handle unexpected changes to unknown block

An unknown block represents a dynamic configuration block with an
unknown for_each value. We were not catching the case where a provider
modified this value unexpectedly, which would crash with block of type
NestingList blocks where the config value has no length for comparison.

internal/plans/objchange/plan_valid.go

@@ -69,7 +69,20 @@ func assertPlanValid(schema *configschema.Block, priorState, config, plannedStat
 			// Easy path: nothing has changed at all
 			continue
 		}
+
+		if !configV.IsKnown() {
+			// An unknown config block represents a dynamic block where the
+			// for_each value is unknown, and therefor cannot be altered by the
+			// provider.
+			errs = append(errs, path.NewErrorf("planned value %#v for unknown dynamic block", plannedV))
+			continue
+		}
+
 		if !plannedV.IsKnown() {
+			// Only dynamic configuration can set blocks to unknown, so this is
+			// not allowed from the provider. This means that either the config
+			// and plan should match, or we have an error where the plan
+			// changed the config value, both of which have been checked.
 			errs = append(errs, path.NewErrorf("attribute representing nested block must not be unknown itself; set nested attribute values to unknown instead"))
 			continue
 		}
@@ -94,6 +107,7 @@ func assertPlanValid(schema *configschema.Block, priorState, config, plannedStat
 				errs = append(errs, path.NewErrorf("block count in plan (%d) disagrees with count in config (%d)", plannedL, configL))
 				continue
 			}
+
 			for it := plannedV.ElementIterator(); it.Next(); {
 				idx, plannedEV := it.Element()
 				path := append(path, cty.IndexStep{Key: idx})

internal/plans/objchange/plan_valid_test.go

@@ -388,6 +388,110 @@ func TestAssertPlanValid(t *testing.T) {
 				`.b: attribute representing a list of nested blocks must be empty to indicate no blocks, not null`,
 			},
 		},
+
+		// blocks can be unknown when using dynamic
+		"nested list, unknown nested dynamic": {
+			&configschema.Block{
+				BlockTypes: map[string]*configschema.NestedBlock{
+					"a": {
+						Nesting: configschema.NestingList,
+						Block: configschema.Block{
+							BlockTypes: map[string]*configschema.NestedBlock{
+								"b": {
+									Nesting: configschema.NestingList,
+									Block: configschema.Block{
+										Attributes: map[string]*configschema.Attribute{
+											"c": {
+												Type:     cty.String,
+												Optional: true,
+											},
+											"computed": {
+												Type:     cty.String,
+												Computed: true,
+											},
+										},
+									},
+								},
+							},
+						},
+					},
+				},
+			},
+
+			cty.ObjectVal(map[string]cty.Value{
+				"a": cty.ListVal([]cty.Value{cty.ObjectVal(map[string]cty.Value{
+					"computed": cty.NullVal(cty.String),
+					"b": cty.ListVal([]cty.Value{cty.ObjectVal(map[string]cty.Value{
+						"c": cty.StringVal("x"),
+					})}),
+				})}),
+			}),
+			cty.ObjectVal(map[string]cty.Value{
+				"a": cty.ListVal([]cty.Value{cty.ObjectVal(map[string]cty.Value{
+					"b": cty.UnknownVal(cty.List(cty.Object(map[string]cty.Type{
+						"c":        cty.String,
+						"computed": cty.String,
+					}))),
+				})}),
+			}),
+			cty.ObjectVal(map[string]cty.Value{
+				"a": cty.ListVal([]cty.Value{cty.ObjectVal(map[string]cty.Value{
+					"b": cty.UnknownVal(cty.List(cty.Object(map[string]cty.Type{
+						"c":        cty.String,
+						"computed": cty.String,
+					}))),
+				})}),
+			}),
+			[]string{},
+		},
+
+		"nested set, unknown dynamic cannot be planned": {
+			&configschema.Block{
+				Attributes: map[string]*configschema.Attribute{
+					"computed": {
+						Type:     cty.String,
+						Computed: true,
+					},
+				},
+				BlockTypes: map[string]*configschema.NestedBlock{
+					"b": {
+						Nesting: configschema.NestingSet,
+						Block: configschema.Block{
+							Attributes: map[string]*configschema.Attribute{
+								"c": {
+									Type:     cty.String,
+									Optional: true,
+								},
+							},
+						},
+					},
+				},
+			},
+
+			cty.ObjectVal(map[string]cty.Value{
+				"computed": cty.NullVal(cty.String),
+				"b": cty.SetVal([]cty.Value{cty.ObjectVal(map[string]cty.Value{
+					"c": cty.StringVal("x"),
+				})}),
+			}),
+			cty.ObjectVal(map[string]cty.Value{
+				"computed": cty.NullVal(cty.String),
+				"b": cty.UnknownVal(cty.Set(cty.Object(map[string]cty.Type{
+					"c": cty.String,
+				}))),
+			}),
+			cty.ObjectVal(map[string]cty.Value{
+				"computed": cty.StringVal("default"),
+				"b": cty.SetVal([]cty.Value{cty.ObjectVal(map[string]cty.Value{
+					"c": cty.StringVal("oops"),
+				})}),
+			}),
+
+			[]string{
+				`.b: planned value cty.SetVal([]cty.Value{cty.ObjectVal(map[string]cty.Value{"c":cty.StringVal("oops")})}) for unknown dynamic block`,
+			},
+		},
+
 		"nested set, null in plan": {
 			&configschema.Block{
 				BlockTypes: map[string]*configschema.NestedBlock{