JNI: Tolerate threading mismatches and improve diagnostics

Amend 944200b5a9705a7617f82cdaf5caf8932380aba4. We have code paths
in Qt that result in a global QJniObject being called from multiple
threads (QtActivity instance, possibly the QAndroidSystemLocale),
so we need to tolerate that until we understand better under which
circumstances this should be allowed, and what tools we can use to
control this better.

Don't instantiate QJniEnvironment to check the JNIEnv, but use raw
JNI calls so that we don't implicitly attach a thread to the JVM
when checking. Use categorized logging to emit log output if we
have an environment/threading mismatch, including the name of the
class we are trying to call, and fall-back to the thread-local JNI
environment pointer instead of the one stored in the QJniObject's
private.

Also, give the threads that we attach to the JVM the name of the
QThread instances if we can, resulting in better logcat output on
the device.

This still results in less overhead than the code had prior to
944200b5a9705a7617f82cdaf5caf8932380aba4, as we don't instantiate
multiple temporary QJniEnvironment objects on the stack for each
call.

Change-Id: I98b92edd29162dccfc5c34e6261d4de219b59531
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
Reviewed-by: Sami Shalayel <sami.shalayel@qt.io>

1 files changed, 5 insertions, 3 deletions

diff --git a/src/corelib/kernel/qjnienvironment.cpp b/src/corelib/kernel/qjnienvironment.cpp
index 97c5584ea2..8d4e386a68 100644
--- a/src/corelib/kernel/qjnienvironment.cpp
+++ b/src/corelib/kernel/qjnienvironment.cpp
@@ -29,8 +29,6 @@ QT_BEGIN_NAMESPACE
     It has not been tested for other platforms.
 */
 
-static const char qJniThreadName[] = "QtThread";
-
 class QJniEnvironmentPrivate
 {
 public:
@@ -61,7 +59,11 @@ QJniEnvironment::QJniEnvironment()
         return;
 
     if (ret == JNI_EDETACHED) { // We need to (re-)attach
-        JavaVMAttachArgs args = { JNI_VERSION_1_6, qJniThreadName, nullptr };
+        const QByteArray threadName = QThread::currentThread()->objectName().toUtf8();
+        JavaVMAttachArgs args = { JNI_VERSION_1_6,
+                                  threadName.isEmpty() ? "QtThread" : threadName.constData(),
+                                  nullptr
+                                };
         if (vm->AttachCurrentThread(&d->jniEnv, &args) != JNI_OK)
             return;