Fix flex checksum nil dereference issue (#2964)

* fix nil deref bug in checksum setter

* add changelog

* update changelog and code comment

* update comment

Author: wty-Bryant

.changelog/2e03eae07dff4beb94963939b6549426.json

@@ -0,0 +1,8 @@
+{
+    "id": "2e03eae0-7dff-4beb-9496-3939b6549426",
+    "type": "bugfix",
+    "description": "Fix nil dereference panic for operations that require checksums, but do not have an input setting for which algorithm to use.",
+    "modules": [
+        "service/internal/checksum"
+    ]
+}

service/internal/checksum/middleware_setup_context.go

@@ -46,9 +46,12 @@ func (m *setupInputContext) HandleInitialize(
 ) (
 	out middleware.InitializeOutput, metadata middleware.Metadata, err error,
 ) {
-	if algorithm, ok := m.GetAlgorithm(in.Parameters); ok {
-		ctx = internalcontext.SetChecksumInputAlgorithm(ctx, algorithm)
-		return next.HandleInitialize(ctx, in)
+	// nil check here is for operations that require checksum but do not have input algorithm setting
+	if m.GetAlgorithm != nil {
+		if algorithm, ok := m.GetAlgorithm(in.Parameters); ok {
+			ctx = internalcontext.SetChecksumInputAlgorithm(ctx, algorithm)
+			return next.HandleInitialize(ctx, in)
+		}
 	}
 
 	if m.RequireChecksum || m.RequestChecksumCalculation == aws.RequestChecksumCalculationWhenSupported {