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diff --git a/background/script_injector.mjs b/background/script_injector.mjs
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+++ b/background/script_injector.mjs
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+/**
+* Myext script injector
+* 
+* Copyright (C) 2021 Wojtek Kosior
+*
+* Dual-licensed under:
+*   - 0BSD license
+*   - GPLv3 or (at your option) any later version
+*/
+
+import {TYPE_PREFIX} from '/common/stored_types.mjs';
+import sha256 from './sha256.mjs';
+import {url_item} from './ResponseProcessor.mjs';
+import get_storage from './storage.mjs';
+
+"use strict";
+
+var storage;
+
+function ajax_callback()
+{
+    if (this.readyState == 4)
+	this.resolve_callback(this);
+}
+
+function initiate_ajax_request(resolve, method, url)
+{
+    var xhttp = new XMLHttpRequest();
+    xhttp.resolve_callback = resolve;
+    xhttp.onreadystatechange = ajax_callback;
+    xhttp.open(method, url, true);
+    xhttp.send();
+}
+
+function make_ajax_request(method, url)
+{
+    return new Promise((resolve, reject) =>
+	initiate_ajax_request(resolve, method, url));
+}
+
+async function fetch_remote_script(script_data)
+{
+    try {
+	let xhttp = await make_ajax_request("GET", script_data.url);
+	if (xhttp.status === 200) {
+	    let computed_hash = sha256(xhttp.responseText);
+	    if (computed_hash !== script_data.hash) {
+		console.log(`Bad hash for ${script_data.url}\n    got ${computed_hash} instead of ${script_data.hash}`);
+		return;
+	    }
+	    return xhttp.responseText;
+	} else {
+	    console.log("script not fetched: " + script_data.url);
+	    return;
+	}
+    } catch (e) {
+	console.log(e);
+    }
+}
+
+async function get_script_text(script_name)
+{
+    try {
+	let script_data = storage.get(TYPE_PREFIX.SCRIPT, script_name);
+	if (script_data === undefined) {
+	    console.log(`missing data for ${script_name}`);
+	    return;
+	}
+	let script_text = script_data.text;
+	if (!script_text)
+	    script_text = await fetch_remote_script(script_data);
+	return script_text;
+    } catch (e) {
+	console.log(e);
+    }
+}
+
+// TODO: parallelize script fetching
+// TODO: guard against infinite recursion
+
+async function inject_scripts_rec(components, doc)
+{
+    for (let [prefix, name] of components) {
+	if (prefix === TYPE_PREFIX.BUNDLE) {
+	    var bundle = storage.get(TYPE_PREFIX.BUNDLE, name);
+
+	    if (bundle === undefined) {
+		console.log(`no bundle in storage for key ${elem_key}`);
+		continue;
+	    }
+	    await inject_scripts_rec(bundle, doc);
+	} else {
+	    let script_text = await get_script_text(name,);
+	    if (script_text === undefined)
+		continue;
+
+	    let script = doc.createElement("script");
+	    script.textContent = script_text;
+	    doc.body.appendChild(script);
+	}
+    }
+}
+
+async function inject_scripts(url, doc)
+{
+    storage = await get_storage();
+
+    url = url_item(url);
+
+    let components = storage.get(TYPE_PREFIX.PAGE, url);
+
+    if (components === undefined) {
+	console.log(`got nothing for ${url}`);
+	return
+    } else {
+	console.log(`got ${components.length} component(s) for ${url}`);
+    }
+
+    await inject_scripts_rec(components, doc);
+}
+
+export default inject_scripts;