Fix image uploads being random data when canvas read access is blocked (#15180)

Fixes #15178

PR #11499 introduced a way to deal with all-white canvas, but newer
Firefox versions set random data instead.

This PR detects whether canvas operations are reliable by comparing
the results on a hardcoded 2×2 pixels image, and memoizing the result.

This should be both more reliable and faster than the previous check.

app/javascript/mastodon/utils/resize_image.js

@@ -41,6 +41,45 @@ const dropOrientationIfNeeded = (orientation) => new Promise(resolve => {
   }
 });
 
+// Some browsers don't allow reading from a canvas and instead return all-white
+// or randomized data. Use a pre-defined image to check if reading the canvas
+// works.
+const checkCanvasReliability = () => new Promise((resolve, reject) => {
+  switch(_browser_quirks['canvas-read-unreliable']) {
+  case true:
+    reject('Canvas reading unreliable');
+    break;
+  case false:
+    resolve();
+    break;
+  default:
+    // 2×2 GIF with white, red, green and blue pixels
+    const testImageURL =
+      'data:image/gif;base64,R0lGODdhAgACAKEDAAAA//8AAAD/AP///ywAAAAAAgACAAACA1wEBQA7';
+    const refData =
+      [255, 255, 255, 255,  255, 0, 0, 255,  0, 255, 0, 255,  0, 0, 255, 255];
+    const img = new Image();
+    img.onload = () => {
+      const canvas  = document.createElement('canvas');
+      const context = canvas.getContext('2d');
+      context.drawImage(img, 0, 0, 2, 2);
+      const imageData = context.getImageData(0, 0, 2, 2);
+      if (imageData.data.every((x, i) => refData[i] === x)) {
+        _browser_quirks['canvas-read-unreliable'] = false;
+        resolve();
+      } else {
+        _browser_quirks['canvas-read-unreliable'] = true;
+        reject('Canvas reading unreliable');
+      }
+    };
+    img.onerror = () => {
+      _browser_quirks['canvas-read-unreliable'] = true;
+      reject('Failed to load test image');
+    };
+    img.src = testImageURL;
+  }
+});
+
 const getImageUrl = inputFile => new Promise((resolve, reject) => {
   if (window.URL && URL.createObjectURL) {
     try {
@@ -110,14 +149,6 @@ const processImage = (img, { width, height, orientation, type = 'image/png' }) =
 
   context.drawImage(img, 0, 0, width, height);
 
-  // The Tor Browser and maybe other browsers may prevent reading from canvas
-  // and return an all-white image instead. Assume reading failed if the resized
-  // image is perfectly white.
-  const imageData = context.getImageData(0, 0, width, height);
-  if (imageData.data.every(value => value === 255)) {
-    throw 'Failed to read from canvas';
-  }
-
   canvas.toBlob(resolve, type);
 });
 
@@ -127,7 +158,8 @@ const resizeImage = (img, type = 'image/png') => new Promise((resolve, reject) =
   const newWidth  = Math.round(Math.sqrt(MAX_IMAGE_PIXELS * (width / height)));
   const newHeight = Math.round(Math.sqrt(MAX_IMAGE_PIXELS * (height / width)));
 
-  getOrientation(img, type)
+  checkCanvasReliability()
+    .then(getOrientation(img, type))
     .then(orientation => processImage(img, {
       width: newWidth,
       height: newHeight,