830716 - DataCloneError is not helpful

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect)

Description

[David Teller [:Yoric] - still alive but not very active]

"DataCloneError: The object could not be cloned." is not a really helpful message. What part of my object could not be cloned? Or is it the transferable list that is somehow wrong?

Several times, I have found myself spending literally hours trying to find out why my object is not clonable. I would really like more helpful messages.

Comment 1

[Kyle Huey (Exited; not receiving bugmail, old account, do not use)]

This would require some JS side assistance.

Comment 2

[Andrew Sutherland [:asuth] (he/him)]

Attachment: Bad idea printfs when structured cloning runs out of ideas, v1

Here's a horrible patch I created.  If you do something like this on the main thread:

  w.postMessage({ f: function() { console.log('I want RPC!'); } })

You get this lovely printf to stdout:

  MT DataCloneError coming up on: function () { console.log('I want RPC!'); }

And if you do something like this to get yourself a fancy worker doing the same thing...

  blobby = new Blob(["postMessage({ f: function() { console.log('I want MTV!'); } })"], { type: 'application/javascript' })
  z = new Worker(URL.createObjectURL(blobby))

You get this equally lovely printf to stdout:

  WT DataCloneError coming up on: function () { console.log('I want MTV!'); }


And if you do something like this:

  sketchyFunc = function() { console.log('harmless'); };
  sketchyFunc.toString = function() { console.log('moohoohahaha'); return 'sucker!'; };

You get "moohoohahaha" in your console plus:

  MT DataCloneError coming up on: sucker!

(In case it's not clear, besides the whole printf thing, JS gets invoked on the stack with structured cloning going on, which seems like a horrible idea.  A less awful patch would just want XPConnect and JS's built-in toString thing going on.  I'm sure there's a way to do that.)


Also, this patch doesn't do anything with the many, many other sources of DataCloneErrors.  (https://dxr.mozilla.org/mozilla-central/search?q=DATA_CLONE_ERR)

Comment 3

[Andrew Sutherland [:asuth] (he/him)]

Also, note that this specific patch only touches DOM code.  I presume the "JS side assistance" khuey is referencing has to do with getting structured cloning to maintain some type of path traversal information and/or having the error type carry a string payload?  (Maybe also the XPConnect classes?  There seem to be classes involved instead of just bare integer NSRESULTs these days, so maybe things are less hopeless than they used to be when it comes to errors.)

Comment 4

[Kyle Huey (Exited; not receiving bugmail, old account, do not use)]

Right, it means that we don't know where the thing we're failing to clone is in relation to the root.  Also note that it's possible for cloning to fail for reasons other than just "unrecognized type" (e.g. you can't clone a Function) and Gecko will not get called about those.

Comment 5

[Ivan Kupalov]

The problem is still there for FF 66.
At least the type of the object would be nice to have ("cannot clone a Function" e.g.). Basically any reason would be better than nothing. Firefox prides itself for good development tools and I hope that it will keep the standard high.

Comment 6

[Tom S. (please needinfo tschuster)]

Attachment: Bug 830716 - Show class-name of object that failed to be cloned. r?smaug

This has one small wrinkle that I might fix in the future: DOM classes that are proxies
like window show up as "Proxy".

Comment 7

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/3bdf9fae7d34
Show class-name of object that failed to be cloned. r=smaug

Comment 8

[Cristina Cozmuta (:CrissCozmuta)]

Backed out for causing xpcshell failures on test_ext_runtime_sendMessage_errors.js. CLOSED TREE
Backout link
Push with failures
Link to failure log
Failure line :
TEST-UNEXPECTED-FAIL | xpcshell.ini:toolkit/components/extensions/test/xpcshell/test_ext_runtime_sendMessage_errors.js | xpcshell return code: 0

Comment 9

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/2e358af06e5a
Show class-name of object that failed to be cloned. r=smaug

Comment 10

[Marian-Vasile Laza]

Backed out changeset 2e358af06e5a (Bug 830716) for causing xpcshell failures on test_promise_unhandled_rejection.js.
Backout link
Push with failures <--> x2
Failure Log

Comment 11

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/6b76fb554f2f
Show class-name of object that failed to be cloned. r=smaug

Comment 12

[Norisz Fay [:noriszfay]]

https://hg.mozilla.org/mozilla-central/rev/6b76fb554f2f