RUMM-506 Add test for malformations in the middle of the file

Tests/DatadogTests/Datadog/Core/Persistence/FileWriterTests.swift

@@ -41,7 +41,7 @@ class FileWriterTests: XCTestCase {
     }
 
     /// NOTE: Test added after incident-4797
-    func testGivenFileContainingData_whenNextDataFails_itDoesNotMalformTheFileContent() throws {
+    func testGivenFileContainingData_whenNextDataFails_itDoesNotMalformTheEndOfTheFile() throws {
         let previousObjcExceptionHandler = objcExceptionHandler
         defer { objcExceptionHandler = previousObjcExceptionHandler }
 
@@ -76,6 +76,38 @@ class FileWriterTests: XCTestCase {
         )
     }
 
+    /// NOTE: Test added after incident-4797
+    func testWhenIOExceptionsHappenRandomly_theFileIsNeverMalformed() throws {
+        let previousObjcExceptionHandler = objcExceptionHandler
+        defer { objcExceptionHandler = previousObjcExceptionHandler }
+
+        let expectation = self.expectation(description: "write completed")
+        let writer = FileWriter(
+            orchestrator: .mockWriteToSingleFile(in: temporaryDirectory),
+            queue: queue
+        )
+
+        objcExceptionHandler = ObjcExceptionHandlerNonDeterministicMock(
+            throwingError: ErrorMock("I/O exception"),
+            withProbability: 0.2
+        )
+
+        struct Foo: Codable {
+            let foo = "bar"
+        }
+
+        (0...500).forEach { _ in writer.write(value: Foo()) }
+
+        waitForWritesCompletion(on: queue, thenFulfill: expectation)
+        waitForExpectations(timeout: 5, handler: nil)
+        XCTAssertEqual(try temporaryDirectory.files().count, 1)
+
+        let fileData = try temporaryDirectory.files()[0].read()
+        let jsonDecoder = JSONDecoder()
+
+        XCTAssertNoThrow(try jsonDecoder.decode([Foo].self, from: "[".utf8Data + fileData + "]".utf8Data))
+    }
+
     func testGivenErrorVerbosity_whenIndividualDataExceedsMaxWriteSize_itDropsDataAndPrintsError() throws {
         let expectation1 = self.expectation(description: "write completed")
         let expectation2 = self.expectation(description: "second write completed")

Tests/DatadogTests/Datadog/Mocks/DatadogPrivateMocks.swift

@@ -44,3 +44,23 @@ class ObjcExceptionHandlerDeferredMock: ObjcExceptionHandler {
         currentCallsCount += 1
     }
 }
+
+/// An `ObjcExceptionHandler` which throws given error with given probability.
+class ObjcExceptionHandlerNonDeterministicMock: ObjcExceptionHandler {
+    private let probability: Int
+    let error: Error
+
+    /// Probability should be described as a number between `0` and `1`
+    init(throwingError: Error, withProbability probability: Double) {
+        self.error = throwingError
+        self.probability = Int(probability * 1_000)
+    }
+
+    override func rethrowToSwift(tryBlock: @escaping () -> Void) throws {
+        if Int.random(in: 0...1_000) < probability {
+            throw error
+        } else {
+            tryBlock()
+        }
+    }
+}