Re: synchronously waiting for a Promise

Cool example. You are right. 

We are migrating our code to asynchronous. :-)

On Thursday, 1 September 2016 13:59:45 UTC+1, Thomas Broyer wrote:

Consider the following code:

class Foo {

  String bar;

  synchronized String getBar() {

    if (bar == null) { // OK, we all know this is bad in the Java world, just bear with me for this sample code

      Future<String> realBar = …; // make a call that returns a CompletableFuture<String>

      bar = realBar.get();

    }

    return bar;

  }

}

You're proposing that it's somehow translated more or less to:

var _symbol$bar = Symbol("Foo.bar");

class Foo {

  async getBar() { // Note: transformed to 'async', no 'synchronized'

    if (this[_symbol$bar] == null) {

      var realBar = …;

      this[_symbol$bar] = await realBar.get(); // transformed to await

    }

    return this[_symbol$bar];

  }

}

Now imagine that while await⋅ing realBar.get(), getBar() is called again, on the same Foo instance (triggered by an event; for example, getBar() is called from a click handler, and the … returning the CompletableFuture<String> fetches some resource through HTTP).

Looking at the original Java code, you'd expect that the second call is blocked until the first one terminates and releases the lock on the Foo object, so the first call would set the bar field and the second call would skip the 'if' branch.

In JS though, the second call would *enter* the 'if' branch and make a second call; then, when each 'realBar' is completed, the private (through Symbol) property is set: twice; and if the call that returned a CompletableFuture<String> is stateful, that means the state has been modified twice.

What kind of JS would you produce that'd prevent this from happening?

I wouldn't trade "emulating CompletableFuture#get" for "bloated JS and a much more complex compiler". The Pareto rule tells us that you should just embrace asynchrony and live with only the non-blocking API (CompletionStage basically, plus getNow() and a few others).

On Thursday, September 1, 2016 at 11:53:45 AM UTC+2, Ian Preston wrote:

Interesting question, Thomas. I believe it should still work. Consider the following: 

1. A 'thread' comes into a method and obtains a lock on A.

2. whilst holding the lock, it makes an async call to CompletableFuture.get()

3. Somewhere down the call stack some state which is guarded by the lock on A is modified

On the JVM, either that lock is not re-entrant, in which case it is a deadlock, or it is re-entrant and modifying the state is fine. We only have one thread in JS so it is fine. 

Correct me if I'm wrong, but I believe this means that GWT could continue to ignore synchronization, and get the same results assuming non deadlocking programs, which can't be translated without a full emulator like doppio anyway. 


On Thursday, 1 September 2016 10:22:08 UTC+1, Thomas Broyer wrote:

On Wednesday, August 31, 2016 at 11:58:51 PM UTC+2, Ian Preston wrote:

One idea, which would be awesome from a user perspective, is the following:

Emulate CompletableFuture with native Promises. Then if the synchronous cf.get() call is used, then translate that to await, and make the function it is in async (in JS land). This would automatically change the signature to a CompletableFuture, and then propagate this up to all callers. If it makes it all the way to a function exposed with JsInterop, then that function changes signature like the rest of them to return a promise. The consumer of this function is obviously in JS and so can handle the promise fine. This would equate to translating the synchronous java calls which wait on a CompletableFuture (Promise) to return promises in JS. This wouldn't require any changes to the Java code - it could continue in its synchronous style, and not disrupt JVM based users of the same code. 

Thoughts?

'await' is not synchronous, which means things can happen while awaiting, which could mutate state you'd rely on being immutable because you 'synchronized' it (in other words: how would your proposal work with the 'synchronized' keyword? GWT currently simply ignores 'synchronized' because JS is single-threaded anyway; now what if you call CompletableFuture#get() within a 'synchronized' function, possibly several levels deep in the call-stack?).

There are good reasons why only async functions can call async functions, and only async functions can use the await keyword: explicit vs. implicit.

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