[compute_ctl] Make role deletion spec processing idempotent (#3380)

Previously, we were trying to re-assign owned objects of the already
deleted role. This were causing a crash loop in the case when compute
was restarted with a spec that includes delta operation for role
deletion. To avoid such cases, check that role is still present before
calling `reassign_owned_objects`.

Resolves neondatabase/cloud#3553

compute_tools/src/compute.rs

@@ -252,7 +252,7 @@ impl ComputeNode {
         // If connection fails,
         // it may be the old node with `zenith_admin` superuser.
         //
-        // In this case we need to connect with old `zenith_admin`name
+        // In this case we need to connect with old `zenith_admin` name
         // and create new user. We cannot simply rename connected user,
         // but we can create a new one and grant it all privileges.
         let mut client = match Client::connect(self.connstr.as_str(), NoTls) {
@@ -278,6 +278,7 @@ impl ComputeNode {
             Ok(client) => client,
         };
 
+        // Proceed with post-startup configuration. Note, that order of operations is important.
         handle_roles(&self.spec, &mut client)?;
         handle_databases(&self.spec, &mut client)?;
         handle_role_deletions(self, &mut client)?;

compute_tools/src/pg_helpers.rs

@@ -130,8 +130,8 @@ impl Role {
     /// Serialize a list of role parameters into a Postgres-acceptable
     /// string of arguments.
     pub fn to_pg_options(&self) -> String {
-        // XXX: consider putting LOGIN as a default option somewhere higher, e.g. in Rails.
-        // For now we do not use generic `options` for roles. Once used, add
+        // XXX: consider putting LOGIN as a default option somewhere higher, e.g. in control-plane.
+        // For now, we do not use generic `options` for roles. Once used, add
         // `self.options.as_pg_options()` somewhere here.
         let mut params: String = "LOGIN".to_string();
 

compute_tools/src/spec.rs

@@ -213,8 +213,20 @@ pub fn handle_role_deletions(node: &ComputeNode, client: &mut Client) -> Result<
     if let Some(ops) = &node.spec.delta_operations {
         // First, reassign all dependent objects to db owners.
         info!("reassigning dependent objects of to-be-deleted roles");
+
+        // Fetch existing roles. We could've exported and used `existing_roles` from
+        // `handle_roles()`, but we only make this list there before creating new roles.
+        // Which is probably fine as we never create to-be-deleted roles, but that'd
+        // just look a bit untidy. Anyway, the entire `pg_roles` should be in shared
+        // buffers already, so this shouldn't be a big deal.
+        let mut xact = client.transaction()?;
+        let existing_roles: Vec<Role> = get_existing_roles(&mut xact)?;
+        xact.commit()?;
+
         for op in ops {
-            if op.action == "delete_role" {
+            // Check that role is still present in Postgres, as this could be a
+            // restart with the same spec after role deletion.
+            if op.action == "delete_role" && existing_roles.iter().any(|r| r.name == op.name) {
                 reassign_owned_objects(node, &op.name)?;
             }
         }