Add binderprocess package for ABinderProcess_* thread pool functions

These functions are not exposed in the NDK public headers but are
available at runtime via dlsym from libbinder_ndk.so.

Author: xaionaro@dx.center

binderprocess/thread_pool.go

@@ -0,0 +1,77 @@
+// Package binderprocess provides access to ABinderProcess_* functions
+// that are not part of the public NDK headers but available at runtime
+// via dlsym in libbinder_ndk.so.
+package binderprocess
+
+/*
+#include <dlfcn.h>
+#include <stdint.h>
+
+static int thread_pool_start(unsigned thread_pool_size)
+{
+	typedef int (*set_thread_pool_max_fn)(uint32_t);
+	typedef void (*start_thread_pool_fn)(void);
+
+	void *h = dlopen("libbinder_ndk.so", RTLD_NOW | RTLD_LOCAL);
+	if (h == NULL)
+		return 1;
+
+	start_thread_pool_fn start_thread_pool =
+		(start_thread_pool_fn)dlsym(h, "ABinderProcess_startThreadPool");
+	if (start_thread_pool == NULL)
+		return 2;
+
+	if (thread_pool_size != 0) {
+		set_thread_pool_max_fn set_thread_pool_max =
+			(set_thread_pool_max_fn)dlsym(h, "ABinderProcess_setThreadPoolMaxThreadCount");
+		if (set_thread_pool_max == NULL)
+			return 3;
+		set_thread_pool_max(thread_pool_size);
+	}
+
+	start_thread_pool();
+	return 0;
+}
+*/
+import "C"
+
+import "fmt"
+
+// ErrDLOpenLibbinderNDK is returned when libbinder_ndk.so cannot be loaded.
+type ErrDLOpenLibbinderNDK struct{}
+
+func (ErrDLOpenLibbinderNDK) Error() string {
+	return "unable to dlopen libbinder_ndk.so"
+}
+
+// ErrSymbolStartThreadPool is returned when ABinderProcess_startThreadPool is not found.
+type ErrSymbolStartThreadPool struct{}
+
+func (ErrSymbolStartThreadPool) Error() string {
+	return "unable to find ABinderProcess_startThreadPool symbol"
+}
+
+// ErrSymbolSetThreadPoolMax is returned when ABinderProcess_setThreadPoolMaxThreadCount is not found.
+type ErrSymbolSetThreadPoolMax struct{}
+
+func (ErrSymbolSetThreadPoolMax) Error() string {
+	return "unable to find ABinderProcess_setThreadPoolMaxThreadCount symbol"
+}
+
+// StartThreadPool starts the binder thread pool with the given number of threads.
+// If threads is 0, the default thread count is used (only startThreadPool is called).
+func StartThreadPool(threads int) error {
+	rc := C.thread_pool_start(C.uint(threads))
+	switch rc {
+	case 0:
+		return nil
+	case 1:
+		return ErrDLOpenLibbinderNDK{}
+	case 2:
+		return ErrSymbolStartThreadPool{}
+	case 3:
+		return ErrSymbolSetThreadPoolMax{}
+	default:
+		return fmt.Errorf("thread_pool_start returned unexpected code %d", rc)
+	}
+}