onSnapshot fromCache always false when change is local

Question

According to the description here I'd expect snapshot.metadata.fromCache to be true when the document being listened to is modified in the same client as the listener, e.g.

1. The local .update(...) immediately triggers the onSnapshot handler (and is handed a snapshot with fromCache set to true)
2. The data is sent to the db
3. The firebase client receives the return message and does nothing (does not trigger a onSnapshot) because the server data agrees with the cache.

Ergo, fromCache should always be true when onSnapshot is trigged by a local change.

However, this only appears to be the case on the first two to three onSnapshot responses, after-which fromCache appears to always be false.

Example Test:

// ... firestore init w/ a test project and with persistence enabled. 

const db = firebase.firestore();
db.settings({
    ignoreUndefinedProperties:true
})

// Where "_test" is an empty collection with full allowance for read/write
await db.collection("_test").doc("deleteme").set({}); 

let doc = db.collection("_test").doc("deleteme") 

// ?! Expect this to be true but after the first one or two occurrences it is always false.
doc.onSnapshot(s=>{console.log("test snapshot change from cache? ",s.metadata.fromCache)}) 

let x = 0;      
let poke = async ()=>{
  doc.update({
    n:Math.random()
  })
  await sleep(3000); // generic custom delay 
  window.requestAnimationFrame(poke)
};
window.requestAnimationFrame(poke);

Edit: The question here is due to similarly missing knowledge as in this other question: Is Firestore onSnapshot update event due to local client Set?

Answer 1

Ergo, fromCache should always be true when onSnapshot is trigged by a local change.

I don't think that is how it is defined; it is more whether the client knows if its local snapshot is up-to-date with the server.

You maybe confusing fromCache with hasPendingWrites. The fromCache name is indeed confusing. I understand fromCache as "may not contain all data from the server yet", while the isPending is "may contain data that the server doesn't know about yet".