Prevent RESET_BLOCKS from affecting reusable blocks

Changes the editor reducer so that RESET_BLOCKS will only remove blocks
that are actually in the post, i.e. blocks that are a descendent of the
`''` `blocks.order` key.

This fixes a bug where editing a post in Text Mode would break any
reusable blocks in the post.

packages/editor/src/store/reducer.js

@@ -101,6 +101,31 @@ function getFlattenedBlocks( blocks ) {
 	return flattenedBlocks;
 }
 
+/**
+ * Given a block order map object, returns *all* of the block client IDs that are
+ * a descendant of the given root client ID.
+ *
+ * Calling this with `rootClientId` set to `''` results in a list of client IDs
+ * that are in the post. That is, it excludes blocks like fetched reusable
+ * blocks which are stored into state but not visible.
+ *
+ * @param {Object}  blocksOrder  Object that maps block client IDs to a list of
+ *                               nested block client IDs.
+ * @param {?string} rootClientId The root client ID to search. Defaults to ''.
+ *
+ * @return {Array} List of descendant client IDs.
+ */
+function getNestedBlockClientIds( blocksOrder, rootClientId = '' ) {
+	const attachedClientIds = [];
+	if ( blocksOrder[ rootClientId ] ) {
+		blocksOrder[ rootClientId ].forEach( ( clientId ) => {
+			attachedClientIds.push( clientId );
+			attachedClientIds.push( ...getNestedBlockClientIds( blocksOrder, clientId ) );
+		} );
+	}
+	return attachedClientIds;
+}
+
 /**
  * Returns an object against which it is safe to perform mutating operations,
  * given the original object and its current working copy.
@@ -211,6 +236,35 @@ const withInnerBlocksRemoveCascade = ( reducer ) => ( state, action ) => {
 	return reducer( state, action );
 };
 
+/**
+ * Higher-order reducer which targets the combined blocks reducer and handles
+ * the `RESET_BLOCKS` action. When dispatched, this action will replace all
+ * blocks that exist in the post, leaving blocks that exist only in state (e.g.
+ * reusable blocks) alone.
+ *
+ * @param {Function} reducer Original reducer function.
+ *
+ * @return {Function} Enhanced reducer function.
+ */
+const withBlockReset = ( reducer ) => ( state, action ) => {
+	if ( state && action.type === 'RESET_BLOCKS' ) {
+		const visibleClientIds = getNestedBlockClientIds( state.order );
+		return {
+			...state,
+			byClientId: {
+				...omit( state.byClientId, visibleClientIds ),
+				...getFlattenedBlocks( action.blocks ),
+			},
+			order: {
+				...omit( state.order, visibleClientIds ),
+				...mapBlockOrder( action.blocks ),
+			},
+		};
+	}
+
+	return reducer( state, action );
+};
+
 /**
  * Undoable reducer returning the editor post state, including blocks parsed
  * from current HTML markup.
@@ -280,6 +334,8 @@ export const editor = flow( [
 	blocks: flow( [
 		combineReducers,
 
+		withBlockReset,
+
 		// Track whether changes exist, resetting at each post save. Relies on
 		// editor initialization firing post reset as an effect.
 		withChangeDetection( {
@@ -289,7 +345,6 @@ export const editor = flow( [
 	] )( {
 		byClientId( state = {}, action ) {
 			switch ( action.type ) {
-				case 'RESET_BLOCKS':
 				case 'SETUP_EDITOR_STATE':
 					return getFlattenedBlocks( action.blocks );
 
@@ -392,7 +447,6 @@ export const editor = flow( [
 
 		order( state = {}, action ) {
 			switch ( action.type ) {
-				case 'RESET_BLOCKS':
 				case 'SETUP_EDITOR_STATE':
 					return mapBlockOrder( action.blocks );
 

packages/editor/src/store/test/reducer.js

@@ -1074,6 +1074,58 @@ describe( 'state', () => {
 		} );
 
 		describe( 'blocks', () => {
+			it( 'should not reset any blocks that are not in the post', () => {
+				const actions = [
+					{
+						type: 'RESET_BLOCKS',
+						blocks: [
+							{
+								clientId: 'block1',
+								innerBlocks: [
+									{ clientId: 'block11', innerBlocks: [] },
+									{ clientId: 'block12', innerBlocks: [] },
+								],
+							},
+						],
+					},
+					{
+						type: 'RECEIVE_BLOCKS',
+						blocks: [
+							{
+								clientId: 'block2',
+								innerBlocks: [
+									{ clientId: 'block21', innerBlocks: [] },
+									{ clientId: 'block22', innerBlocks: [] },
+								],
+							},
+						],
+					},
+				];
+				const original = deepFreeze( actions.reduce( editor, undefined ) );
+
+				const state = editor( original, {
+					type: 'RESET_BLOCKS',
+					blocks: [
+						{
+							clientId: 'block3',
+							innerBlocks: [
+								{ clientId: 'block31', innerBlocks: [] },
+								{ clientId: 'block32', innerBlocks: [] },
+							],
+						},
+					],
+				} );
+
+				expect( state.present.blocks.byClientId ).toEqual( {
+					block2: { clientId: 'block2' },
+					block21: { clientId: 'block21' },
+					block22: { clientId: 'block22' },
+					block3: { clientId: 'block3' },
+					block31: { clientId: 'block31' },
+					block32: { clientId: 'block32' },
+				} );
+			} );
+
 			describe( 'byClientId', () => {
 				it( 'should return with attribute block updates', () => {
 					const original = deepFreeze( editor( undefined, {