Remove promise-to-callback, reimplement createAsyncMiddleware (#46)

* remove promise-to-callback, reimplement createAsyncMiddleware

* fix middleware plural copy

package.json

@@ -17,7 +17,6 @@
   },
   "dependencies": {
     "eth-rpc-errors": "^2.1.1",
-    "promise-to-callback": "^1.0.0",
     "safe-event-emitter": "^1.0.1"
   },
   "devDependencies": {

src/createAsyncMiddleware.js

@@ -1,39 +1,60 @@
-const promiseToCallback = require('promise-to-callback')
+/**
+ * JsonRpcEngine only accepts callback-based middleware directly.
+ * createAsyncMiddleware exists to enable consumers to pass in async middleware
+ * functions.
+ *
+ * Async middleware have no "end" function. Instead, they "end" if they return
+ * without calling "next". Rather than passing in explicit return handlers,
+ * async middleware can simply await "next", and perform operations on the
+ * response object when execution resumes.
+ *
+ * To accomplish this, createAsyncMiddleware passes the async middleware a
+ * wrapped "next" function. That function calls the internal JsonRpcEngine
+ * "next" function with a return handler that resolves a promise when called.
+ *
+ * The return handler will always be called. Its resolution of the promise
+ * enables the control flow described above.
+ */
 
 module.exports = function createAsyncMiddleware (asyncMiddleware) {
   return (req, res, next, end) => {
-    let nextDonePromise = null
-    const finishedPromise = asyncMiddleware(req, res, getNextPromise)
-    promiseToCallback(finishedPromise)((err) => {
-      // async middleware ended
-      if (nextDonePromise) {
-        // next handler was called - complete nextHandler
-        promiseToCallback(nextDonePromise)((nextErr, nextHandlerSignalDone) => {
-          // nextErr is only present if something went really wrong
-          // if an error is thrown after `await next()` it appears as `err` and not `nextErr`
-          if (nextErr) {
-            console.error(nextErr)
-            return end(nextErr)
-          }
-          nextHandlerSignalDone(err)
-          return undefined
-        })
-      } else {
-        // next handler was not called - complete middleware
-        end(err)
-      }
+
+    // nextPromise is the key to the implementation
+    // it is resolved by the return handler passed to the
+    // "next" function
+    let resolveNextPromise
+    const nextPromise = new Promise((resolve) => {
+      resolveNextPromise = resolve
     })
 
-    async function getNextPromise () {
-      nextDonePromise = getNextDoneCallback()
-      await nextDonePromise
-      return undefined
+    let returnHandlerCallback, nextWasCalled
+
+    const asyncNext = async () => {
+
+      nextWasCalled = true
+
+      next((callback) => { // eslint-disable-line callback-return
+        returnHandlerCallback = callback
+        resolveNextPromise()
+      })
+      await nextPromise
     }
 
-    function getNextDoneCallback () {
-      return new Promise((resolve) => {
-        next((cb) => resolve(cb))
+    asyncMiddleware(req, res, asyncNext)
+      .then(async () => {
+        if (nextWasCalled) {
+          await nextPromise // we must wait until the return handler is called
+          returnHandlerCallback(null)
+        } else {
+          end(null)
+        }
+      })
+      .catch((error) => {
+        if (returnHandlerCallback) {
+          returnHandlerCallback(error)
+        } else {
+          end(error)
+        }
       })
-    }
   }
 }

test/createAsyncMiddleware.spec.js

@@ -92,7 +92,7 @@ describe('createAsyncMiddleware tests', function () {
     })
   })
 
-  it('doesnt await next', function (done) {
+  it('doesn\'t await next', function (done) {
     const engine = new RpcEngine()
 
     engine.push(createAsyncMiddleware(async (_req, _res, next) => {

yarn.lock

@@ -1131,11 +1131,6 @@ is-extglob@^2.1.1:
   resolved "https://registry.yarnpkg.com/is-extglob/-/is-extglob-2.1.1.tgz#a88c02535791f02ed37c76a1b9ea9773c833f8c2"
   integrity sha1-qIwCU1eR8C7TfHahueqXc8gz+MI=
 
-is-fn@^1.0.0:
-  version "1.0.0"
-  resolved "https://registry.yarnpkg.com/is-fn/-/is-fn-1.0.0.tgz#9543d5de7bcf5b08a22ec8a20bae6e286d510d8c"
-  integrity sha1-lUPV3nvPWwiiLsiiC65uKG1RDYw=
-
 is-fullwidth-code-point@^1.0.0:
   version "1.0.0"
   resolved "https://registry.yarnpkg.com/is-fullwidth-code-point/-/is-fullwidth-code-point-1.0.0.tgz#ef9e31386f031a7f0d643af82fde50c457ef00cb"
@@ -1778,14 +1773,6 @@ progress@^2.0.0:
   resolved "https://registry.yarnpkg.com/progress/-/progress-2.0.3.tgz#7e8cf8d8f5b8f239c1bc68beb4eb78567d572ef8"
   integrity sha512-7PiHtLll5LdnKIMw100I+8xJXR5gW2QwWYkT6iJva0bXitZKa/XMrSbdmg3r2Xnaidz9Qumd0VPaMrZlF9V9sA==
 
-promise-to-callback@^1.0.0:
-  version "1.0.0"
-  resolved "https://registry.yarnpkg.com/promise-to-callback/-/promise-to-callback-1.0.0.tgz#5d2a749010bfb67d963598fcd3960746a68feef7"
-  integrity sha1-XSp0kBC/tn2WNZj805YHRqaP7vc=
-  dependencies:
-    is-fn "^1.0.0"
-    set-immediate-shim "^1.0.1"
-
 punycode@^2.1.0:
   version "2.1.1"
   resolved "https://registry.yarnpkg.com/punycode/-/punycode-2.1.1.tgz#b58b010ac40c22c5657616c8d2c2c02c7bf479ec"
@@ -1934,10 +1921,15 @@ set-blocking@^2.0.0:
   resolved "https://registry.yarnpkg.com/set-blocking/-/set-blocking-2.0.0.tgz#045f9782d011ae9a6803ddd382b24392b3d890f7"
   integrity sha1-BF+XgtARrppoA93TgrJDkrPYkPc=
 
-set-immediate-shim@^1.0.1:
-  version "1.0.1"
-  resolved "https://registry.yarnpkg.com/set-immediate-shim/-/set-immediate-shim-1.0.1.tgz#4b2b1b27eb808a9f8dcc481a58e5e56f599f3f61"
-  integrity sha1-SysbJ+uAip+NzEgaWOXlb1mfP2E=
+set-value@^2.0.0, set-value@^2.0.1:
+  version "2.0.1"
+  resolved "https://registry.yarnpkg.com/set-value/-/set-value-2.0.1.tgz#a18d40530e6f07de4228c7defe4227af8cad005b"
+  integrity sha512-JxHc1weCN68wRY0fhCoXpyK55m/XPHafOmK4UWD7m2CI14GMcFypt4w/0+NV5f/ZMby2F6S2wwA7fgynh9gWSw==
+  dependencies:
+    extend-shallow "^2.0.1"
+    is-extendable "^0.1.1"
+    is-plain-object "^2.0.3"
+    split-string "^3.0.1"
 
 shebang-command@^1.2.0:
   version "1.2.0"