store: skip chains in pool_size = 0 shards instead of panicking at startup (#6195)

store/postgres/src/block_store.rs

@@ -253,7 +253,27 @@ impl BlockStore {
         const CHAIN_HEAD_CACHE_TTL: Duration = Duration::from_secs(2);
 
         let mirror = PrimaryMirror::new(&pools);
-        let existing_chains = mirror.read_async(primary::load_chains).await?;
+        // Treat a chain whose shard has no connection pool (i.e. a shard
+        // configured with `pool_size = 0`, which `store_builder` drops) as if it
+        // doesn't exist: skip it here so we neither create a chain store for it
+        // nor fail startup. See issue #6195.
+        let existing_chains: Vec<_> = mirror
+            .read_async(primary::load_chains)
+            .await?
+            .into_iter()
+            .filter(|chain| {
+                let has_pool = pools.contains_key(&chain.shard);
+                if !has_pool {
+                    warn!(
+                        &logger,
+                        "Ignoring chain `{}`: its shard `{}` has no connection pool (pool_size = 0)",
+                        chain.name,
+                        chain.shard,
+                    );
+                }
+                has_pool
+            })
+            .collect();
         let chain_head_cache = TimedCache::new(CHAIN_HEAD_CACHE_TTL);
         let chains = shards.clone();