sql: Fix race condition in the query cache

The PreparedMetadata was being put into the cache, but then the
InferredTypes field was being set after that by execPrepare. The fix
is to make a copy of PreparedMetadata before entering it into the cache.

Release note: None

pkg/sql/plan_opt.go

@@ -110,13 +110,17 @@ func (p *planner) prepareUsingOptimizer(
 	if opc.allowMemoReuse {
 		stmt.Prepared.Memo = memo
 		if opc.useCache {
+			// execPrepare sets the PrepareMetadata.InferredTypes field after this
+			// point. However, once the PrepareMetadata goes into the cache, it
+			// can't be modified without causing race conditions. So make a copy of
+			// it now.
+			// TODO(radu): Determine if the extra object allocation is really
+			// necessary.
+			pm := stmt.Prepared.PrepareMetadata
 			cachedData := querycache.CachedData{
-				SQL:  stmt.SQL,
-				Memo: memo,
-				// We rely on stmt.Prepared.PrepareMetadata not being subsequently modified.
-				// TODO(radu): this also holds on to the memory referenced by other
-				// PreparedStatement fields.
-				PrepareMetadata: &stmt.Prepared.PrepareMetadata,
+				SQL:             stmt.SQL,
+				Memo:            memo,
+				PrepareMetadata: &pm,
 			}
 			p.execCfg.QueryCache.Add(&p.queryCacheSession, &cachedData)
 		}