strongloop / loopback-next / 3274

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feat(context): forbid bind().to() a Promise instance (#854)

Promises are a construct primarily intended for flow control:
In an algorithm with steps 1 and 2, we want to wait for the outcome
of step 1 before starting step 2.

Promises are NOT a tool for storing values that may become available
in the future, depending on the success or a failure of a background
async task.

Values stored in bindings are typically accessed only later,
in a different turn of the event loop or the Promise micro-queue.
As a result, when a promise is stored via `.to()` and is rejected
later, then more likely than not, there will be no error (catch)
handler registered yet, and Node.js will print
"Unhandled Rejection Warning".

BREAKING CHANGE: It is no longer possible to pass a promise instance
to `.to()` method of a Binding. Use `.toDynamicValue()` instead.
Consider deferring the async computation (that produced the promise
instance you are binding) into the dynamic value getter function,
i.e. start the async computation only from the getter function.

An example diff showing how to upgrade your existing code:

-    ctx.bind('bar').to(Promise.resolve('BAR'));
+    ctx.bind('bar').toDynamicValue(() => Promise.resolve('BAR'));

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