fix: shorten HandledPromises to propagate handlers

packages/eventual-send/src/index.js

@@ -54,15 +54,46 @@ export function makeHandledPromise(Promise) {
   let presenceToPromise;
   let promiseToHandler;
   let promiseToPresence; // only for HandledPromise.unwrap
+  let promiseToForwardedPromise; // forwarding, union-find-ish
   function ensureMaps() {
     if (!presenceToHandler) {
       presenceToHandler = new WeakMap();
       presenceToPromise = new WeakMap();
       promiseToHandler = new WeakMap();
       promiseToPresence = new WeakMap();
+      promiseToForwardedPromise = new WeakMap();
     }
   }
 
+  function shorten(target, error = false) {
+    let p = target;
+    // target -> resolved1 -> resolved2
+    while (promiseToForwardedPromise.has(p)) {
+      p = promiseToForwardedPromise.get(p);
+    }
+    const presence = promiseToPresence.get(p);
+    if (presence) {
+      // Presences are final, so it is ok to propagate
+      // this upstream.
+      while (target !== p) {
+        const parent = promiseToForwardedPromise.get(target);
+        promiseToForwardedPromise.delete(target);
+        promiseToPresence.set(target, presence);
+        target = parent;
+      }
+    } else {
+      // Even if p has an unfulfilledHandler, we don't care.
+      // We still propagate only p upstream since
+      // unfulfulled handlers are transient.
+      while (target !== p) {
+        const parent = promiseToForwardedPromise.get(target);
+        promiseToForwardedPromise.set(target, p);
+        target = parent;
+      }
+    }
+    return target;
+  }
+
   // This special handler accepts Promises, and forwards
   // handled Promises to their corresponding fulfilledHandler.
   let forwardingHandler;
@@ -75,33 +106,47 @@ export function makeHandledPromise(Promise) {
     }
     let handledResolve;
     let handledReject;
-    let fulfilled = false;
+    let resolved = false;
+    let handledP;
     const superExecutor = (resolve, reject) => {
       handledResolve = value => {
-        fulfilled = true;
+        if (promiseToForwardedPromise.has(handledP)) {
+          throw new TypeError('internal: already forwarded');
+        }
+        value = shorten(value);
+        resolved = true;
+        let targetP;
+        if (promiseToHandler.has(value) || promiseToPresence.has(value)) {
+          targetP = value;
+        } else {
+          targetP = presenceToPromise.get(value);
+        }
+        if (targetP && targetP !== handledP) {
+          promiseToForwardedPromise.set(handledP, targetP);
+        } else {
+          promiseToForwardedPromise.delete(handledP);
+        }
         resolve(value);
       };
       handledReject = err => {
-        fulfilled = true;
+        if (promiseToForwardedPromise.has(handledP)) {
+          throw new TypeError('internal: already forwarded');
+        }
+        promiseToHandler.delete(handledP);
+        resolved = true;
         reject(err);
       };
     };
-    const handledP = harden(
-      Reflect.construct(Promise, [superExecutor], new.target),
-    );
+    handledP = harden(Reflect.construct(Promise, [superExecutor], new.target));
 
     ensureMaps();
-    let continueForwarding = () => {};
 
-    if (!unfulfilledHandler) {
-      // Create a simple unfulfilledHandler that just postpones until the
+    const makePostponedHandler = () => {
+      // Create a simple postponedHandler that just postpones until the
       // fulfilledHandler is set.
-      //
-      // This is insufficient for actual remote handled Promises
-      // (too many round-trips), but is an easy way to create a
-      // local handled Promise.
+      let donePostponing;
       const interlockP = new Promise((resolve, reject) => {
-        continueForwarding = (err = null, targetP = undefined) => {
+        donePostponing = (err = null, targetP = undefined) => {
           if (err !== null) {
             reject(err);
             return;
@@ -115,7 +160,7 @@ export function makeHandledPromise(Promise) {
       // It will bubble up to the HandledPromise itself.
       interlockP.catch(_ => {});
 
