Pad: New `padDefaultContent` hook

CHANGELOG.md

@@ -39,6 +39,7 @@
 #### For plugin authors
 
 * New `expressPreSession` server-side hook.
+* New `padDefaultContent` server-side hook.
 * New APIs for processing attributes: `ep_etherpad-lite/static/js/attributes`
   (low-level API) and `ep_etherpad-lite/static/js/AttributeMap` (high-level
   API).

doc/api/hooks_server-side.md

@@ -216,6 +216,31 @@ Context properties:
 * `authorId`: The ID of the author who created the pad.
 * `author` (**deprecated**): Synonym of `authorId`.
 
+## `padDefaultContent`
+
+Called from `src/node/db/Pad.js`
+
+Called to obtain a pad's initial content, unless the pad is being created with
+specific content. The return value is ignored; to change the content, modify the
+`content` context property.
+
+This hook is run asynchronously. All registered hook functions are run
+concurrently (via `Promise.all()`), so be careful to avoid race conditions when
+reading and modifying the context properties.
+
+Context properties:
+
+* `pad`: The newly created Pad object.
+* `authorId`: The author ID of the user that is creating the pad.
+* `type`: String identifying the content type. Currently this is `'text'` and
+  must not be changed. Future versions of Etherpad may add support for HTML,
+  jsdom objects, or other formats, so plugins must assert that this matches a
+  supported content type before reading `content`.
+* `content`: The pad's initial content. Change this property to change the pad's
+  initial content. If the content type is changed, the `type` property must also
+  be updated to match. Plugins must check the value of the `type` property
+  before reading this value.
+
 ## `padLoad`
 
 Called from: `src/node/db/PadManager.js`

src/node/db/Pad.js

@@ -372,11 +372,6 @@ Pad.prototype.getChatMessages = async function (start, end) {
 };
 
 Pad.prototype.init = async function (text, authorId = '') {
-  // replace text with default text if text isn't set
-  if (text == null) {
-    text = settings.defaultPadText;
-  }
-
   // try to load the pad
   const value = await this._db.get(`pad:${this.id}`);
 
@@ -391,9 +386,18 @@ Pad.prototype.init = async function (text, authorId = '') {
       }
     }
   } else {
-    // this pad doesn't exist, so create it
-    const firstChangeset = Changeset.makeSplice('\n', 0, 0, exports.cleanText(text));
-
+    if (text == null) {
+      const context = {
+        pad: this,
+        authorId,
+        type: 'text',
+        content: exports.cleanText(settings.defaultPadText),
+      };
+      await hooks.aCallAll('padDefaultContent', context);
+      if (context.type !== 'text') throw new Error(`unsupported content type: ${context.type}`);
+      text = context.content;
+    }
+    const firstChangeset = Changeset.makeSplice('\n', 0, 0, text);
     await this.appendRevision(firstChangeset, authorId);
   }
 };