fix potential nil deref in waiter path matcher (#563)

Author: lucix-aws

codegen/smithy-go-codegen/src/main/java/software/amazon/smithy/go/codegen/GoJmespathExpressionGenerator.java

@@ -16,6 +16,7 @@
 package software.amazon.smithy.go.codegen;
 
 import static software.amazon.smithy.go.codegen.GoWriter.goTemplate;
+import static software.amazon.smithy.go.codegen.SymbolUtils.isNilable;
 import static software.amazon.smithy.go.codegen.SymbolUtils.isPointable;
 import static software.amazon.smithy.go.codegen.SymbolUtils.sliceOf;
 import static software.amazon.smithy.go.codegen.util.ShapeUtil.BOOL_SHAPE;
@@ -280,7 +281,21 @@ private Variable visitProjection(ProjectionExpression expr, Variable current) {
 
     private Variable visitSub(Subexpression expr, Variable current) {
         var left = visit(expr.getLeft(), current);
-        return visit(expr.getRight(), left);
+        if (!isNilable(left.type)) {
+            return visit(expr.getRight(), left);
+        }
+
+        var lookahead = new GoJmespathExpressionGenerator(ctx, new GoWriter(""))
+                .generate(expr.getRight(), left);
+        var ident = nextIdent();
+        writer.write("var $L $P", ident, lookahead.type);
+        writer.write("if $L != nil {", left.ident);
+        writer.indent();
+        var inner = visit(expr.getRight(), left);
+        writer.write("$L = $L", ident, inner.ident);
+        writer.dedent();
+        writer.write("}");
+        return new Variable(inner.shape, ident, inner.type);
     }
 
     private Variable visitField(FieldExpression expr, Variable current) {

codegen/smithy-go-codegen/src/test/java/software/amazon/smithy/go/codegen/GoJmespathExpressionGeneratorTest.java

@@ -124,7 +124,11 @@ public void testSubexpression() {
         assertThat(actual.ident(), Matchers.equalTo("v2"));
         assertThat(writer.toString(), Matchers.containsString("""
                 v1 := input.Nested
-                v2 := v1.NestedField
+                var v2 *string
+                if v1 != nil {
+                    v3 := v1.NestedField
+                    v2 = v3
+                }
                 """));
     }
 
@@ -304,14 +308,18 @@ public void testComparatorStringLHSNil() {
                 "input"
         ));
         assertThat(actual.shape(), Matchers.equalTo(ShapeUtil.BOOL_SHAPE));
-        assertThat(actual.ident(), Matchers.equalTo("v4"));
+        assertThat(actual.ident(), Matchers.equalTo("v5"));
         assertThat(writer.toString(), Matchers.containsString("""
                 v1 := input.Nested
-                v2 := v1.NestedField
-                v3 := "foo"
-                var v4 bool
+                var v2 *string
+                if v1 != nil {
+                    v3 := v1.NestedField
+                    v2 = v3
+                }
+                v4 := "foo"
+                var v5 bool
                 if v2 != nil   {
-                    v4 = string(*v2) == string(v3)
+                    v5 = string(*v2) == string(v4)
                 }
                 """));
     }
@@ -327,14 +335,18 @@ public void testComparatorStringRHSNil() {
                 "input"
         ));
         assertThat(actual.shape(), Matchers.equalTo(ShapeUtil.BOOL_SHAPE));
-        assertThat(actual.ident(), Matchers.equalTo("v4"));
+        assertThat(actual.ident(), Matchers.equalTo("v5"));
         assertThat(writer.toString(), Matchers.containsString("""
                 v1 := "foo"
                 v2 := input.Nested
-                v3 := v2.NestedField
-                var v4 bool
+                var v3 *string
+                if v2 != nil {
+                    v4 := v2.NestedField
+                    v3 = v4
+                }
+                var v5 bool
                 if   v3 != nil {
-                    v4 = string(v1) == string(*v3)
+                    v5 = string(v1) == string(*v3)
                 }
                 """));
     }
@@ -350,14 +362,18 @@ public void testComparatorStringBothNil() {
                 "input"
         ));
         assertThat(actual.shape(), Matchers.equalTo(ShapeUtil.BOOL_SHAPE));
-        assertThat(actual.ident(), Matchers.equalTo("v4"));
+        assertThat(actual.ident(), Matchers.equalTo("v5"));
         assertThat(writer.toString(), Matchers.containsString("""
                 v1 := input.Nested
-                v2 := v1.NestedField
-                v3 := input.SimpleShape
-                var v4 bool
-                if v2 != nil && v3 != nil {
-                    v4 = string(*v2) == string(*v3)
+                var v2 *string
+                if v1 != nil {
+                    v3 := v1.NestedField
+                    v2 = v3
+                }
+                v4 := input.SimpleShape
+                var v5 bool
+                if v2 != nil && v4 != nil {
+                    v5 = string(*v2) == string(*v4)
                 }
                 """));
     }