feat(HandledPromise): add sync unwrap() to get presences

Closes #412

Author: michaelfig

packages/eventual-send/src/E.js

@@ -68,6 +68,7 @@ export default function makeE(HandledPromise) {
 
   E.G = makeEGetterProxy;
   E.resolve = HandledPromise.resolve;
+  E.unwrap = HandledPromise.unwrap;
 
   return harden(E);
 }

packages/eventual-send/src/index.js

@@ -53,11 +53,13 @@ export function makeHandledPromise(Promise) {
   let presenceToHandler;
   let presenceToPromise;
   let promiseToHandler;
+  let promiseToPresence; // only for HandledPromise.unwrap
   function ensureMaps() {
     if (!presenceToHandler) {
       presenceToHandler = new WeakMap();
       presenceToPromise = new WeakMap();
       promiseToHandler = new WeakMap();
+      promiseToPresence = new WeakMap();
     }
   }
 
@@ -109,6 +111,9 @@ export function makeHandledPromise(Promise) {
           // Return undefined.
         };
       });
+      // A failed interlock should not be recorded as an unhandled rejection.
+      // It will bubble up to the HandledPromise itself.
+      interlockP.catch(_ => {});
 
       const makePostponed = postponedOperation => {
         // Just wait until the handler is resolved/rejected.
@@ -167,6 +172,7 @@ export function makeHandledPromise(Promise) {
         // Create table entries for the presence mapped to the
         // fulfilledHandler.
         presenceToPromise.set(resolvedPresence, handledP);
+        promiseToPresence.set(handledP, resolvedPresence);
         presenceToHandler.set(resolvedPresence, presenceHandler);
 
         // Remove the mapping, as our presenceHandler should be
@@ -206,10 +212,16 @@ export function makeHandledPromise(Promise) {
         }
 
         // See if the target is a presence we already know of.
-        const presence = await target;
+        let presence;
+        try {
+          presence = HandledPromise.unwrap(target);
+        } catch (e) {
+          presence = await target;
+        }
         const existingPresenceHandler = presenceToHandler.get(presence);
         if (existingPresenceHandler) {
           promiseToHandler.set(handledP, existingPresenceHandler);
+          promiseToPresence.set(handledP, presence);
           return continueForwarding(null, handledP);
         }
 
@@ -284,6 +296,30 @@ export function makeHandledPromise(Promise) {
         promiseResolve().then(_ => new HandledPromise(executeThen)),
       );
     },
+    // TODO verify that this is safe to provide universally, i.e.,
+    // that by itself it doesn't provide access to mutable state in
+    // ways that violate normal ocap module purity rules. The claim
+    // that it does not rests on the handled promise itself being
+    // necessary to perceive this mutable state. In that sense, we
+    // can think of the right to perceive it, and of access to the
+    // target, as being in the handled promise. Note that a .then on
+    // the handled promise will already provide async access to the
+    // target, so the only additional authorities are: 1)
+    // synchronous access for handled promises only, and thus 2) the
+    // ability to tell, from the client side, whether a promise is
+    // handled. Or, at least, the ability to tell given that the
+    // promise is already fulfilled.
+    unwrap(value) {
+      ensureMaps();
+      const pr = presenceToPromise.get(value) || value;
+      const presence = promiseToPresence.get(pr);
+      if (!presence) {
+        throw TypeError(
+          `Value is not a presence nor a HandledPromise resolved to a presence`,
+        );
+      }
+      return presence;
+    },
   });
 
   defineProperties(HandledPromise, getOwnPropertyDescriptors(staticMethods));

packages/eventual-send/test/test-e.js

@@ -1,5 +1,16 @@
 import test from 'tape-promise/tape';
-import { E } from '../src/index';
+import { E, HandledPromise } from '../src/index';
+
+test('E reexports', async t => {
+  try {
+    t.equals(E.resolve, HandledPromise.resolve, 'E reexports resolve');
+    t.equals(E.unwrap, HandledPromise.unwrap, 'E reexports unwrap');
+  } catch (e) {
+    t.isNot(e, e, 'unexpected exception');
+  } finally {
+    t.end();
+  }
+});
 
 test('E method calls', async t => {
   try {

packages/eventual-send/test/test-hp.js

@@ -45,3 +45,88 @@ test('chained properties', async t => {
     t.end();
   }
 });
+
+test('HandledPromise.unwrap', async t => {
+  try {
+    t.throws(
+      () => HandledPromise.unwrap({}),
+      TypeError,
+      `unwrapped non-presence throws`,
+    );
+    const p0 = new Promise(_ => {});
+    t.throws(
+      () => HandledPromise.unwrap(p0),
+      TypeError,
+      `unwrapped unfulfilled Promise throws`,
+    );
+    const p1 = new Promise(resolve => {
+      resolve({});
+    });
+    t.throws(
+      () => HandledPromise.unwrap(p1),
+      TypeError,
+      `unwrapped resolved Promise throws`,
+    );
+    const p2 = new Promise((_, reject) => {
+      reject(Error('p2'));
+    });
+    // Prevent unhandled promise rejection.
+    p2.catch(_ => {});
+    t.throws(
+      () => HandledPromise.unwrap(p2),
+      TypeError,
+      `unwrapped rejected Promise throws`,
+    );
+    const hp0 = new HandledPromise(_ => {});
+    t.throws(
+      () => HandledPromise.unwrap(hp0),
+      TypeError,
+      'unfulfilled HandledPromise throws',
+    );
+    const hp1 = new HandledPromise(resolve => {
+      resolve({});
+    });
+    t.throws(
+      () => HandledPromise.unwrap(hp1),
+      TypeError,
+      'resolved HandledPromise throws',
+    );
+    const hp2 = new HandledPromise((_, reject) => {
+      reject(Error('hp2'));
+    });
+    // Prevent unhandled promise rejection.
+    hp2.catch(_ => {});
+    t.throws(
+      () => HandledPromise.unwrap(hp2),
+      TypeError,
+      'rejected HandledPromise throws',
+    );
+    let presence;
+    const hp3 = new HandledPromise((_res, _rej, resolveWithPresence) => {
+      presence = resolveWithPresence({});
+    });
+    t.equals(typeof presence, 'object', `typeof presence is object`);
+    t.equals(
+      HandledPromise.unwrap(hp3),
+      presence,
+      `unwrapped HandledPromise is presence`,
+    );
+    t.equals(
+      HandledPromise.unwrap(presence),
+      presence,
+      `unwrapped presence is presence`,
+    );
+    const hp4 = new HandledPromise(resolve => {
+      resolve(hp3);
+    });
+    t.equals(
+      HandledPromise.unwrap(hp4),
+      presence,
+      `unwrapped forwarded HandledPromise is presence`,
+    );
+  } catch (e) {
+    t.isNot(e, e, 'unexpected exception');
+  } finally {
+    t.end();
+  }
+});