Make server able to run old or new binary so that we can compare the actix version with this one

backend/test_utils/src/workspace/start_server.rs

@@ -1,30 +1,45 @@
 use super::commands;
 use crate::server::{self, Server, Command};
 
-/// Start a telemetry core server. We'll use `cargo run` by default, to ensure that
-/// the code we run is uptodate, but you can also provide env vars to configure the binary
-/// that runs for the shard and core process:
+/// Start a telemetry server. We'll use `cargo run` by default, but you can also provide
+/// env vars to configure the binary that runs for the shard and core process. Either:
 ///
-/// TELEMETRY_SHARD_BIN - path to telemetry_shard binary
-/// TELEMETRY_CORE_BIN - path to telemetry_core binary
-async fn start_server(release_mode: bool) -> Server {
+/// - `TELEMETRY_BIN` - path to the telemetry binary (which can function as shard _and_ core)
+///
+/// Or alternately neither/one/both of:
+///
+/// - `TELEMETRY_SHARD_BIN` - path to telemetry_shard binary
+/// - `TELEMETRY_CORE_BIN` - path to telemetry_core binary
+///
+/// Whatever is not provided will be substituted with a `cargo run` variant instead.
+pub async fn start_server(release_mode: bool) -> Server {
+
+    if let Ok(bin) = std::env::var("TELEMETRY_BIN") {
+        return Server::start(server::StartOpts::SingleProcess {
+            command: Command::new(bin)
+        }).await.unwrap();
+    }
+
     let shard_command = std::env::var("TELEMETRY_SHARD_BIN")
         .map(|val| Command::new(val))
-        .unwrap_or_else(|_| commands::cargo_run_telemetry_shard(release_mode).expect("valid shard command"));
+        .unwrap_or_else(|_| commands::cargo_run_telemetry_shard(release_mode).expect("must be in rust workspace to run shard command"));
 
     let core_command = std::env::var("TELEMETRY_CORE_BIN")
         .map(|val| Command::new(val))
-        .unwrap_or_else(|_| commands::cargo_run_telemetry_core(release_mode).expect("valid core command"));
+        .unwrap_or_else(|_| commands::cargo_run_telemetry_core(release_mode).expect("must be in rust workspace to run core command"));
 
-    Server::start(server::StartOpts { shard_command, core_command }).await.unwrap()
+    Server::start(server::StartOpts::ShardAndCore {
+        shard_command,
+        core_command
+    }).await.unwrap()
 }
 
-/// Start a telemetry server using debug builds for compile speed
+/// Start a telemetry core server in debug mode. see [`start_server`] for details.
 pub async fn start_server_debug() -> Server {
     start_server(false).await
 }
 
-/// Start a telemetry server using release builds for performance accuracy
+/// Start a telemetry core server in release mode. see [`start_server`] for details.
 pub async fn start_server_release() -> Server {
     start_server(true).await
 }