Thread detach hook for Java JNI on Android

[johnno1962]

This is a resubmission of a callback hook required when mixing multi threaded Swift and Java on Android. When Java JNI has been used on a thread the thread needs to be detached from JNI before the thread exits to free local variables or the application will crash e.g. https://github.com/SwiftJava/java_swift/blob/master/Sources/JavaJNI.swift#L25.
Introducing this hook gives any future Java integration code a chance to do this.

[MadCoder]

@swift-ci please test

[johnno1962]

Thanks @MadCoder, good to have this tied down. While you’re here, there are in fact three problems building libdispatch on Android which you can probably provide an authoritative answer on.

The first is membarrier is not available:

diff --git a/src/shims/lock.h b/src/shims/lock.h
index 99c5563..34e1559 100644
--- a/src/shims/lock.h
+++ b/src/shims/lock.h
@@ -57,11 +57,11 @@ _dispatch_lock_owner(dispatch_lock lock_value)
 }
 
 #elif defined(__linux__)
 
 #include <linux/futex.h>
-#if !defined(__x86_64__) && !defined(__i386__) && !defined(__s390x__)
+#if !defined(__x86_64__) && !defined(__i386__) && !defined(__s390x__) && !defined(__ANDROID__)
 #include <linux/membarrier.h>
 #endif
 #include <unistd.h>
 #include <sys/syscall.h>   /* For SYS_xxx definitions */
 
@@ -471,11 +471,11 @@ _dispatch_once_gate_tryenter(dispatch_once_gate_t l)
 
 DISPATCH_ALWAYS_INLINE
 static inline dispatch_once_t
 _dispatch_once_xchg_done(dispatch_once_t *pred)
 {
-#if defined(__i386__) || defined(__x86_64__) || defined(__s390x__)
+#if defined(__i386__) || defined(__x86_64__) || defined(__s390x__) || defined(__ANDROID__)
        // On Intel, any load is a load-acquire, so we don't need to be fancy
        // same for s390x
        return os_atomic_xchg(pred, DLOCK_ONCE_DONE, release);
 #elif defined(__linux__)
        if (unlikely(syscall(__NR_membarrier, MEMBARRIER_CMD_SHARED, 0) < 0)) {

And the seems to be some problem with the MACRO “TRASHIT”

diff --git a/src/internal.h b/src/internal.h
index 0536db1..ecbfc8f 100644
--- a/src/internal.h
+++ b/src/internal.h
@@ -377,13 +377,15 @@ DISPATCH_EXPORT DISPATCH_NOTHROW void dispatch_atfork_child(void);
 // sys/queue.h debugging
 #if defined(__linux__)
 #define QUEUE_MACRO_DEBUG 1
 #else
 #undef TRASHIT
-#define TRASHIT(x) do {(x) = (void *)-1;} while (0)
 #endif
 #endif // DISPATCH_DEBUG
+#ifndef TRASHIT
+#define TRASHIT(x) do {(x) = (void *)-1;} while (0)
+#endif
 #define _TAILQ_TRASH_ENTRY(elm, field) do { \
                        TRASHIT((elm)->field.tqe_next); \
                        TRASHIT((elm)->field.tqe_prev); \
                } while (0)
 #define _TAILQ_TRASH_HEAD(head) do { \

And finally libdispatch needs to be linked to libswiftCore.so while it is being built.

Do you want me to file a new PR for the solutions to the first two problems?

[MadCoder]

This review is not the place to discuss it. however, the dispatch once thing is wrong on android, arm gives no such promise.

[johnno1962]

Any suggestions as to what the proper solution might be?

[MadCoder]

see #228 . If you aren't fluent with arm memory model, you should leave that patch to someone who does.

[johnno1962]

Fair enough. WIll do. Thanks for your help