1556457 - Screenshot node fails with dead object error

Status: RESOLVED DUPLICATE of bug 1528036

(DevTools :: Inspector, defect, P1)

Description

[:Harald Kirschner :digitarald]

69.0a1 (2019-05-31) (64-bit)

What were you doing?

1. Open a tweet
2. Take screenshot
3. Paste in another URL to navigate to a new site
4. Try another screenshot

What happened?

Second screenshot fails.

Error while calling actor 'screenshot's method 'capture' can't access dead object protocol.js:1042:13
    writeError resource://devtools/shared/protocol.js:1042
    handler resource://devtools/shared/protocol.js:1226
createScreenshotDataURL@resource://devtools/shared/screenshot/capture.js:51:16
captureScreenshot@resource://devtools/shared/screenshot/capture.js:41:10
capture@resource://devtools/server/actors/screenshot.js:26:12
handler@resource://devtools/shared/protocol.js:1189:37
onPacket@resource://devtools/server/main.js:1291:58
receiveMessage@resource://devtools/shared/transport/child-transport.js:66:16
protocol.js:1045:15
    writeError resource://devtools/shared/protocol.js:1045
    handler resource://devtools/shared/protocol.js:1226
Protocol error (unknownError): can't access dead object

What should have happened?

Screenshot being saved.

Comment 1

[Razvan Caliman [:rcaliman]]

We're having difficulty reproducing this. Are you able to record a video of the steps?

Comment 2

[:Harald Kirschner :digitarald]

Attachment: screenshot-dead.mp4

Added an STR to the first comment, and for completeness, a video.

Comment 3

[Razvan Caliman [:rcaliman]]

The culprit seems to be the this.document dead object in the ScreenshotActor.

The ScreenshotActor gets instantiated and this.document is set against the current targetActor. After navigating away while still keeping the DevTools open, then attempting the screenshot node workflow again, the ScreenshotActor seems to be recycled (its initialize() doesn't get called again) and the this.document reference points to the previous, now dead, object.

Sounds like we should query the targetActor on-demand, not on init, but I don't know how I could get to it otherwise. I don't have a good grasp of the Actor lifecycle.

Yulia, can you please advise if there's an Actor event I could rely on or another mechanism available to circumvent this issue?

Comment 4

[Yulia Startsev [:yulia]]

you can listen to the "navigate" event, it has the window object:

https://searchfox.org/mozilla-central/rev/153172de0c5bfca31ef861bd8fc0995f44cada6a/devtools/server/actors/targets/browsing-context.js#1378

You can listen to it like so: https://searchfox.org/mozilla-central/rev/153172de0c5bfca31ef861bd8fc0995f44cada6a/devtools/server/actors/animation.js#541

you can add the target actor code in screenshot
https://searchfox.org/mozilla-central/source/devtools/server/actors/screenshot.js#14

something like so

initialize(targetActor) {
  this.targetActor = targetActor;
  this.onNavigate = this.onNavigate.bind(this);
  this.document = this.target.window.document;
  this.targetActor.on("navigate", this.onNavigate);
}

onNavigate({window}) {
  this.document = window.document
} 

let me know if it doesnt work.

Comment 5

[Razvan Caliman [:rcaliman]]

This works great. Thank you, Yulia! I will submit a patch ASAP.

A tangential issue: after the node screenshot is made, an error is thrown (it doesn't affect the screenshot functionality):

PageInfo: Invalid value for property 'url': 
"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAACSwAAC8JCAYAAAALXbl0AAAgAElEQVR4nOzd+3tcBZ348e+
/UK4rqLi6i/J81e8TD20pZQC5iAQWlAUj90sHAZFChQUXSFi8cGmQCsgmC6yotCAX0RbEhUK7CHIx5SYXU64KJFwKFBKg
lNJ+vj/gmZ6ZOTOZpMkkpa/X87yfR5tz5hLmZGbOfOac… PlacesUtils.jsm:772
    validateItemProperties resource://gre/modules/PlacesUtils.jsm:772
    validatePageInfo resource://gre/modules/PlacesUtils.jsm:1125
    update resource://gre/modules/History.jsm:687
    update self-hosted:1003
    setMetadata resource://gre/modules/DownloadHistory.jsm:299
    InterpretGeneratorResume self-hosted:1284
    AsyncFunctionNext self-hosted:839

The cause seems to be a validation which fails because the data-uri length for the screenshot exceeds the maximum allowed URL length defined in DB_URL_LENGTH_MAX.

It throws only for screenshots which result in a large file, for example when using the <body> of a large document as the node to screenshot.

The error doesn't seem to affect the screenshot flow and I'm wary of touching that code to increase or circumvent limits for data-uri. It seems to be used in a much broader context within Firefox and I don't know what knock-on effects that would have. I'll leave that error untouched.

Comment 6

[Razvan Caliman [:rcaliman]]

Attachment: Bug 1556457 - Ensure node screenshots still work after page navigation. r=yulia

When navigating away from the current page, ensure the Screenshot actor has the correct reference to the current window.document so subsequent screenshot actions don't reference a dead node pointing to the previous window.document.

Comment 7

[Yulia Startsev [:yulia]]

Regarding that error -- the code that we currently use for screenshots will likely be replaced with a fission compatible version that is the same across firefox.