set default max_lifetime in pool::Options to 30 minutes and explain why

sqlx-core/src/cache.rs

@@ -2,6 +2,14 @@ use std::collections::hash_map::{HashMap, Entry};
 use std::cmp::Ordering;
 use futures_core::Future;
 
+// TODO: figure out a cache eviction strategy
+// we currently naively cache all prepared statements which could live-leak memory
+// on both the client and server if the user is synthesizing queries that are different each time
+
+// We put an upper bound on this by setting a default max connection lifetime in pool::Options but
+// that's only a band-aid
+
+/// Per-connection prepared statement cache.
 pub struct StatementCache<Id> {
     statements: HashMap<String, Id>
 }

sqlx-core/src/pool/options.rs

@@ -68,7 +68,10 @@ impl Default for Options {
             max_size: 10,
             min_idle: 0,
             connect_timeout: Duration::from_secs(30),
-            max_lifetime: None,
+            // 30 minutes
+            // prevents unbounded live-leaking of memory due to naive prepared statement caching
+            // see src/cache.rs for context
+            max_lifetime: Some(Duration::from_secs(1800)),
             idle_timeout: None,
         }
     }