Add example for wrapping PostgresConnectionManager

[film42]

With this example we show how easy it is to wrap the postgres connection
pool to prepare several queries when a connection is created. Then, once
a connection is checked out, this examples shows how easy it is to use
the custom state to pull a prepared statement.

Inspired by #110 which requests this kind of behavior in bb8 directly,
but for now it's easy to extend any connection manager with custom
needs.

Happy hacktoberfest!

[djc]

I think this might be helpful, so thanks for putting it together!

With some style tweaks, I'm happy to merge this.

[djc]

It's actually not alphabetic order but top-down order. But in this case they somewhat coincide.

[seanlinsley]

I'm trying to do something similar to this, but it doesn't seem like it works. For me at least, pool.get() returns a PooledConnection not a CustomPostgresConnection so there's no way to access the statement cache.

[djc]

I recommend you start from this exact example, which is compile checked in CI and thus should work.

[seanlinsley]

Ah, it looks like the connection having a type of PooledConnection is expected. I get the same error when trying to specify the type in the example code:

let connection: CustomPostgresConnection = pool.get().await.expect("pool error");

So my issue seems to be more related to #57. It's not clear what type annotations are needed so bb8 pooled connections can be passed to a function. This for example fails with:

no method named query found for struct PooledConnection<'_, M> in the current scope

async fn run_query<M: bb8::ManageConnection>(connection: PooledConnection<'_, M>) {
    connection.query("", &[]).await.unwrap();
}

Sorry if this has been asked somewhere else, but if it has I can't find it.

[djc]

Your code sample presupposes the existence of a query() method on M::Connection. You'll need to explain to the compiler why it does. For example, M is some specific type, like CustomPostgresConnectionManager, or M::Connection implements some trait that provides a query() method of the appropriate type.

[seanlinsley]

Right, so how do I do that? 😅 The postgres glue code isn't implementing a trait from the tokio_postgres crate so it's not clear how these function calls are passed through to the postgres connection as it is.

Edit: so, this does work. But now every function call (I have many) is specifying NoTls when the use of TLS would be decided at runtime.

async fn run_query(connection: PooledConnection<'_, CustomPostgresConnectionManager<NoTls>>) {
    connection.query("", &[]).await.unwrap();
}

[djc]

Make it abstract over dyn MakeTlsConnect<Socket>?

[seanlinsley]

dyn MakeTlsConnect<Socket> gets me to:

help: specify the associated types: MakeTlsConnect<Socket, Stream = Type, TlsConnect = Type, Error = Type>

I tried multiple versions of that but couldn't work around this error:

(dyn MakeTlsConnect<...> + 'static) cannot be shared between threads safely

I also tried defining a type like #57 (comment) but ran into the same error.

[djc]

Add a Send bound. But all of this ends up being Rust knowledge/experience, so maybe better on Stack Overflow or users.rust-lang.org.

[seanlinsley]

Yes I don't have much experience with Rust, but this is a basic usability / documentation issue with bb8. I've tried multiple variations here, including adding Send, and I'm thrown into an ever-deeper hole of errors.

Re: posting this elsewhere, I doubt anyone's going to put in the time to meaningfully help if they're not already familiar with this project.