distsql: reduce allocations in setupFlow

Previously, the variable `evalCtx *eval.Context` escaped to the
heap because it was captured by an `onFlowCleanupEnd` closure in one
branch of a conditional and modified in another branch. Go's escape
analysis cannot determine that the capture and modification are mutually
exclusive, so the pointer is heap allocated. This commit eliminates the
allocation by using a new `*eval.Context` variable in the closure that
is never modified.

Release note: None

pkg/sql/distsql/server.go

@@ -269,13 +269,20 @@ func (ds *ServerImpl) setupFlow(
 		// this allows us to avoid an unnecessary deserialization of the eval
 		// context proto.
 		evalCtx = localState.EvalContext
+		// We create an eval context variable scoped to this block and reference
+		// it in the onFlowCleanupEnd closure. If the closure referenced
+		// evalCtx, then the pointer would be heap allocated because it is
+		// modified in the other branch of the conditional, and Go's escape
+		// analysis cannot determine that the capture and modification are
+		// mutually exclusive.
+		localEvalCtx := evalCtx
 		// We're about to mutate the evalCtx and we want to restore its original
 		// state once the flow cleans up. Note that we could have made a copy of
 		// the whole evalContext, but that isn't free, so we choose to restore
 		// the original state in order to avoid performance regressions.
-		origTxn := evalCtx.Txn
+		origTxn := localEvalCtx.Txn
 		onFlowCleanupEnd = func() {
-			evalCtx.Txn = origTxn
+			localEvalCtx.Txn = origTxn
 			reserved.Close(ctx)
 		}
 		if localState.MustUseLeafTxn() {
@@ -286,7 +293,7 @@ func (ds *ServerImpl) setupFlow(
 			}
 			// Update the Txn field early (before f.SetTxn() below) since some
 			// processors capture the field in their constructor (see #41992).
-			evalCtx.Txn = leafTxn
+			localEvalCtx.Txn = leafTxn
 		}
 	} else {
 		onFlowCleanupEnd = func() {