What and where exactly are privileged code, chrome code, Gecko? - firefox-addon-webextensions

Question

In reading about the File API and wanting to write data directly from an indexedDB database to the client disk instead of first building and holding a large blob in RAM to download to disk, there are a few basic items I'm not understanding.
In the MDN documents these two statements are found:
In Gecko, privileged code can create File objects representing any local file without user interaction.
If you want to use the DOM File API in chrome code, you can do so without restriction. In fact, you get one bonus feature: you can create File objects specifying the path of the file on the user's computer. This only works from privileged code, so web content can't do it.
Where exactly does one write chrome code and/or Gecko priveleged code? Is this beyond a web extension? I've read and experimented with extensions; so, I'm not asking specifically about how to access them.
I'm not concerned about a 'normal' web page and server accessing the client disk. I know that it's not permitted inorder to protect the individual.
I'm interested in what can be done offline through the browser--with the aid of web extensions and/or a separate profile granting special permissions but without node.js, electron, etc.-- by an individual who knowingly wants to use the browser to do maybe what they should have built in the OS rather than the browser.
Put another way, if I want to use the browser just to run my javascript code to perform tasks all offline on my own machine, where is the privileged code written that gives access to these types of APIs that aren't subject to the security issues of a normal web page?
Is it still javascript or C++ in these areas?
Thank you.
This old question provides a link to their extension which includes the File API that writes to disk in a way that appears to provide a means to bypass the creation of a large blob of data. It's six years old but appears to contain what is needed, at least to get started.
I'm not referring to their trying to get around using indexedDB, but just that using this type of extension could allow for writing each object from the database directly to the client disk without first having to generate a large blob to download.
Attempt at employing Andrew Swan's suggestion
I'm trying to put the pieces together but have reached a point where am not sure how to continue. I wrote the code below in the background script of an extension. In attempting to employ Andrew Swan's suggestion, the plan is to intitiate a GET request for a text/csv file, which is intercepted and replaced by data extracted from database and written to the GET request by the stream filter.
First, make a GET request to a bogus url and listen for response, as follows:
let request = new XMLHttpRequest();
request.open("GET", url );
request.setRequestHeader( "Content-Type", "text/csv" );
request.send(null);
request.onreadystatechange = () =>
{
portFromCS.postMessage( { 'func' : 'disp_result', 'args' : { 'msg' : "request.status :", 'value' : request.status + ' : ' + request.statusText} } );
};
Second, intercept the request and write to the GET, as follows:
browser.webRequest.onBeforeRequest.addListener(
listener,
{ urls : ["<all_urls>"] },
["blocking"] );
function listener( details )
{
let filter = browser.webRequest.filterResponseData(details.requestId);
let decoder = new TextDecoder("utf-8");
let encoder = new TextEncoder();
filter.onstart = event =>
{
let str = decoder.decode(event.data, {stream: true});
str = '' +
'HTTP/1.1 200 OK \r\n' +
'Content-Length: 17 \r\n' +
'Content-Type: text/csv \r\n\r\n\r\n' +
'This is a string.';
filter.write(encoder.encode(str));
filter.disconnect();
};
}
The message sent from the background script in the request.onreadystatechange function is received in the content script, and the request.status is '0'.
The filter.onstart is used because the ondata event will never fire since the url is bogus. Also, that means there will be no converting of data from the url, but only the writing of new data through the filter.
The str data is written and received by the request, but only as responseText and not as a response header. The request.status remains '0' instead of '200'.
It seems that can't change the response header unless in onHeadersReceived which will never take place, it appears, for a bogus url. However, I tried this on a real url and, even though the event fired, an error of webRequest.HttpHeaders is not a function was thrown. I had "responseHeaders" in the webRequest extraInfoSpec at that time.
My questions are:
Can a response header be written to set the request.status to '200' and then start writing the database data through an async function in small blocks as retrieved?
Can the Content Disposition section of the header response be set such that it automatically starts the download of the response.text and allows the user to select the file name and save location, and stay "open" as keep writing to the file as the data is extracted from the database and passed to the GET request through the filter.write()?
Thank you.
Conclusion
It was a good idea but I don't think it is possible for at least two reasons.
One is that webRequest doesn't appear to intercept a downloads.download() function at all, or any download event; so, you can't intercept a download, and an event with a Content Disposition of 'attachment' is needed to even try to write to it with a stream. I could intercept a forced click to an anchor tag href but no other events fired beyond onBeforeRequest.
The other is that a response header can't be modified until an onHeadersReceived event, which means the fake URL has to return something. You can't just cancel it in onBeforeRequest. So, this wouldn't work offline. But, even if you let it process online to an existing URL that returns a reponse header, it won't accept a modification. I tried repeatedly to modify the response header and it just won't work. I tried an XMLHttpRequest GET and can intercept the events that fire but can't modify the response header; so, can't set Content Disposition to 'attachment', with or without file, to start a download. I can write to it but it's no good unless it is going to download what is written. It would be ok if the written content was going to a web page.
Also, if you redirect the URL along the way to anything other than a webRequest acceptable URL, the other events won't be interceptable. So, if redirect to an object URL in onBeforeRequest, you won't intercept the response headers stage in webRequest but can view threm in the onreadystatechange event of the XMLHttpRequest.
So, the upshot is that it appears the response headers cannot be modified even though the MDN Web Docs say it is possible. And, this idea of using awebRequest stream filter to stream data generated on the client or extracted from an indexedDB database, as opposed to building one large blob for download, won't work because can't intercept a download or change the response headers to trigger a download into which to write via the stream filter.
It was an interesting idea though. I still wonder whether or not the download would remain 'open', so to speak, while the data was being written on the client and passed in blocks or chunks. Perhaps, if that part of the response headers that states how data is to be passed and received was modified also it would work.
For now, I am no longer pursuing this approach. One of the Web Docs or a bug records stated that it is planned to allow a data URL to be intercepted. Perhaps, for an offline download to the client, that would be preferrable to a fake URL.
If anyone gets this to work, please let us know. Thank you.

Answer 1

A couple of terms:
"Gecko" is the rendering engine on which Firefox (and a few other applications like Thunderbird) is built
"Chrome" in this context means the browser user interface and features, as opposed to the contents of a web page being displayed by the browser.
In Firefox, much of the browser chrome is implemented in Javascript. The code that implements the user interface needs to be able to do things that normal web pages cannot do (such as reading and writing the local filesystem). Therefore, this code runs with different privileges than Javascript that runs as part of a web page. The terms "privileged code", "chrome privileged code", "Gecko privileged code" are all different ways to describe the same thing: Javascript code that is built in to the browser and has access to capabilities that web pages do not have.
Prior to the Firefox Quantum (version 57) release, Firefox extensions were allowed to run privileged Javascript code. As you might imagine, this was fraught with problems for security, performance, and stability, among other things. With WebExtensions, extensions now run with the same level of privilege as regular web content (ie, they do not execute with elevated privileges). Some browser features are exposed to extensions through extension APIs.
So, if you're interested in what you can do from an extension, any documents on MDN that reference privileged code, are effectively irrelevant. There are not currently any APIs available to WebExtensions that would allow you to directly access the filesystem, but there is an open bug to add some this capability. (that bug has existed for quite some time, but I suspect there will be progress relatively soon...)