Improve error handling

Author: rdingman

Sources/Subprocess/AsyncBufferSequence.swift

@@ -73,7 +73,21 @@ public struct AsyncBufferSequence: AsyncSequence, Sendable {
                 diskIO.stream(upToLength: readBufferSize, continuation: continuation)
             }
 
-            if let buffer = try await streamIterator.next() {
+            let buffer: Buffer?
+
+            do {
+                buffer = try await streamIterator.next()
+            } catch {
+                #if os(Windows)
+                try self.diskIO.close()
+                #else
+                self.diskIO.close()
+                #endif
+
+                throw error
+            }
+
+            if let buffer {
                 bytesRemaining -= buffer.count
                 return buffer
             } else {
@@ -82,6 +96,7 @@ public struct AsyncBufferSequence: AsyncSequence, Sendable {
                 #else
                 self.diskIO.close()
                 #endif
+
                 return nil
             }
         }

Sources/Subprocess/Platforms/Subprocess+Unix.swift

@@ -421,19 +421,26 @@ extension DispatchIO {
             length: maxLength,
             queue: .global()
         ) { done, data, error in
-            if error != 0 {
+            guard error == 0 else {
                 continuation.finish(throwing: SubprocessError(
                     code: .init(.failedToReadFromSubprocess),
                     underlyingError: .init(rawValue: error)
                 ))
                 return
             }
 
-            // Treat empty data and nil as the same
-            if let data = data.map({ $0.isEmpty ? nil : $0 }) ?? nil {
+            guard let data else {
+                fatalError("Unexpectedly received nil data from DispatchIO with error == 0.")
+            }
+
+            if !data.isEmpty {
+                // We have non-empty data. Yield it to the continuation
                 continuation.yield(AsyncBufferSequence.Buffer(data: data))
             } else if done {
+                // Receiving an empty data and done == true means we've reached the end of the file.
                 continuation.finish()
+            } else {
+                fatalError("Unexpectedly received no data from DispatchIO with it indicating it is not done.")
             }
         }
     }

Sources/Subprocess/Platforms/Subprocess+Windows.swift

@@ -417,7 +417,6 @@ public struct PlatformOptions: Sendable {
     /// process in case the parent task is cancelled before
     /// the child proces terminates.
     /// Always ends in forcefully terminate at the end.
-    internal var streamOptions: StreamOptions = .init()
     public var teardownSequence: [TeardownStep] = []
     /// A closure to configure platform-specific
     /// spawning constructs. This closure enables direct
@@ -442,12 +441,6 @@ public struct PlatformOptions: Sendable {
     public init() {}
 }
 
-extension PlatformOptions {
-    internal struct StreamOptions: Sendable {
-        internal init() {}
-    }
-}
-
 extension PlatformOptions: CustomStringConvertible, CustomDebugStringConvertible {
     internal func description(withIndent indent: Int) -> String {
         let indent = String(repeating: " ", count: indent * 4)