Implement Lisp threading on Android

Much like the NS port, only the main thread receives input from
the user interface, which is fortunately not a major problem for
packages such as lsp-mode that create Lisp threads.

* configure.ac: Enable with_threads under Android.

* src/android.c (android_init_events): Set `main_thread_id' to
the ID of the main thread.
(setEmacsParams): Set new global variable `android_jvm' to the
JVM object, for the purpose of attaching Lisp threads to the
JVM.
(android_select): [THREADS_ENABLED]: If the caller isn't the
main thread, resort to pselect.  Don't check query before select
returns.
(android_check_query): Export.

* src/android.h (_ANDROID_H_): Define new macro and update
prototypes.

* src/process.c (android_select_wrapper): New function.
(wait_reading_process_output): If THREADS_ENABLED, call
thread_select through the Android select wrapper.

* src/thread.c (post_acquire_global_lock): Call
android_check_query; replace android_java_env with the incoming
Lisp thread's.
(run_thread): Attach and detach the thread created to the JVM.
(init_threads): Set the main thread's JNI environment object.

* src/thread.h (struct thread_state) <java_env>: New field.

1 files changed, 30 insertions, 3 deletions

diff --git a/src/process.c b/src/process.c
index ddab9ed6c01..48a2c0c8e53 100644
--- a/src/process.c
+++ b/src/process.c
@@ -5209,6 +5209,27 @@ wait_reading_process_output_1 (void)
 {
 }
 
+#if defined HAVE_ANDROID && !defined ANDROID_STUBIFY	\
+  && defined THREADS_ENABLED
+
+/* Wrapper around `android_select' that exposes a calling interface with
+   an extra argument for compatibility with `thread_pselect'.  */
+
+static int
+android_select_wrapper (int nfds, fd_set *readfds, fd_set *writefds,
+			fd_set *exceptfds, const struct timespec *timeout,
+			const sigset_t *sigmask)
+{
+  /* sigmask is not supported.  */
+  if (sigmask)
+    emacs_abort ();
+
+  return android_select (nfds, readfds, writefds, exceptfds,
+			 (struct timespec *) timeout);
+}
+
+#endif /* HAVE_ANDROID && !ANDROID_STUBIFY && THREADS_ENABLED */
+
 /* Read and dispose of subprocess output while waiting for timeout to
    elapse and/or keyboard input to be available.
 
@@ -5701,13 +5722,19 @@ wait_reading_process_output (intmax_t time_limit, int nsecs, int read_kbd,
 	    timeout = short_timeout;
 #endif
 
-	  /* Android doesn't support threads and requires using a
-	     replacement for pselect in android.c to poll for
-	     events.  */
+	  /* Android requires using a replacement for pselect in
+	     android.c to poll for events.  */
 #if defined HAVE_ANDROID && !defined ANDROID_STUBIFY
+#ifndef THREADS_ENABLED
 	  nfds = android_select (max_desc + 1,
 				 &Available, (check_write ? &Writeok : 0),
 				 NULL, &timeout);
+#else /* THREADS_ENABLED */
+	  nfds = thread_select (android_select_wrapper,
+				max_desc + 1,
+				&Available, (check_write ? &Writeok : 0),
+				NULL, &timeout, NULL);
+#endif /* THREADS_ENABLED */
 #else
 
 	  /* Non-macOS HAVE_GLIB builds call thread_select in