Add retry to fetching basebackup (#6537)

## Problem
Currently we have no retry mechanism for fetching basebackup. If there's
an unstable connection, starting compute will just fail.

## Summary of changes
Adds an exponential backoff with 7 retries to get the basebackup.

compute_tools/src/compute.rs

@@ -319,7 +319,7 @@ impl ComputeNode {
     // Get basebackup from the libpq connection to pageserver using `connstr` and
     // unarchive it to `pgdata` directory overriding all its previous content.
     #[instrument(skip_all, fields(%lsn))]
-    fn get_basebackup(&self, compute_state: &ComputeState, lsn: Lsn) -> Result<()> {
+    fn try_get_basebackup(&self, compute_state: &ComputeState, lsn: Lsn) -> Result<()> {
         let spec = compute_state.pspec.as_ref().expect("spec must be set");
         let start_time = Instant::now();
 
@@ -390,6 +390,34 @@ impl ComputeNode {
         Ok(())
     }
 
+    // Gets the basebackup in a retry loop
+    #[instrument(skip_all, fields(%lsn))]
+    pub fn get_basebackup(&self, compute_state: &ComputeState, lsn: Lsn) -> Result<()> {
+        let mut retry_period_ms = 500;
+        let mut attempts = 0;
+        let max_attempts = 5;
+        loop {
+            let result = self.try_get_basebackup(compute_state, lsn);
+            match result {
+                Ok(_) => {
+                    return result;
+                }
+                Err(ref e) if attempts < max_attempts => {
+                    warn!(
+                        "Failed to get basebackup: {} (attempt {}/{})",
+                        e, attempts, max_attempts
+                    );
+                    std::thread::sleep(std::time::Duration::from_millis(retry_period_ms));
+                    retry_period_ms *= 2;
+                }
+                Err(_) => {
+                    return result;
+                }
+            }
+            attempts += 1;
+        }
+    }
+
     pub async fn check_safekeepers_synced_async(
         &self,
         compute_state: &ComputeState,

test_runner/regress/test_bad_connection.py

@@ -9,14 +9,14 @@ def test_compute_pageserver_connection_stress(neon_env_builder: NeonEnvBuilder):
     env = neon_env_builder.init_start()
     env.pageserver.allowed_errors.append(".*simulated connection error.*")
 
+    # Enable failpoint before starting everything else up so that we exercise the retry
+    # on fetching basebackup
     pageserver_http = env.pageserver.http_client()
+    pageserver_http.configure_failpoints(("simulated-bad-compute-connection", "50%return(15)"))
+
     env.neon_cli.create_branch("test_compute_pageserver_connection_stress")
     endpoint = env.endpoints.create_start("test_compute_pageserver_connection_stress")
 
-    # Enable failpoint after starting everything else up so that loading initial
-    # basebackup doesn't fail
-    pageserver_http.configure_failpoints(("simulated-bad-compute-connection", "50%return(15)"))
-
     pg_conn = endpoint.connect()
     cur = pg_conn.cursor()