Possibly unnecessary Sync

[jokil123]

The following code:

use std::io::Read;

use firestore::{FirestoreDb, FirestoreListenerTarget};
use tower_controller_rs::temp_file_token_storage::TempFileTokenStorage;

#[tokio::main]
async fn main() {
    let db = FirestoreDb::new("my-project-id").await.unwrap();

    let mut listener = db.create_listener(TempFileTokenStorage).await.unwrap();

    db.fluent()
        .select()
        .from("my-col")
        .listen()
        .add_target(FirestoreListenerTarget::new(1), &mut listener)
        .unwrap();

    listener
        .start(move |r| {
            let db = db.clone();
            async move {
                db.fluent()
                    .select()
                    .by_id_in("mycol")
                    .one("mydoc")
                    .await
                    .unwrap();
                Ok(())
            }
        })
        .await
        .unwrap();

    std::io::stdin().read(&mut [1]).unwrap();

    listener.shutdown().await.unwrap();
}

Fails to compile with the following error:

error: future cannot be shared between threads safely
   --> src\bin\error_example.rs:20:10
    |
20  |         .start(move |r| {
    |          ^^^^^ future created by async block is not `Sync`
    |
    = help: the trait `Sync` is not implemented for `dyn Future<Output = Result<gcloud_sdk::apis::google::firestore::v1::Document, FirestoreError>> + Send`
note: future is not `Sync` as this value is used across an await
   --> D:\Repos\firestore-rs\src\fluent_api\select_builder.rs:330:17
    |
322 |               match self
    |  ___________________-
323 | |                 .db
324 | |                 .get_doc_at::<S>(
325 | |                     parent.as_str(),
...   |
328 | |                     self.return_only_fields,
329 | |                 )
    | |_________________- has type `Pin<Box<dyn Future<Output = Result<gcloud_sdk::apis::google::firestore::v1::Document, FirestoreError>> + Send>>` which is not `Sync`
330 |                   .await
    |                   ^^^^^^ await occurs here, with the value maybe used later
...
338 |           } else {
    |           - the value is later dropped here
note: required by a bound in `FirestoreListener::<D, S>::start`
   --> D:\Repos\firestore-rs\src\db\listen_changes.rs:231:57
    |
231 |         F: Future<Output = BoxedErrResult<()>> + Send + Sync + 'static,
    |                                                         ^^^^ required by this bound in `FirestoreListener::<D, S>::start`

This is due to the start function requiring a Sync future and the fluent api not returning a future with Sync. It is possible to remove this requirement from the start function (and a function it calls) without it causing an error. Not guaranteed everything still works. I will open a draft pull request (#71 ).

[abdolence]

Hmm, interesting. It is possibly easier to just propagate db instances to the async function as additional argument - maybe useful for many cases.

[abdolence]

This is maybe more convenient for people in general:
#73

[abdolence]

Still considering if #73 is needed or not. Until then I've released your fix in 0.26.2. Thanks for the PR!