storage_controller: wait for db on startup (#7479)

## Problem

In some dev/test environments, there aren't health checks to guarantee
the database is available before starting the controller. This creates
friction for the developer.

## Summary of changes

- Wait up to 5 seconds for the database to become available on startup

storage_controller/src/main.rs

@@ -5,6 +5,7 @@ use diesel::Connection;
 use metrics::launch_timestamp::LaunchTimestamp;
 use metrics::BuildInfo;
 use std::sync::Arc;
+use std::time::Duration;
 use storage_controller::http::make_router;
 use storage_controller::metrics::preinitialize_metrics;
 use storage_controller::persistence::Persistence;
@@ -245,6 +246,8 @@ async fn async_main() -> anyhow::Result<()> {
     };
 
     // After loading secrets & config, but before starting anything else, apply database migrations
+    Persistence::await_connection(&secrets.database_url, Duration::from_secs(5)).await?;
+
     migration_run(&secrets.database_url)
         .await
         .context("Running database migrations")?;

storage_controller/src/persistence.rs

@@ -2,6 +2,7 @@ pub(crate) mod split_state;
 use std::collections::HashMap;
 use std::str::FromStr;
 use std::time::Duration;
+use std::time::Instant;
 
 use self::split_state::SplitState;
 use camino::Utf8Path;
@@ -144,6 +145,31 @@ impl Persistence {
         }
     }
 
+    /// A helper for use during startup, where we would like to tolerate concurrent restarts of the
+    /// database and the storage controller, therefore the database might not be available right away
+    pub async fn await_connection(
+        database_url: &str,
+        timeout: Duration,
+    ) -> Result<(), diesel::ConnectionError> {
+        let started_at = Instant::now();
+        loop {
+            match PgConnection::establish(database_url) {
+                Ok(_) => {
+                    tracing::info!("Connected to database.");
+                    return Ok(());
+                }
+                Err(e) => {
+                    if started_at.elapsed() > timeout {
+                        return Err(e);
+                    } else {
+                        tracing::info!("Database not yet available, waiting... ({e})");
+                        tokio::time::sleep(Duration::from_millis(100)).await;
+                    }
+                }
+            }
+        }
+    }
+
     /// Wraps `with_conn` in order to collect latency and error metrics
     async fn with_measured_conn<F, R>(&self, op: DatabaseOperation, func: F) -> DatabaseResult<R>
     where