select: Allow registering cleanup method for the callback

This commit adds the registerCleanupFunc method for the callback
passed to the stream implementation. It allows the stream
implementation to optionally register a cleanup function that will be
invoked when select is no longer interested in the stream events. This
enables the stream implementation to perform additional cleanups such
as discarding the reference to the event callback.

Signed-off-by: Kohei Tokunaga <ktokunaga.mail@gmail.com>

Author: ktock

src/lib/libsyscall.js

@@ -702,6 +702,7 @@ var SyscallsLibrary = {
 #if PROXY_TO_PTHREAD
     SYSCALLS.deactivateSelectCallbacks(fdIdx); // deactivate all old callbacks
     var makeNotifyCallback = (fd) => null;
+    var cleanupFuncs = [];
     if (timeoutInMillis != 0) {
       Atomics.store(HEAP32, fdIdx, -1); // Initialize the shared region
       Atomics.store(HEAP32, flagsIdx, -1); // Initialize the shared region
@@ -711,10 +712,14 @@ var SyscallsLibrary = {
             return; // This callback is no longer active.
           }
           SYSCALLS.deactivateSelectCallbacks(fdIdx); // Only the first event is notified.
+          cleanupFuncs.forEach(cb => cb());
           Atomics.store(HEAP32, flagsIdx, flags);
           Atomics.store(HEAP32, fdIdx, fd);
           Atomics.notify(HEAP32, fdIdx);
         }
+        cb.registerCleanupFunc = (f) => {
+          if (f != null) cleanupFuncs.push(f);
+        }
         SYSCALLS.activateSelectCallback(fdIdx, cb);
         return cb;
       }
@@ -752,6 +757,7 @@ var SyscallsLibrary = {
       fdSet.commit(fd, flags);
       // No wait will happen in the caller. Deactivate all callbacks.
       SYSCALLS.deactivateSelectCallbacks(fdIdx);
+      cleanupFuncs.forEach(f => f());
     }
 #else
     fdSet.commit(fd, flags);