } catch {
-            _ = try? fd.close()
-            throw error
         }
-        try fd.close()
     }
 
     func append(_ path: Path, contents: ByteString) throws {

Tests/SWBUtilTests/FileHandleTests.swift

@@ -69,28 +69,30 @@ import SystemPackage
             // Read while triggering an I/O error
             do {
                 let fd = try Path.root.str.withPlatformString { try FileDescriptor.open($0, .readOnly) }
-                let fh = FileHandle(fileDescriptor: fd.rawValue, closeOnDealloc: false)
+                try await fd.closeAfter {
+                    let fh = FileHandle(fileDescriptor: fd.rawValue, closeOnDealloc: false)
 
-                if #available(macOS 15, iOS 18, tvOS 18, watchOS 11, visionOS 2, *) {
-                    var it = fh.bytes().makeAsyncIterator()
-                    try fd.close()
+                    if #available(macOS 15, iOS 18, tvOS 18, watchOS 11, visionOS 2, *) {
+                        var it = fh.bytes().makeAsyncIterator()
 
-                    await #expect(throws: (any Error).self) {
-                        while let _ = try await it.next() {
+                        // Can't read a directory
+                        await #expect(throws: (any Error).self) {
+                            while let _ = try await it.next() {
+                            }
                         }
-                    }
-                } else {
-                    var it = fh._bytes().makeAsyncIterator()
-                    try fd.close()
+                    } else {
+                        var it = fh._bytes().makeAsyncIterator()
 
-                    await #expect(throws: (any Error).self) {
-                        while let _ = try await it.next() {
+                        // Can't read a directory
+                        await #expect(throws: (any Error).self) {
+                            while let _ = try await it.next() {
+                            }
                         }
                     }
                 }
             }
 
-            // Read part of a file, then close the handle
+            // Read part of a file, then cancel the task
             do {
                 let condition = CancellableWaitCondition()