From 32bb24ad4142c125981cfa87285a7431232b88f4 Mon Sep 17 00:00:00 2001
From: ETolboom <e.s.tolboom@student.tue.nl>
Date: Tue, 29 Sep 2026 12:47:25 +0200
Subject: [PATCH] fix(logging): stop os_log crashing on more than five
 arguments

OSLog.log handed the [CVarArg] array to os_log for 6+ arguments, which
reads garbage pointers for each %@ and segfaults in
_os_log_fmt_flatten_object_impl. The seven-argument "Backfill
acknowledged" line hit this on every backfill, crash-looping the app.

The fallback now asserts in debug and logs the pre-rendered line as a
single argument in release. The backfill ack line is collapsed to one
argument.
---
 .../Session/TransmitterSession.swift          |  8 ++++----
 Sources/G6SensorCore/Transport/OSLog.swift    | 20 ++++++++++++-------
 2 files changed, 17 insertions(+), 11 deletions(-)

diff --git a/Sources/G6SensorCore/Session/TransmitterSession.swift b/Sources/G6SensorCore/Session/TransmitterSession.swift
index 3176be4..17d77a9 100644
--- a/Sources/G6SensorCore/Session/TransmitterSession.swift
+++ b/Sources/G6SensorCore/Session/TransmitterSession.swift
@@ -466,10 +466,10 @@ public final class TransmitterSession: TransmitterConnectionDelegate {
 
         let ack = try peripheral.requestBackfill(startTime: startTime, endTime: endTime)
 
-        log.default("Backfill acknowledged: status %{public}@, backfillStatus %{public}@, id %{public}@, range %{public}@-%{public}@, length %{public}@, crc %{public}@",
-                    String(ack.status), String(ack.backfillStatus), String(ack.identifier),
-                    String(ack.startTime), String(ack.endTime),
-                    String(ack.bufferLength), String(format: "%04x", ack.bufferCRC))
+        log.default("Backfill acknowledged: %{public}@",
+                    "status \(ack.status), backfillStatus \(ack.backfillStatus), id \(ack.identifier), " +
+                    "range \(ack.startTime)-\(ack.endTime), length \(ack.bufferLength), " +
+                    "crc \(String(format: "%04x", ack.bufferCRC))")
 
         // Nothing stored for the window: an empty result, not a failure.
         guard ack.bufferLength > 0 else {
diff --git a/Sources/G6SensorCore/Transport/OSLog.swift b/Sources/G6SensorCore/Transport/OSLog.swift
index 847d5be..999052d 100644
--- a/Sources/G6SensorCore/Transport/OSLog.swift
+++ b/Sources/G6SensorCore/Transport/OSLog.swift
@@ -34,7 +34,8 @@ extension OSLog {
     }
 
     private func log(_ message: StaticString, type: OSLogType, _ args: [CVarArg]) {
-        mirrorToFile(message, type: type, args)
+        let rendered = render(message, args)
+        mirrorToFile(rendered, type: type)
 
         switch args.count {
         case 0:
@@ -50,7 +51,10 @@ extension OSLog {
         case 5:
             os_log(message, log: self, type: type, args[0], args[1], args[2], args[3], args[4])
         default:
-            os_log(message, log: self, type: type, args)
+            // os_log can't take the array as varargs: handing it over reads
+            // garbage pointers for each %@ and crashes. Log the rendered line.
+            assertionFailure("OSLog supports at most 5 arguments, got \(args.count): \(message)")
+            os_log("%{public}@", log: self, type: type, rendered)
         }
     }
 
@@ -65,15 +69,17 @@ extension OSLog {
         return Self.categories[ObjectIdentifier(self)] ?? "G6SensorKit"
     }
 
-    /// Mirrors every OSLog line into the exportable file log. Format
-    /// specifiers are OSLog-flavoured (%{public}@), so the privacy qualifier
-    /// is stripped before String(format:) sees them.
-    private func mirrorToFile(_ message: StaticString, type: OSLogType, _ args: [CVarArg]) {
+    /// Format specifiers are OSLog-flavoured (%{public}@), so the privacy
+    /// qualifier is stripped before String(format:) sees them.
+    private func render(_ message: StaticString, _ args: [CVarArg]) -> String {
         let template = "\(message)"
             .replacingOccurrences(of: "%{public}", with: "%")
             .replacingOccurrences(of: "%{private}", with: "%")
-        let rendered = args.isEmpty ? template : String(format: template, arguments: args)
+        return args.isEmpty ? template : String(format: template, arguments: args)
+    }
 
+    /// Mirrors every OSLog line into the exportable file log.
+    private func mirrorToFile(_ rendered: String, type: OSLogType) {
         let level: String
         switch type {
         case .debug: level = "DEBUG"