From 3146ae0adcd5cbce048c5db17a52dd119624222f Mon Sep 17 00:00:00 2001
From: Julien Letessier <julien.letessier@gmail.com>
Date: Thu, 15 Jan 2026 12:59:03 +0100
Subject: [PATCH] Handle vsnprintf errors in logger

---
 src/Logger.cpp             | 10 ++++++++
 test/logunit_vsnprintf.cpp | 14 ++++++++++++
 test/logunit_vsnprintf.py  | 47 ++++++++++++++++++++++++++++++++++++++
 test/run.sh                |  1 +
 4 files changed, 72 insertions(+)
 create mode 100644 test/logunit_vsnprintf.cpp
 create mode 100644 test/logunit_vsnprintf.py

diff --git a/src/Logger.cpp b/src/Logger.cpp
index b819224..ced0e53 100644
--- a/src/Logger.cpp
+++ b/src/Logger.cpp
@@ -59,6 +59,16 @@ void LogUnit::vformat(LogLevel::Type level, const char* file, int line, const ch
     mLen += n;
     len -= n;
     n = vsnprintf(p, len, fmt, ap);
+    if (n < 0) {
+        // Formatting failed; keep existing prefix and finish the line.
+        if (mLen < MaxLogLen - 1) {
+            mBuf[mLen++] = '\n';
+        } else {
+            mBuf[MaxLogLen - 1] = '\n';
+            mLen = MaxLogLen;
+        }
+        return;
+    }
     mLen += n;
     if (mLen >= MaxLogLen) {
         mLen = MaxLogLen - 1;
diff --git a/test/logunit_vsnprintf.cpp b/test/logunit_vsnprintf.cpp
new file mode 100644
index 0000000..f61f9dc
--- /dev/null
+++ b/test/logunit_vsnprintf.cpp
@@ -0,0 +1,14 @@
+/*
+ * Minimal test for LogUnit::vformat negative vsnprintf handling.
+ */
+
+#include "../src/Logger.h"
+
+int main() {
+    LogUnit unit;
+    unit.format(LogLevel::Info, __FILE__, __LINE__, "%");
+    if (unit.length() < 0) {
+        return 1;
+    }
+    return 0;
+}
diff --git a/test/logunit_vsnprintf.py b/test/logunit_vsnprintf.py
new file mode 100644
index 0000000..d2adfba
--- /dev/null
+++ b/test/logunit_vsnprintf.py
@@ -0,0 +1,47 @@
+#!/usr/bin/env python3
+#
+# Build and run the LogUnit::vformat vsnprintf edge-case test.
+#
+
+import os
+import subprocess
+import tempfile
+from test_util import parse_args, exit_with_result
+
+
+def run_test(project_root):
+    src = os.path.join(project_root, "test", "logunit_vsnprintf.cpp")
+    if not os.path.exists(src):
+        print("FAIL: logunit_vsnprintf.cpp not found")
+        return False
+
+    with tempfile.TemporaryDirectory() as tmp:
+        exe = os.path.join(tmp, "logunit_vsnprintf")
+        cmd = [
+            "g++",
+            "-std=c++11",
+            src,
+            os.path.join(project_root, "src", "Logger.cpp"),
+            os.path.join(project_root, "src", "LogFileSink.cpp"),
+            os.path.join(project_root, "src", "Timer.cpp"),
+            "-o",
+            exe,
+        ]
+        try:
+            subprocess.check_call(cmd, stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)
+        except Exception as exc:
+            print("FAIL: compile logunit_vsnprintf:", exc)
+            return False
+        try:
+            subprocess.check_call([exe], stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)
+        except subprocess.CalledProcessError as exc:
+            print("FAIL: logunit_vsnprintf returned", exc.returncode)
+            return False
+    return True
+
+
+if __name__ == "__main__":
+    _ = parse_args("LogUnit vsnprintf test")
+    root = os.path.abspath(os.path.join(os.path.dirname(__file__), ".."))
+    success = run_test(root)
+    exit_with_result(success, "logunit vsnprintf", "logunit vsnprintf")
diff --git a/test/run.sh b/test/run.sh
index 5a2d465..d2baf7b 100755
--- a/test/run.sh
+++ b/test/run.sh
@@ -146,6 +146,7 @@ TESTS=(
     "test/request_parser_long_command.py"
     "test/signal_handling.py"
     "test/buffer_vsnprintf.py"
+    "test/logunit_vsnprintf.py"
     "test/pubsub_long_name.py"
     "test/pubsub_large_message.py"
     "test/transaction_forbid.py"