refactor(tvix/castore/tonic): make async, support wait-connect=?

This moves the sync `channel::from_url` to a async
`tonic::channel_from_url`. It now allows connecting non-lazily if `wait-
connect=1` is set in the URL params.

Also, make the pingpong tests for blobsvc and directorysvc use the wait-
connect=1 codepath.

Change-Id: Ibeea33117c8121814627e7f6aba0e943ae2e92ca
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10030
Tested-by: BuildkiteCI
Reviewed-by: Connor Brewster <cbrewster@hey.com>

tvix/castore/src/tonic.rs

@@ -0,0 +1,115 @@
+use tokio::net::UnixStream;
+use tonic::transport::{Channel, Endpoint};
+
+fn url_wants_wait_connect(url: &url::Url) -> bool {
+    url.query_pairs()
+        .filter(|(k, v)| k == "wait-connect" && v == "1")
+        .count()
+        > 0
+}
+
+/// Turn a [url::Url] to a [Channel] if it can be parsed successfully.
+/// It supports `grpc+unix:/path/to/socket`, as well as the regular schemes supported
+/// by tonic, for example `grpc+http://[::1]:8000`.
+/// It supports wait-connect=1 as a URL parameter, in which case we don't connect lazily.
+pub async fn channel_from_url(url: &url::Url) -> Result<Channel, self::Error> {
+    // Stringify the URL and remove the grpc+ prefix.
+    // We can't use `url.set_scheme(rest)`, as it disallows
+    // setting something http(s) that previously wasn't.
+    let unprefixed_url_str = match url.to_string().strip_prefix("grpc+") {
+        None => return Err(Error::MissingGRPCPrefix()),
+        Some(url_str) => url_str.to_owned(),
+    };
+
+    if url.scheme() == "grpc+unix" {
+        if url.host_str().is_some() {
+            return Err(Error::HostSetForUnixSocket());
+        }
+
+        let connector = tower::service_fn({
+            let url = url.clone();
+            move |_: tonic::transport::Uri| UnixStream::connect(url.path().to_string().clone())
+        });
+
+        let channel = if url_wants_wait_connect(url) {
+            Endpoint::from_static("http://[::]:50051")
+                .connect_with_connector(connector)
+                .await?
+        } else {
+            Endpoint::from_static("http://[::]:50051").connect_with_connector_lazy(connector)
+        };
+
+        return Ok(channel);
+    }
+
+    // ensure path is empty, not supported with gRPC.
+    if !url.path().is_empty() {
+        return Err(Error::PathMayNotBeSet());
+    }
+
+    // Use the regular tonic transport::Endpoint logic, but unprefixed_url_str,
+    // as tonic doesn't know about grpc+http[s].
+    let endpoint = Endpoint::try_from(unprefixed_url_str)?;
+    let channel = if url_wants_wait_connect(url) {
+        endpoint.connect().await?
+    } else {
+        endpoint.connect_lazy()
+    };
+
+    Ok(channel)
+}
+
+/// Errors occuring when trying to connect to a backend
+#[derive(Debug, thiserror::Error)]
+pub enum Error {
+    #[error("grpc+ prefix is missing from URL")]
+    MissingGRPCPrefix(),
+
+    #[error("host may not be set for unix domain sockets")]
+    HostSetForUnixSocket(),
+
+    #[error("path may not be set")]
+    PathMayNotBeSet(),
+
+    #[error("transport error: {0}")]
+    TransportError(tonic::transport::Error),
+}
+
+impl From<tonic::transport::Error> for Error {
+    fn from(value: tonic::transport::Error) -> Self {
+        Self::TransportError(value)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::channel_from_url;
+    use test_case::test_case;
+    use url::Url;
+
+    /// Correct scheme to connect to a unix socket.
+    #[test_case("grpc+unix:///path/to/somewhere", true; "grpc valid unix socket")]
+    /// Connecting with wait-connect set to 0 succeeds, as that's the default.
+    #[test_case("grpc+unix:///path/to/somewhere?wait-connect=0", true; "grpc valid unix wait-connect=0")]
+    /// Connecting with wait-connect set to 1 fails, as the path doesn't exist.
+    #[test_case("grpc+unix:///path/to/somewhere?wait-connect=1", false; "grpc valid unix wait-connect=1")]
+    /// Correct scheme for unix socket, but setting a host too, which is invalid.
+    #[test_case("grpc+unix://host.example/path/to/somewhere", false; "grpc invalid unix socket and host")]
+    /// Correct scheme to connect to localhost, with port 12345
+    #[test_case("grpc+http://[::1]:12345", true; "grpc valid IPv6 localhost port 12345")]
+    /// Correct scheme to connect to localhost over http, without specifying a port.
+    #[test_case("grpc+http://localhost", true; "grpc valid http host without port")]
+    /// Correct scheme to connect to localhost over http, without specifying a port.
+    #[test_case("grpc+https://localhost", true; "grpc valid https host without port")]
+    /// Correct scheme to connect to localhost over http, but with additional path, which is invalid.
+    #[test_case("grpc+http://localhost/some-path", false; "grpc valid invalid host and path")]
+    /// Connecting with wait-connect set to 0 succeeds, as that's the default.
+    #[test_case("grpc+http://localhost?wait-connect=0", true; "grpc valid host wait-connect=0")]
+    /// Connecting with wait-connect set to 1 fails, as the host doesn't exist.
+    #[test_case("grpc+http://nonexist.invalid?wait-connect=1", false; "grpc valid host wait-connect=1")]
+    #[tokio::test]
+    async fn test_from_addr_tokio(uri_str: &str, is_ok: bool) {
+        let url = Url::parse(uri_str).expect("must parse");
+        assert_eq!(channel_from_url(&url).await.is_ok(), is_ok)
+    }
+}