Proper use of errorBuffer for expanded error messages

Author: johnno1962

Foundation/URLSession/http/EasyHandle.swift

@@ -56,7 +56,7 @@ internal final class _EasyHandle {
     fileprivate var headerList: _CurlStringList?
     fileprivate var pauseState: _PauseState = []
     internal var timeoutTimer: _TimeoutSource!
-    private static var errorBuffer = [UInt8](repeating: 0, count: Int(CFURLSessionEasyErrorSize))
+    internal lazy var errorBuffer = [UInt8](repeating: 0, count: Int(CFURLSessionEasyErrorSize))
     #if os(Android)
     static fileprivate var _CAInfoFile: UnsafeMutablePointer<Int8>?
     #endif
@@ -147,7 +147,7 @@ extension _EasyHandle {
     /// Set error buffer for error messages
     /// - SeeAlso: https://curl.haxx.se/libcurl/c/CURLOPT_ERRORBUFFER.html
     func set(errorBuffer buffer: UnsafeMutableBufferPointer<UInt8>?) {
-        let buffer = buffer ?? _EasyHandle.errorBuffer.withUnsafeMutableBufferPointer { $0 }
+        let buffer = buffer ?? errorBuffer.withUnsafeMutableBufferPointer { $0 }
         try! CFURLSession_easy_setopt_ptr(rawHandle, CFURLSessionOptionERRORBUFFER, buffer.baseAddress).asError()
     }
     /// Request failure on HTTP response >= 400

Foundation/URLSession/http/MultiHandle.swift

@@ -214,9 +214,13 @@ fileprivate extension URLSession._MultiHandle {
         }
         let easyHandle = easyHandles[idx]
         // Find the NSURLError code
-        let error = easyHandle.urlErrorCode(for: easyCode).map {
-            NSError(domain: NSURLErrorDomain, code: $0, userInfo: [
-                NSLocalizedDescriptionKey: CFURLSessionCreateErrorDescription(easyCode.value)._swiftObject
+        var error: NSError?
+        if let errorCode = easyHandle.urlErrorCode(for: easyCode) {
+            let errorDescription = easyHandle.errorBuffer[0] != 0 ?
+                String(cString: easyHandle.errorBuffer) :
+                CFURLSessionCreateErrorDescription(easyCode.value)._swiftObject
+            error = NSError(domain: NSURLErrorDomain, code: errorCode, userInfo: [
+                NSLocalizedDescriptionKey: errorDescription
             ])
         }
         completedTransfer(forEasyHandle: easyHandle, error: error)