Detect when Xcode is presenting os_log as structured log messages

In that case, just like when os_log mirrors to stderr by itself, we
want to disable Qt's fallback stderr handler.

Pick-to: 6.6 6.5
Change-Id: Ia373b19788edbce616d4f0d3d9f0b217ddc1e5c0
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

3 files changed, 32 insertions, 14 deletions

diff --git a/src/corelib/kernel/qcore_mac.mm b/src/corelib/kernel/qcore_mac.mm
index 2ad835b754..12bf81f092 100644
--- a/src/corelib/kernel/qcore_mac.mm
+++ b/src/corelib/kernel/qcore_mac.mm
@@ -86,17 +86,35 @@ QCFString::operator CFStringRef() const
 
 #if defined(QT_USE_APPLE_UNIFIED_LOGGING)
 
-bool AppleUnifiedLogger::willMirrorToStderr()
-{
-    // When running under Xcode or LLDB, one or more of these variables will
-    // be set, which triggers libsystem_trace.dyld to log messages to stderr
-    // as well, via_os_log_impl_mirror_to_stderr. Un-setting these variables
-    // is not an option, as that would silence normal NSLog or os_log calls,
-    // so instead we skip our own stderr output. See rdar://36919139.
+bool AppleUnifiedLogger::preventsStderrLogging()
+{
+    // os_log will mirror to stderr if OS_ACTIVITY_DT_MODE is set,
+    // regardless of its value. OS_ACTIVITY_MODE then controls whether
+    // to include info and/or debug messages in this mirroring.
+    // For some reason, when launched under lldb (via Xcode or not),
+    // all levels are included.
+
+    // CFLog will normally log to both stderr, and via os_log.
+    // Setting CFLOG_FORCE_DISABLE_STDERR disables the stderr
+    // logging. Setting CFLOG_FORCE_STDERR will both duplicate
+    // CFLog's output to stderr, and trigger OS_ACTIVITY_DT_MODE,
+    // resulting in os_log calls also being mirrored to stderr.
+    // Setting ACTIVITY_LOG_STDERR has the same effect.
+
+    // NSLog is plumbed to CFLog, and will respond to the same
+    // environment variables as CFLog.
+
+    // We want to disable Qt's default stderr log handler when
+    // os_log has already mirrored to stderr.
     static bool willMirror = qEnvironmentVariableIsSet("OS_ACTIVITY_DT_MODE")
-                                 || qEnvironmentVariableIsSet("ACTIVITY_LOG_STDERR")
-                                 || qEnvironmentVariableIsSet("CFLOG_FORCE_STDERR");
-    return willMirror;
+                          || qEnvironmentVariableIsSet("ACTIVITY_LOG_STDERR")
+                          || qEnvironmentVariableIsSet("CFLOG_FORCE_STDERR");
+
+    // As well as when we suspect that Xcode is going to present os_log
+    // as structured log messages.
+    static bool disableStderr = qEnvironmentVariableIsSet("CFLOG_FORCE_DISABLE_STDERR");
+
+    return willMirror || disableStderr;
 }
 
 QT_MAC_WEAK_IMPORT(_os_log_default);
@@ -137,7 +155,7 @@ bool AppleUnifiedLogger::messageHandler(QtMsgType msgType, const QMessageLogCont
     // system from redacting our log message.
     os_log_with_type(log, logType, "%{public}s", qPrintable(message));
 
-    return willMirrorToStderr();
+    return preventsStderrLogging();
 }
 
 os_log_type_t AppleUnifiedLogger::logTypeForMessageType(QtMsgType msgType)
diff --git a/src/corelib/kernel/qcore_mac_p.h b/src/corelib/kernel/qcore_mac_p.h
index 39ffe831bb..22c045f30c 100644
--- a/src/corelib/kernel/qcore_mac_p.h
+++ b/src/corelib/kernel/qcore_mac_p.h
@@ -235,7 +235,7 @@ class Q_CORE_EXPORT AppleUnifiedLogger
 public:
     static bool messageHandler(QtMsgType msgType, const QMessageLogContext &context, const QString &message,
         const QString &subsystem = QString());
-    static bool willMirrorToStderr();
+    static bool preventsStderrLogging();
 private:
     static os_log_type_t logTypeForMessageType(QtMsgType msgType);
     static os_log_t cachedLog(const QString &subsystem, const QString &category);
diff --git a/src/testlib/qtestcase.cpp b/src/testlib/qtestcase.cpp
index c6d9dc0340..fa0042f59f 100644
--- a/src/testlib/qtestcase.cpp
+++ b/src/testlib/qtestcase.cpp
@@ -1050,10 +1050,10 @@ Q_TESTLIB_EXPORT void qtest_qParseArgs(int argc, const char *const argv[], bool
 
 #if defined(QT_USE_APPLE_UNIFIED_LOGGING)
     // Any explicitly requested loggers will be added by now, so we can check if they use stdout
-    const bool safeToAddAppleLogger = !AppleUnifiedLogger::willMirrorToStderr() || !QTestLog::loggerUsingStdout();
+    const bool safeToAddAppleLogger = !AppleUnifiedLogger::preventsStderrLogging() || !QTestLog::loggerUsingStdout();
     if (safeToAddAppleLogger && QAppleTestLogger::debugLoggingEnabled()) {
         QTestLog::addLogger(QTestLog::Apple, nullptr);
-        if (AppleUnifiedLogger::willMirrorToStderr() && !logFilename)
+        if (AppleUnifiedLogger::preventsStderrLogging() && !logFilename)
             addFallbackLogger = false; // Prevent plain test logger fallback below
     }
 #endif