Ensure encoded multipart body stream has appropriate length. (#3420)

* Ensure that the encoded body stream data has expected length (#3380)

* Ensure that the encoded body stream data has expected length

* Using a custom error for unexpected stream length instead of extending MultipartEncodingFailureReason

* Making `bytesExpected` and `bytesRead` of `UnexpectedInputStreamLengthError` public

* Slight cleanup.
Co-authored-by: NYu Ao <YuAo@users.noreply.github.com>

Source/AFError.swift

@@ -57,6 +57,15 @@ public enum AFError: Error {
         case inputStreamReadFailed(error: Error)
     }
 
+    /// Represents unexpected input stream length that occur when encoding the `MultipartFormData`. Instances will be
+    /// embedded within an `AFError.multipartEncodingFailed` `.inputStreamReadFailed` case.
+    public struct UnexpectedInputStreamLength: Error {
+        /// The expected byte count to read.
+        public var bytesExpected: UInt64
+        /// The actual byte count read.
+        public var bytesRead: UInt64
+    }
+
     /// The underlying reason the `.parameterEncodingFailed` error occurred.
     public enum ParameterEncodingFailureReason {
         /// The `URLRequest` did not have a `URL` to encode.

Source/MultipartFormData.swift

@@ -419,6 +419,12 @@ open class MultipartFormData {
             }
         }
 
+        guard UInt64(encoded.count) == bodyPart.bodyContentLength else {
+            let error = AFError.UnexpectedInputStreamLength(bytesExpected: bodyPart.bodyContentLength,
+                                                            bytesRead: UInt64(encoded.count))
+            throw AFError.multipartEncodingFailed(reason: .inputStreamReadFailed(error: error))
+        }
+
         return encoded
     }
 

Tests/MultipartFormDataTests.swift

@@ -918,4 +918,31 @@ class MultipartFormDataFailureTestCase: BaseTestCase {
         XCTAssertNotNil(encodingError, "encoding error should not be nil")
         XCTAssertEqual(encodingError?.asAFError?.isOutputStreamURLInvalid, true)
     }
+
+    func testThatStreamBodyPartHasUnexpectedLength() {
+        // Given
+        let multipartFormData = MultipartFormData()
+        let data = Data("Lorem ipsum dolor sit amet.".utf8)
+        multipartFormData.append(data, withName: "data")
+        var firstError: Error?
+        var secondError: Error?
+
+        // When
+        do {
+            _ = try multipartFormData.encode()
+        } catch {
+            firstError = error
+        }
+
+        do {
+            _ = try multipartFormData.encode()
+        } catch {
+            secondError = error
+        }
+
+        XCTAssertNil(firstError, "firstError should be nil")
+        XCTAssertNotNil(secondError, "secondError should not be nil")
+        XCTAssertEqual(secondError?.asAFError?.isInputStreamReadFailed, true)
+        XCTAssert(secondError?.asAFError?.underlyingError is AFError.UnexpectedInputStreamLength)
+    }
 }