@uppy/core: make upload() idempotent

[Murderlon]

Closes #5669
Closes #5676

Before

1. Add a file
2. Call uppy.upload() but make it fail (set throttling to offline in your browser)
3. Call uppy.upload() again but no throttling
4. Events are not fired, endless uploading state.

You must call retryAll() instead but it's better DX if you can simply call upload() again and let us figure it out.

After

upload() behaves like retryAll() when errors occurred in a backwards compatible way.

What if an upload fails and you also add a new file?

Backwards compatible behaviour similar to how it currently works when using the dashboard, first call to upload() only retries the failed files. You have to call upload() again to upload the new files. Making upload() do both at once is hard and makes the events a bit unclear, are emitting 'upload' or 'retry-all'?

Question

In this case:

1. Add a file
2. Call upload()` but make it fail (set throttling to offline in your browser)
3. Add another file
4. Call upload() to successfully upload the failed file
5. Call `upload() again to upload the new file
   • should the 'completed' event here contain both files or only the last one? Currently it's the former. Maybe a breaking change if we change it?

[github-actions]

Diff output files

diff --git a/packages/@uppy/core/lib/Uppy.js b/packages/@uppy/core/lib/Uppy.js
index 1ba8ade..87e04ba 100644
--- a/packages/@uppy/core/lib/Uppy.js
+++ b/packages/@uppy/core/lib/Uppy.js
@@ -994,6 +994,29 @@ export class Uppy {
     let {
       files,
     } = this.getState();
+    const filesToRetry = Object.keys(files).filter(fileID => files[fileID].error);
+    const hasFilesToRetry = filesToRetry.length > 0;
+    if (hasFilesToRetry) {
+      const updatedFiles = {
+        ...files,
+      };
+      filesToRetry.forEach(fileID => {
+        updatedFiles[fileID] = {
+          ...updatedFiles[fileID],
+          isPaused: false,
+          error: null,
+        };
+      });
+      this.setState({
+        files: updatedFiles,
+        error: null,
+      });
+      this.emit("retry-all", Object.values(updatedFiles));
+      const uploadID = _classPrivateFieldLooseBase(this, _createUpload)[_createUpload](filesToRetry, {
+        forceAllowNewUpload: true,
+      });
+      return _classPrivateFieldLooseBase(this, _runUpload)[_runUpload](uploadID);
+    }
     const onBeforeUploadResult = this.opts.onBeforeUpload(files);
     if (onBeforeUploadResult === false) {
       return Promise.reject(new Error("Not starting the upload because onBeforeUpload returned false"));

[Murderlon]

One thing I'm not too happy about yet is that in the case of a failed upload and adding a new file before uploading again, is that you don't really know you have to call upload() twice. Maybe upload() should return something to indicate that or further exploration is needed to upload both at once.