-      const makePostponed = postponedOperation => {
+      const makePostponedOperation = postponedOperation => {
         // Just wait until the handler is resolved/rejected.
         return function postpone(x, ...args) {
           // console.log(`forwarding ${postponedOperation} ${args[0]}`);
@@ -133,10 +178,19 @@ export function makeHandledPromise(Promise) {
         };
       };
 
-      unfulfilledHandler = {
-        get: makePostponed('get'),
-        applyMethod: makePostponed('applyMethod'),
+      const postponedHandler = {
+        get: makePostponedOperation('get'),
+        applyMethod: makePostponedOperation('applyMethod'),
       };
+      return [postponedHandler, donePostponing];
+    };
+
+    let continueForwarding = () => {};
+    if (!unfulfilledHandler) {
+      // This is insufficient for actual remote handled Promises
+      // (too many round-trips), but is an easy way to create a
+      // local handled Promise.
+      [unfulfilledHandler, continueForwarding] = makePostponedHandler();
     }
 
     const validateHandler = h => {
@@ -150,18 +204,24 @@ export function makeHandledPromise(Promise) {
     promiseToHandler.set(handledP, unfulfilledHandler);
 
     const rejectHandled = reason => {
-      if (fulfilled) {
+      if (resolved) {
         return;
       }
+      if (promiseToForwardedPromise.has(handledP)) {
+        throw new TypeError('internal: already forwarded');
+      }
       handledReject(reason);
       continueForwarding(reason);
     };
 
     let resolvedPresence = null;
     const resolveWithPresence = presenceHandler => {
-      if (fulfilled) {
+      if (resolved) {
         return resolvedPresence;
       }
+      if (promiseToForwardedPromise.has(handledP)) {
+        throw new TypeError('internal: already forwarded');
+      }
       try {
         // Sanity checks.
         validateHandler(presenceHandler);
@@ -191,16 +251,21 @@ export function makeHandledPromise(Promise) {
     };
 
     const resolveHandled = async (target, deprecatedPresenceHandler) => {
-      if (fulfilled) {
+      if (resolved) {
         return undefined;
       }
+      if (promiseToForwardedPromise.has(handledP)) {
+        throw new TypeError('internal: already forwarded');
+      }
       try {
         if (deprecatedPresenceHandler) {
           throw TypeError(
             `resolveHandled no longer accepts a handler; use resolveWithPresence`,
           );
         }
 
+        target = shorten(target);
+
         // Resolve with the target when it's ready.
         handledResolve(target);
 
@@ -211,21 +276,29 @@ export function makeHandledPromise(Promise) {
           return continueForwarding(null, target);
         }
 
-        // See if the target is a presence we already know of.
-        let presence;
-        try {
-          presence = HandledPromise.unwrap(target);
-        } catch (e) {
-          presence = await target;
-        }
-        const existingPresenceHandler = presenceToHandler.get(presence);
-        if (existingPresenceHandler) {
+        const receivePresence = presence => {
+          const existingPresenceHandler = presenceToHandler.get(presence);
+          if (!existingPresenceHandler) {
+            return false;
+          }
           promiseToHandler.set(handledP, existingPresenceHandler);
           promiseToPresence.set(handledP, presence);
-          return continueForwarding(null, handledP);
+          continueForwarding(null, handledP);
+          return true;
+        };
+
+        // See if the target is a presence we already know of.
+        if (receivePresence(promiseToPresence.get(target))) {
+          return undefined;
+        }
+
+        // Wait for the target before continuing.
+        const presence = await target;
+        if (receivePresence(presence)) {
+          return undefined;
         }
 
-        // Remove the mapping, as we don't need a handler.
+        // Remove the mapping, as we don't need a handler anymore.
         promiseToHandler.delete(handledP);
         return continueForwarding();
       } catch (e) {
@@ -371,6 +444,7 @@ export function makeHandledPromise(Promise) {
 
   handle = (p, operation, ...args) => {
     ensureMaps();
+    p = shorten(p);
     const unfulfilledHandler = promiseToHandler.get(p);
     let executor;
     if (