Better cross platform font handling (#2104)

* fix: better handling of custom font

Practically speaking, custom font seems to have only worked on Linux, because
`net.fetch` would include the mime type in the response headers which could validate the payload.
This doesn't appear to be the case on windows/macOS. Instead:

1. On Linux (or if some other system supports it), check the content type. If good, serve as normal
2. Otherwise, fetch the payload. Read the first four to five bytes and check for a valid magic number.

Additionally, to prevent arbitrary requests fetching other paths via injected content, sync the custom font path
to the main process, and then make _every_ request to `feishin:/` point to the same renderer path.
When setting the font, first send the path to the main process. This will register `feishin:/` to point
to the path provided. This is done via a promise-based set.

Finally, provide a default value for the file input (a best effort approximation for the last part of the file path)
on the file input component.

* make the linter happy

src/main/features/core/settings/index.ts

@@ -141,6 +141,14 @@ ipcMain.on('settings-set', (__event, data: { property: string; value: any }) =>
     }
 });
 
+ipcMain.handle('settings-set-sync', (__event, data: { property: string; value: any }) => {
+    if (data.value === null) {
+        store.delete(data.property);
+    } else {
+        store.set(data.property, data.value);
+    }
+});
+
 ipcMain.handle('password-get', (_event, server: string): null | string => {
     if (safeStorage.isEncryptionAvailable()) {
         const servers = store.get('server') as Record<string, string> | undefined;

src/main/index.ts

@@ -252,7 +252,9 @@ function createAlphaUpdaterInstance(): AppImageUpdater | MacUpdater | NsisUpdate
     return new NsisUpdater(ALPHA_UPDATER_CONFIG);
 }
 
-protocol.registerSchemesAsPrivileged([{ privileges: { bypassCSP: true }, scheme: 'feishin' }]);
+protocol.registerSchemesAsPrivileged([
+    { privileges: { bypassCSP: true, corsEnabled: true }, scheme: 'feishin' },
+]);
 
 process.on('uncaughtException', (error: any) => {
     console.error('Error in main process', error);
@@ -989,14 +991,33 @@ app.on('window-all-closed', () => {
     }
 });
 
-const FONT_HEADERS = [
+const FONT_HEADERS = new Set([
     'font/collection',
     'font/otf',
     'font/sfnt',
     'font/ttf',
     'font/woff',
     'font/woff2',
-];
+]);
+
+const bytesToInt = (array: Uint8Array, length: number): number => {
+    let value = 0;
+    for (let i = 0; i < length; i++) {
+        value = (value << 8) + array[i];
+    }
+
+    return value;
+};
+
+const FONT_FOUR_BYTE_MAGIC_NUMBERS = new Set([
+    0x4f54544f, // font/otf
+    0x774f4632, // font/woff2
+    0x774f4646, // font/woff
+]);
+
+const FONT_FIVE_BYTE_MAGIC_NUMBERS = new Set([
+    0x0001000000, // ttf, collection, sfnt
+]);
 
 const singleInstance = isDevelopment ? true : app.requestSingleInstanceLock();
 
@@ -1017,12 +1038,9 @@ if (!singleInstance) {
 
     app.whenReady()
         .then(() => {
-            protocol.handle('feishin', async (request) => {
-                const filePath = `file:${request.url.slice('feishin:'.length)}`;
-                const response = await net.fetch(filePath);
-                const contentType = response.headers.get('content-type');
-
-                if (!contentType || !FONT_HEADERS.includes(contentType)) {
+            protocol.handle('feishin', async () => {
+                const filePath = store.get('local_font_path');
+                if (typeof filePath !== 'string') {
                     getMainWindow()?.webContents.send('custom-font-error', filePath);
 
                     return new Response(null, {
@@ -1031,7 +1049,38 @@ if (!singleInstance) {
                     });
                 }
 
-                return response;
+                const response = await net.fetch('file:' + filePath);
+                const contentType = response.headers.get('content-type');
+
+                // On Linux, the mime type is included in the response header
+                // In this case, we can forward the response with no further processing
+                if (contentType && FONT_HEADERS.has(contentType)) {
+                    return response;
+                }
+
+                // Otherwise, let's check the magic number to see if
+                // the file is a font type. This is either four or five bytes
+                const payload = await response.arrayBuffer();
+                const magicNumber = new Uint8Array(payload.slice(0, 5));
+                const fiveHex = bytesToInt(magicNumber, 5);
+                const fourHex = bytesToInt(magicNumber, 4);
+
+                if (
+                    FONT_FIVE_BYTE_MAGIC_NUMBERS.has(fiveHex) ||
+                    FONT_FOUR_BYTE_MAGIC_NUMBERS.has(fourHex)
+                ) {
+                    // We have to create a new response with the payload, since it has been read now
+                    return new Response(payload, {
+                        headers: response.headers,
+                    });
+                }
+
+                getMainWindow()?.webContents.send('custom-font-error', filePath);
+
+                return new Response(null, {
+                    status: 403,
+                    statusText: 'Forbidden',
+                });
             });
 
             session.defaultSession.webRequest.onHeadersReceived((details, callback) => {