From adf534f7a527e1ad8d22b872379535ea9cacfe0c Mon Sep 17 00:00:00 2001
From: Martin Prusa <martin@martinprusa.com>
Date: Thu, 10 Sep 2026 14:35:22 +0200
Subject: [PATCH] FileLogger: use throwing FileHandle API so a failed write
 cannot crash the app

`seekToEndOfFile()` and `write(_:)` raise an Objective-C `NSFileHandleOperationException`
when the underlying write fails (e.g. "No space left on device"). Such an exception cannot
be caught from Swift, so a full device storage took the whole app down.

Switch to `seekToEnd()` / `write(contentsOf:)` / `close()`, which throw a Swift error that
lands in the existing `catch` and is reported through `loggerForInternalErrors` instead.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
---
 .../Loggers/FileLogger/FileLogger.swift       |  8 ++---
 .../LoggerTests/Loggers/FileLoggerTests.swift | 35 +++++++++++++++++++
 2 files changed, 39 insertions(+), 4 deletions(-)

diff --git a/Sources/Logger/Loggers/FileLogger/FileLogger.swift b/Sources/Logger/Loggers/FileLogger/FileLogger.swift
index f116ed9..24a5c8a 100644
--- a/Sources/Logger/Loggers/FileLogger/FileLogger.swift
+++ b/Sources/Logger/Loggers/FileLogger/FileLogger.swift
@@ -68,10 +68,10 @@ public class FileLogger: Logging {
         }
     }
     
-    private var currentWritableFileHandle: FileHandle? {
+    var currentWritableFileHandle: FileHandle? {
         willSet {
             if currentWritableFileHandle != newValue {
-                currentWritableFileHandle?.closeFile()
+                try? currentWritableFileHandle?.close()
             }
         }
     }
@@ -264,8 +264,8 @@ public class FileLogger: Logging {
                     throw FileLoggerError.stringToDataConversionFailure
                 }
 
-                fileHandle.seekToEndOfFile()
-                fileHandle.write(data)
+                try fileHandle.seekToEnd()
+                try fileHandle.write(contentsOf: data)
             } catch let error {
                 self.externalLogger("Failed to write to a log file with error: \(error)!")
             }
diff --git a/Tests/LoggerTests/Loggers/FileLoggerTests.swift b/Tests/LoggerTests/Loggers/FileLoggerTests.swift
index b5b63f8..7664db2 100644
--- a/Tests/LoggerTests/Loggers/FileLoggerTests.swift
+++ b/Tests/LoggerTests/Loggers/FileLoggerTests.swift
@@ -196,6 +196,41 @@ class FileLoggerTests: XCTestCase {
         }
     }
 
+    func test_failed_write_is_reported_instead_of_crashing() throws {
+        var internalErrors: [String] = []
+
+        let fileLogger = try FileLogger(
+            appName: nil,
+            fileManager: fileManager,
+            userDefaults: userDefaults,
+            logDirURL: logDirURL,
+            namespace: nil,
+            numberOfLogFiles: 3,
+            dateFormatter: DateFormatter.dateFormatter,
+            fileHeaderContent: "",
+            lineSeparator: "\n",
+            logEntryEncoder: LogEntryEncoder(),
+            logEntryDecoder: LogEntryDecoder(),
+            externalLogger: { internalErrors.append($0) },
+            fileAccessQueue: .syncMock
+        )
+
+        fileLogger.log(.mock("First message"))
+
+        // Simulate an I/O failure (e.g. "No space left on device") with a handle that cannot be written to.
+        // The legacy `seekToEndOfFile()` / `write(_:)` API raised an uncatchable NSException here and crashed the app.
+        fileLogger.currentWritableFileHandle = try FileHandle(forReadingFrom: fileLogger.currentLogFileUrl)
+
+        fileLogger.log(.mock("Second message"))
+
+        XCTAssertEqual(internalErrors.count, 1)
+        XCTAssertTrue(internalErrors.first?.hasPrefix("Failed to write to a log file") == true, internalErrors.description)
+
+        let content = try String(contentsOf: fileLogger.currentLogFileUrl, encoding: .utf8)
+        XCTAssertTrue(content.contains("First message"), content)
+        XCTAssertFalse(content.contains("Second message"), content)
+    }
+
     func test_file_rotation() throws {
         let fileLogger = try FileLogger(
             appName: nil,