Saving: Post with only "Classic" block fails to autosave

[aduth]

When a post only contains a "Classic" block, it attempts to autosave but fails.

Steps to reproduce:

1. Navigate to Posts > Add New
2. Insert a "Classic" block
3. Wait a few seconds
4. Autosave kicks in, then fails with notice

Suspected cause:

There is special handling for the classic block to omit its comment demarcation wrapper, so while the editor knows that a block exists and thus thinks the post is saveable, when it's the only content, it will be serialized as the empty string, which is not valid for save.

gutenberg/blocks/api/serializer.js

Lines 274 to 287 in 622bbfb

	export function serializeBlock( block ) {
	const blockName = block.name;
	const blockType = getBlockType( blockName );
	const saveContent = getBlockContent( block );
	const saveAttributes = getCommentAttributes( block.attributes, blockType );
	switch ( blockName ) {
	case getUnknownTypeHandlerName():
	return saveContent;
	default:
	return getCommentDelimitedContent( blockName, saveAttributes, saveContent );
	}
	}

gutenberg/editor/store/selectors.js

Lines 351 to 364 in 622bbfb

	/**
	* Returns true if the edited post has content. A post has content if it has at
	* least one block or otherwise has a non-empty content property assigned.
	*
	* @param {Object} state Global application state.
	*
	* @return {boolean} Whether post has content.
	*/
	export function isEditedPostEmpty( state ) {
	return (
	! getBlockCount( state ) &&
	! getEditedPostAttribute( state, 'content' )
	);
	}

It is not a safe assumption to imply that the presence of blocks means that content is non-empty. isEditedPostEmpty should at least be updated to account for this special Classic block behavior.

[danielbachhuber]

Duplicate of #6556 (comment)

[olygoelements]

i have the same error.
gutenberg have a conflict
i have upgrade php 7.1 to 7.2
and now is work

[aduth]

@olygoelements The "Updating failed" message is a pretty common error that applies broadly to issues where the request to save the post via the REST API fails for some reason. I expect your issue may not be quite the same as what was described here.