Improve error case in Base64 encode w/ large input

`result` can be an empty string even when `onerror` does not fire if the
input is too large to represent as a string. In that case, the function
was returning undefined, which would certainly error any caller.

Instead throw an explicit error and document that callers are expected
to handle if the input is large.

Bug: 436482118
Change-Id: I18330da4b17ff76310cd3141075a86d3df8718b2
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/7455444
Auto-Submit: Connor Clark <cjamcl@chromium.org>
Commit-Queue: Connor Clark <cjamcl@chromium.org>
Commit-Queue: Paul Irish <paulirish@chromium.org>
Reviewed-by: Paul Irish <paulirish@chromium.org>

Author: Connor Clark

front_end/core/common/Base64.ts

@@ -32,11 +32,23 @@ export function decode(input: string): Uint8Array<ArrayBuffer> {
   return bytes;
 }
 
+/**
+ * Note: if input can be very large (larger than the max string size), callers should
+ * expect this to throw an error.
+ */
 export function encode(input: BlobPart): Promise<string> {
   return new Promise((resolve, reject) => {
     const reader = new FileReader();
-    reader.onerror = () => reject(new Error('failed to convert to base64'));
+    reader.onerror = () => reject(new Error('failed to convert to base64: internal error'));
     reader.onload = () => {
+      // The input was too large to encode as a string. The caller should anticipate
+      // this and use a workaround. See TimelinePanel.ts innerSaveToFile for an example.
+      // For more information, see crbug.com/436482118.
+      if (reader.result === '') {
+        reject(new Error('failed to convert to base64: input too large to encode as base64 string'));
+        return;
+      }
+
       // This string can be very large, so take care to not double memory. `split`
       // was used here before, which always results in new strings in V8. By using
       // slice instead, we leverage the sliced string optimization in V8 and avoid

front_end/panels/timeline/TimelinePanel.ts

@@ -1564,21 +1564,23 @@ export class TimelinePanel extends Common.ObjectWrapper.eventMixin<EventTypes, t
     const blobType = blob.type;  // blob may be reassigned later.
 
     // In some cases Base64.encode() can return undefined; see crbug.com/436482118 for details.
-    // TODO(crbug.com/436482118): understand this edge case and fix the Base64.encode method to not just return undefined.
     let bytesAsB64: string|null = null;
     try {
       // The maximum string length in v8 is `2 ** 29 - 23`, aka 538 MB.
       // If the gzipped&base64-encoded trace is larger than that, this'll throw a RangeError.
       this.statusDialog.updateStatus(i18nString(UIStrings.encodingTraceForDownload));
       this.statusDialog.updateProgressBar(i18nString(UIStrings.encodingTraceForDownload), 100);
       bytesAsB64 = await Common.Base64.encode(blob);
-      if (bytesAsB64.length) {
-        blob = new Blob();  // Don't retain this large object for the remaining lifetime of this function.
+      blob = new Blob();  // Don't retain this large object for the remaining lifetime of this function.
+    } catch (err) {
+      if (err instanceof Error && err.message.startsWith('failed to convert to base64')) {
+        // Expected and handled below.
+      } else {
+        throw err;
       }
-    } catch {
     }
 
-    if (bytesAsB64?.length) {
+    if (bytesAsB64) {
       const contentData = new TextUtils.ContentData.ContentData(bytesAsB64, /* isBase64=*/ true, blobType);
       await Workspace.FileManager.FileManager.instance().save(fileName, contentData, /* forceSaveAs=*/ true);
       Workspace.FileManager.FileManager.instance().close(fileName);