Use Vec1 for list of sharded sql connections

Summary: We always have at least one connection, so let's guarantee it on compile time. Some places did manual checks on runtime, now that's not necessary anymore!

Reviewed By: liubov-dmitrieva

Differential Revision: D40637483

fbshipit-source-id: 057f6464a016b6c2fa196e6b83db6e7cbc05756d

Author: yancouto

shed/sql/common/Cargo.toml

@@ -26,6 +26,7 @@ stats = { version = "0.1.0", path = "../../stats" }
 thiserror = "1.0.36"
 time_ext = { version = "0.1.0", path = "../../time_ext" }
 tokio_shim = { version = "0.1.0", path = "../../tokio_shim" }
+vec1 = { version = "1", features = ["serde"] }
 
 [dev-dependencies]
 sql = { version = "0.1.0", path = ".." }

shed/sql/common/lib.rs

@@ -26,6 +26,7 @@ use anyhow::bail;
 use anyhow::format_err;
 use anyhow::Context;
 use anyhow::Error;
+use vec1::Vec1;
 
 // Used in docs
 #[cfg(test)]
@@ -114,35 +115,32 @@ impl From<SqlConnectionsWithSchema> for SqlConnections {
 #[derive(Clone)]
 pub struct SqlShardedConnections {
     /// Write connections to the master for each shard
-    pub write_connections: Vec<Connection>,
+    pub write_connections: Vec1<Connection>,
     /// Read connections for each shard
-    pub read_connections: Vec<Connection>,
+    pub read_connections: Vec1<Connection>,
     /// Read master connections for each shard
-    pub read_master_connections: Vec<Connection>,
+    pub read_master_connections: Vec1<Connection>,
 }
 
-impl SqlShardedConnections {
-    /// Check if the struct is empty.
-    pub fn is_empty(&self) -> bool {
-        self.write_connections.is_empty()
-    }
-}
-
-impl From<Vec<SqlConnections>> for SqlShardedConnections {
-    fn from(shard_connections: Vec<SqlConnections>) -> Self {
-        let mut write_connections = Vec::with_capacity(shard_connections.len());
-        let mut read_connections = Vec::with_capacity(shard_connections.len());
-        let mut read_master_connections = Vec::with_capacity(shard_connections.len());
-        for connections in shard_connections.into_iter() {
+impl From<Vec1<SqlConnections>> for SqlShardedConnections {
+    fn from(shard_connections: Vec1<SqlConnections>) -> Self {
+        let (head, last) = shard_connections.split_off_last();
+        let mut write_connections = Vec::with_capacity(head.len());
+        let mut read_connections = Vec::with_capacity(head.len());
+        let mut read_master_connections = Vec::with_capacity(head.len());
+        for connections in head {
             write_connections.push(connections.write_connection);
             read_connections.push(connections.read_connection);
             read_master_connections.push(connections.read_master_connection);
         }
 
         Self {
-            read_connections,
-            read_master_connections,
-            write_connections,
+            read_connections: Vec1::from_vec_push(read_connections, last.read_connection),
+            read_master_connections: Vec1::from_vec_push(
+                read_master_connections,
+                last.read_master_connection,
+            ),
+            write_connections: Vec1::from_vec_push(write_connections, last.write_connection),
         }
     }
 }