objchange: fix ProposedNew from null objects

The codepath for AllAttributesNull was not correct for any nested object
types with collections, and should create single null values for the
correct NestingMode rather than a single object with null attributes.

Since there is no reason to descend into nested object types to create
nullv alues, we can drop the AllAttributesNull function altogether and
create null values as needed during ProposedNew.

The corresponding AllBlockAttributesNull was only called internally in 1
location, and simply delegated to schema.EmptyValue. We can reduce the
package surface area by dropping that function too and calling
EmptyValue directly.

internal/plans/objchange/objchange.go

@@ -37,7 +37,10 @@ func ProposedNew(schema *configschema.Block, prior, config cty.Value) cty.Value
 		// similar to the result of decoding an empty configuration block,
 		// which simplifies our handling of the top-level attributes/blocks
 		// below by giving us one non-null level of object to pull values from.
-		prior = AllBlockAttributesNull(schema)
+		//
+		// "All attributes null" happens to be the definition of EmptyValue for
+		// a Block, so we can just delegate to that
+		prior = schema.EmptyValue()
 	}
 	return proposedNew(schema, prior, config)
 }
@@ -258,12 +261,15 @@ func proposedNewNestedBlock(schema *configschema.NestedBlock, prior, config cty.
 }
 
 func proposedNewAttributes(attrs map[string]*configschema.Attribute, prior, config cty.Value) map[string]cty.Value {
-	if prior.IsNull() {
-		prior = AllAttributesNull(attrs)
-	}
 	newAttrs := make(map[string]cty.Value, len(attrs))
 	for name, attr := range attrs {
-		priorV := prior.GetAttr(name)
+		var priorV cty.Value
+		if prior.IsNull() {
+			priorV = cty.NullVal(prior.Type().AttributeType(name))
+		} else {
+			priorV = prior.GetAttr(name)
+		}
+
 		configV := config.GetAttr(name)
 		var newV cty.Value
 		switch {

internal/plans/objchange/objchange_test.go

@@ -1536,6 +1536,110 @@ func TestProposedNew(t *testing.T) {
 				}),
 			}),
 		},
+		"prior null nested objects": {
+			&configschema.Block{
+				Attributes: map[string]*configschema.Attribute{
+					"single": {
+						NestedType: &configschema.Object{
+							Nesting: configschema.NestingSingle,
+							Attributes: map[string]*configschema.Attribute{
+								"list": {
+									NestedType: &configschema.Object{
+										Nesting: configschema.NestingList,
+										Attributes: map[string]*configschema.Attribute{
+											"foo": {
+												Type: cty.String,
+											},
+										},
+									},
+									Optional: true,
+								},
+							},
+						},
+						Optional: true,
+					},
+					"map": {
+						NestedType: &configschema.Object{
+							Nesting: configschema.NestingMap,
+							Attributes: map[string]*configschema.Attribute{
+								"map": {
+									NestedType: &configschema.Object{
+										Nesting: configschema.NestingList,
+										Attributes: map[string]*configschema.Attribute{
+											"foo": {
+												Type: cty.String,
+											},
+										},
+									},
+									Optional: true,
+								},
+							},
+						},
+						Optional: true,
+					},
+				},
+			},
+			cty.NullVal(cty.Object(map[string]cty.Type{
+				"single": cty.Object(map[string]cty.Type{
+					"list": cty.List(cty.Object(map[string]cty.Type{
+						"foo": cty.String,
+					})),
+				}),
+				"map": cty.Map(cty.Object(map[string]cty.Type{
+					"list": cty.List(cty.Object(map[string]cty.Type{
+						"foo": cty.String,
+					})),
+				})),
+			})),
+			cty.ObjectVal(map[string]cty.Value{
+				"single": cty.ObjectVal(map[string]cty.Value{
+					"list": cty.ListVal([]cty.Value{
+						cty.ObjectVal(map[string]cty.Value{
+							"foo": cty.StringVal("a"),
+						}),
+						cty.ObjectVal(map[string]cty.Value{
+							"foo": cty.StringVal("b"),
+						}),
+					}),
+				}),
+				"map": cty.MapVal(map[string]cty.Value{
+					"one": cty.ObjectVal(map[string]cty.Value{
+						"list": cty.ListVal([]cty.Value{
+							cty.ObjectVal(map[string]cty.Value{
+								"foo": cty.StringVal("a"),
+							}),
+							cty.ObjectVal(map[string]cty.Value{
+								"foo": cty.StringVal("b"),
+							}),
+						}),
+					}),
+				}),
+			}),
+			cty.ObjectVal(map[string]cty.Value{
+				"single": cty.ObjectVal(map[string]cty.Value{
+					"list": cty.ListVal([]cty.Value{
+						cty.ObjectVal(map[string]cty.Value{
+							"foo": cty.StringVal("a"),
+						}),
+						cty.ObjectVal(map[string]cty.Value{
+							"foo": cty.StringVal("b"),
+						}),
+					}),
+				}),
+				"map": cty.MapVal(map[string]cty.Value{
+					"one": cty.ObjectVal(map[string]cty.Value{
+						"list": cty.ListVal([]cty.Value{
+							cty.ObjectVal(map[string]cty.Value{
+								"foo": cty.StringVal("a"),
+							}),
+							cty.ObjectVal(map[string]cty.Value{
+								"foo": cty.StringVal("b"),
+							}),
+						}),
+					}),
+				}),
+			}),
+		},
 	}
 
 	for name, test := range tests {