fix(screenshots): Split bytes into smaller chunks to run through from…

…CharCode (#3101)

system-addon/lib/Screenshots.jsm

@@ -20,6 +20,24 @@ XPCOMUtils.defineLazyModuleGetter(this, "OS",
   "resource://gre/modules/osfile.jsm");
 
 this.Screenshots = {
+  /**
+   * Convert bytes to a string using extremely fast String.fromCharCode without
+   * exceeding the max number of arguments that can be provided to a function.
+   */
+  _bytesToString(bytes) {
+    // NB: This comes from js/src/vm/ArgumentsObject.h ARGS_LENGTH_MAX
+    const ARGS_LENGTH_MAX = 500 * 1000;
+    let i = 0;
+    let str = "";
+    let {length} = bytes;
+    while (i < length) {
+      const start = i;
+      i += ARGS_LENGTH_MAX;
+      str += String.fromCharCode.apply(null, bytes.slice(start, i));
+    }
+    return str;
+  },
+
   async getScreenshotForURL(url) {
     let screenshot = null;
     try {
@@ -34,7 +52,7 @@ this.Screenshots = {
 
       const contentType = MIMEService.getTypeFromFile(nsFile);
       const bytes = await file.read();
-      const encodedData = btoa(String.fromCharCode.apply(null, bytes));
+      const encodedData = btoa(this._bytesToString(bytes));
       file.close();
       screenshot = `data:${contentType};base64,${encodedData}`;
     } catch (err) {

system-addon/test/unit/lib/Screenshots.test.js

@@ -48,4 +48,27 @@ describe("Screenshots", () => {
       assert.equal(screenshot, null);
     });
   });
+
+  describe("#_bytesToString", () => {
+    it("should convert no bytes to empty string", () => {
+      assert.equal(Screenshots._bytesToString([]), "");
+    });
+    it("should convert bytes to a string", () => {
+      const str = Screenshots._bytesToString([104, 101, 108, 108, 111, 32, 119, 111, 114, 108, 100]);
+
+      assert.equal(str, "hello world");
+    });
+    it("should convert very many bytes to a long string", () => {
+      const bytes = [];
+      for (let i = 0; i < 1000 * 1000; i++) {
+        bytes.push(9);
+      }
+
+      const str = Screenshots._bytesToString(bytes);
+
+      assert.propertyVal(str, 0, "\t");
+      assert.propertyVal(str, "length", 1000000);
+      assert.propertyVal(str, 999999, "\u0009");
+    });
+  });
 });