Getting the source HTML of the current page from chrome extension

I have a chrome extension. I need to analyse from the HTML source of the current page. I found here all kinds of solutions with background pages and content scripts but none helped me. here is what I have so far:

manifest.json

{
  "name": "Extension",
  "version": "1.0",
  "description": "Extension",
  "browser_action": {
    "default_icon": "bmarkred.ico",
    "popup": "Test.html"
  },
  "content_scripts": [
    {
      "matches": ["http://*/*"],
      "js": ["content.js"]
    }
  ],
  "background": {
    "page": "backgroundPage.html"
  },
  "permissions": [
    "cookies",
    "tabs",
    "http://*/*", 
    "https://*/*"
  ]
}

background.html

<html>
<head>
<script type="text/javascript">
    try {
        chrome.tabs.getSelected(null, function (tab) {
            chrome.tabs.sendRequest(tab.id, {action: "getSource"}, function(source) {
                alert(source);
            });
        });
    }
    catch (ex) {
        alert(ex);
    }
</script>
</head>
</html>

content.js

chrome.extension.onRequest.addListener(function(request, sender, callback) {
    if (request.action == "getSource") {
        callback(document.getElementsByTagName('html')[0].innerHTML);
    }
});

The alert always alerts undefined. even if i change in the content.js file the callback function to:

callback('hello');

still the same result. What am I doing wrong? maybe I'm going at this the wrong way. What I really need is this: When the user opens the extension popup (and only then), I need HTML of the current page so I can analyse it.

Solution

Inject a script into the page you want to get the source from and message it back to the popup....

manifest.json

{
    "name": "Get pages source",
    "version": "1.1",
    "manifest_version": 3,
    "description": "Get active tabs or element on that pages source from a popup",

    "action": {
        "default_title": "Get pages source",
        "default_popup": "popup.html"
    },

    "permissions": [
        "scripting",
        "activeTab"
    ]
}

popup.html

function onWindowLoad() {
    var message = document.querySelector('#message');

    chrome.tabs.query({ active: true, currentWindow: true }).then(function (tabs) {
        var activeTab = tabs[0];
        var activeTabId = activeTab.id;

        return chrome.scripting.executeScript({
            target: { tabId: activeTabId },
            // injectImmediately: true,  // uncomment this to make it execute straight away, other wise it will wait for document_idle
            func: DOMtoString,
            // args: ['body']  // you can use this to target what element to get the html for
        });

    }).then(function (results) {
        message.innerText = results[0].result;
    }).catch(function (error) {
        message.innerText = 'There was an error injecting script : \n' + error.message;
    });
}

window.onload = onWindowLoad;

function DOMtoString(selector) {
    if (selector) {
        selector = document.querySelector(selector);
        if (!selector) return "ERROR: querySelector failed to find node"
    } else {
        selector = document.documentElement;
    }
    return selector.outerHTML;
}