Ionic/Cordova File plugin fails with to write file when downloading a large number of documents in succession on iOS device - ios

Question

The problem as the user sees it:
User has long list of documents they need to download to iOS device (200+)
User starts the download, with each file downloaded in succession.
At the end of the download queue, they discover that one of the files fails (and its always one of two specific files that are 25MB+)
They retry the job (which only downloads the failed document) and it succeeds
What I'm seeing as a developer:
My app pulls down the document as a blob
When I inspect the blob (within my Typescript app code), it has a size > 0
I call this.file.writeFile(directoryPath, fileName, blob, {replace: true}), which calls the Ionic File wrapper around Cordova File Plugin
However, when I look at the blob in write of FileWriter.js, it has a size of zero
This all results in the error spitting out as:
{"type":"error","bubbles":false,"cancelBubble":false,"cancelable":false,"lengthComputable":false,"loaded":0,"total":0,"target":{"fileName":"","length":0,"localURL":"cdvfile://localhost/persistent/downloaded-assets/9ce34f8a-6201-4023-9f5b-de6133bd5699/{{redacted}}","position":0,"readyState":2,"result":null,"error":{},"onwritestart":null,"onprogress":null,"onwriteend":null,"onabort":null}}
What I'm getting from this is that somewhere between calling file.writeFile on the Ionic File wrapper in my typescript code and the FileWriter.write method in the Cordova package, my blob is getting corrupted, lost, or emptied somehow.
It's difficult to debug as that layer in-between these two points is minified in the xCode debugger, so it would also be nice to hear some suggestions about how I might be able to debug this myself better.
Do we have any idea what might be going on here? Is it a memory issue on iOS? Does Cordova timeout over multiple repeated requests?
A few things to note:
The full list of files download fine every time on I try within the iOS xCode simulator. This is leading me to believe it might be a memory issue, but I'm not sure.
The failure always happens after about 200 files, and on one of two files that are 25-30MB+
As far as debugging goes, the earliest I can see my blob reduced to 0 is here https://github.com/apache/cordova-plugin-file/blob/4a92bbbea755aa9e5bf8cfd160fb9da1cd3287cd/www/FileWriter.js#L107 (though I might be debugging incorrectly)
EDIT - After a little more digging, I was able to see exactly where the Ionic plugin went wrong:
The code I used:
private writeFileInChunks(writer: FileWriter, file: Blob) {
console.log('SIZE OF FILE AT START', file.size);
const BLOCK_SIZE = 1024 * 1024;
let writtenSize = 0;
function writeNextChunk() {
const size = Math.min(BLOCK_SIZE, file.size - writtenSize);
console.log('CALCULATED SIZE:', size);
const chunk = file.slice(writtenSize, writtenSize + size);
console.log('SIZE OF CHUNK TO WRITE', chunk.size)
writtenSize += size;
writer.write(chunk);
}
return getPromise<any>((resolve, reject) => {
writer.onerror = reject as (event: ProgressEvent) => void;
writer.onwrite = () => {
if (writtenSize < file.size) {
writeNextChunk();
} else {
resolve();
}
};
writeNextChunk();
});
}
The output for the failed document:
SIZE OF FILE AT START: 34012899
CALCULATED SIZE: 1048576
SIZE OF CHUNK TO WRITE: 0
On retry:
SIZE OF FILE AT START: 34012899
CALCULATED SIZE: 1048576
SIZE OF CHUNK TO WRITE: 1048576
CALCULATED SIZE: 1048576
SIZE OF CHUNK TO WRITE: 1048576
...
...
...
CALCULATED SIZE: 458467
SIZE OF CHUNK TO WRITE: 458467
So for whatever reason, after a large number of previous downloads, that file.slice step results in an empty/corrupted blob.
Any ideas on how to correct this?
Ran another test with some expanded logging:
private writeFileInChunks(writer: FileWriter, file: Blob) {
...
function writeNextChunk() {
const size = Math.min(BLOCK_SIZE, file.size - writtenSize);
console.log('CALCULATED SIZE:', size);
console.log('WRITTEN SIZE', writtenSize);
console.log('SUMS TO:', writtenSize + size)
console.log('FILE SIZE BEFORE SLICE:', file.size);
const chunk = file.slice(writtenSize, writtenSize + size);
console.log('SIZE OF CHUNK TO WRITE', chunk.size);
writtenSize += size;
writer.write(chunk);
}
...
...
}
Output came to:
CALCULATED SIZE: 1048576
WRITTEN SIZE: 0
SUMS TO: 1048576
FILE SIZE BEFORE SLICE: 34012899
SIZE OF CHUNK TO WRITE 0
Further confirming the issue