From a3d64ab239b40a53c6af1d51428a76ee05fafcd5 Mon Sep 17 00:00:00 2001
From: Malcolm James MacLeod <malcolm@gulden.com>
Date: Sun, 30 Dec 2018 14:23:46 +0200
Subject: [PATCH] Implement basic automatic attachment/detachment of c++
 threads to JVM. (#405)

* Implement basic automatic attachment/detachment of c++ threads to JVM.

No side effects if not enabled
Compile with -DEXPERIMENTAL_AUTO_CPP_THREAD_ATTACH to enable
Requires a compiler with C++11 support
Borrows code/idea from https://github.com/dropbox/djinni/issues/372#issuecomment-394629525

* Place assignment outside of define.
---
 support-lib/jni/djinni_support.cpp | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/support-lib/jni/djinni_support.cpp b/support-lib/jni/djinni_support.cpp
index 142b9e220..22324e5a6 100644
--- a/support-lib/jni/djinni_support.cpp
+++ b/support-lib/jni/djinni_support.cpp
@@ -72,7 +72,17 @@ void jniShutdown() {
 JNIEnv * jniGetThreadEnv() {
     assert(g_cachedJVM);
     JNIEnv * env = nullptr;
-    const jint get_res = g_cachedJVM->GetEnv(reinterpret_cast<void**>(&env), JNI_VERSION_1_6);
+    jint get_res = g_cachedJVM->GetEnv(reinterpret_cast<void**>(&env), JNI_VERSION_1_6);
+    #ifdef EXPERIMENTAL_AUTO_CPP_THREAD_ATTACH
+    if (get_res == JNI_EDETACHED) {
+        get_res = g_cachedJVM->AttachCurrentThread(&env, nullptr);
+        thread_local struct DetachOnExit {
+            ~DetachOnExit() {
+                g_cachedJVM->DetachCurrentThread();
+            }
+        } detachOnExit;
+    }
+    #endif
     if (get_res != 0 || !env) {
         // :(
         std::abort();