Adding reject-client-connections doc. (#2973)

# Description of Changes

This originally was a PR in the Docs repo:
#352

This is the work portion of
clockworklabs/SpacetimeDBPrivate#1700

This adds a "How To Reject Client Connections" guide in both Rust and C#
using the language toggle functionality.

Testing:
- [X] Validated code behavior presented in guide resulted in described
behavior.

---------

Co-authored-by: Zeke Foppa <[email protected]>
Co-authored-by: Zeke Foppa <[email protected]>

Author: rekhoff

docs/how-to/reject-client-connections.md

@@ -0,0 +1,56 @@
+# Rejecting Client Connections
+
+SpacetimeDB provides a way to disconnect a client during a client connection attempt.
+
+:::server-rust
+In Rust, if we returned and error (or a panic) during the `client_connected` reducer, the client will be disconnected.
+
+Here is a simple example where the server module throws an error for all incoming client connections.
+```rust
+#[reducer(client_connected)]
+pub fn client_connected(_ctx: &ReducerContext) -> Result<(), String> {
+    let client_is_rejected = true;
+    if client_is_rejected {
+        Err("The client connection was rejected. With our current code logic, all clients will be rejected.".to_string())
+    } else {
+        Ok(())
+    }
+}
+```
+
+Client behavior can vary by client type. For example:
+* **C# clients**: Client disconnection behavior is currently undefined and will generate an error reading:
+  `Disconnected abnormally: System.Net.WebSockets.WebSocketException (0x80004005): The remote party closed the WebSocket connection without completing the close handshake.`
+
+* **Rust clients**: Client disconnection behavior is currently undefined and will generate an error reading:
+  `Unable to send subscribe message: WS sender loop has dropped its recv channel: TrySendError { kind: Disconnected }`
+
+* **TypeScript clients**: Client will receive an `Error connecting to SpacetimeDB:` and a `CloseEvent` with a code of 1006.
+
+Regardless of the client type, from the rust server's perspective, the client will be disconnected and the server module's logs will contain an entry reading:
+`ERROR: : The client connection was rejected. With our current code logic, all clients will be rejected.`
+:::
+:::server-csharp
+In C#, if we throw an exception during the `ClientConnected` reducer, the client will be disconnected.
+
+Here is a simple example where the server module throws an error for all incoming client connections.
+```csharp
+[Reducer(ReducerKind.ClientConnected)]
+// Called when a client connects to a SpacetimeDB database server
+public static void ClientConnected(ReducerContext ctx)
+{
+    throw new Exception("The client connection was rejected. With our current code logic, all clients will be rejected.");
+}
+```
+
+Client behavior can vary by client type. For example:
+* **C# clients**: Client disconnection behavior is currently undefined and will generate an error reading:
+`Disconnected abnormally: System.Net.WebSockets.WebSocketException (0x80004005): The remote party closed the WebSocket connection without completing the close handshake.`
+
+* **Rust clients**: Client will receive an `on_disconnected` event with no error message.
+
+* **TypeScript clients**: Client will receive an `Error connecting to SpacetimeDB:` and a `CloseEvent` with a code of 1006.
+
+Regardless of the client type, from the C# server's perspective, the client will be disconnected and the server module's logs will contain an entry reading:
+`ERROR: : System.Exception: The client connection was rejected. With our current code logic, all clients will be rejected.`
+:::

docs/nav.js

@@ -44,6 +44,7 @@ const nav = {
         page('Row Level Security', 'rls', 'rls/index.md'),
         section('How To'),
         page('Incremental Migrations', 'how-to/incremental-migrations', 'how-to/incremental-migrations.md'),
+        page('Reject Client Connections', 'how-to/reject-client-connections', 'how-to/reject-client-connections.md'),
         section('HTTP API'),
         page('Authorization', 'http/authorization', 'http/authorization.md'),
         page('`/identity`', 'http/identity', 'http/identity.md'),

nav.ts

@@ -96,6 +96,7 @@ const nav: Nav = {
 
     section('How To'),
     page('Incremental Migrations', 'how-to/incremental-migrations', 'how-to/incremental-migrations.md'),
+    page('Reject Client Connections', 'how-to/reject-client-connections', 'how-to/reject-client-connections.md'),
 
     section('HTTP API'),
     page('Authorization', 'http/authorization', 'http/authorization.md'),