It looks like only the GTK code uses nsIPrintSetting.duplex. As far as I can tell, we don't do anything if this is set on macOS or Windows. We'll need to fix that in order for the new printing UI to implement the "Print on both sides" option and have it actually work.

Depends on D87958

Depends on D87959

I've attached patches that I think ought to make this work for Windows and macOS, but they are untested -- I don't have access to a printer with duplexing options, so it's hard to confirm whether they result in the expected behavior!

Erik, Emily: are either of you able to test this at all, by any chance (for either platform)? If so, that'd be awesome.

I read my printer's manual and it says that it supports Duplex printing for Windows, but not on macOS, unfortunately.

Sean, you said that you have a pretty fancy printer, right? Do you think you could test the duplex printing on macOS here?

Sorry, no fancy printer here. I used to have access to a color laser that did duplex, but no more.

OK, I suggest we just get this landed on the platform side, even though it's currently untested, so that as soon as it's exposed in the front-end anyone (QA?) with a duplex-capable printer can try it out and confirm if all works as expected.

Backed out for multiple perma failures.

Push with failure: https://treeherder.mozilla.org/#/jobs?repo=autoland&selectedTaskRun=SiLftjdTTKGyWpjJ3dS1Hw.0&resultStatus=pending%2Crunning%2Csuccess%2Ctestfailed%2Cbusted%2Cexception&searchStr=os%2Cx%2C10.14%2Cdebug%2Cweb%2Cplatform%2Ctests%2Ctest-macosx1014-64%2Fdebug-web-platform-tests-wdspec-e10s%2Cwd1&revision=b61a789151df460acffd32964a87719845db3f9a

Logs:
https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=313985450&repo=autoland&lineNumber=71272
https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=313992031&repo=autoland&lineNumber=812
https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=313992032&repo=autoland&lineNumber=6116
https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=313992034&repo=autoland&lineNumber=1436

Backout: https://hg.mozilla.org/integration/autoland/rev/c832b3901d262c04479934f930e09b3032955e4d

Depends on D87960

https://hg.mozilla.org/mozilla-central/rev/0451aa9d80c7
https://hg.mozilla.org/mozilla-central/rev/8387257bb40e
https://hg.mozilla.org/mozilla-central/rev/5d6896aa46ad

Reminder to request Beta approval.

Do we need to uplift this? I thought duplex was targeted for 82.

I guess not. It should be harmless to uplift though since it's currently unused code, and uplifting has the benefit that it makes it less likely that other printing patches that we want to uplift will encounter conflicts. I'd lean towards uplift, but up to you.

Comment on attachment 9171543 [details]
Bug 1659856 - patch 1 - Add IDL constants for duplexing options to nsIPrintSettings.

Beta/Release Uplift Approval Request

• User impact if declined: No direct user impact (unused code), but may make other uplifts more difficult over the next few weeks as there are likely to be other patches in nearby code that are needed for the Firefox 81 new printing UI.
• Is this code covered by automated tests?: No
• Has the fix been verified in Nightly?: No
• Needs manual test from QE?: No
• If yes, steps to reproduce:
• List of other uplifts needed: None
• Risk to taking this patch: Low
• Why is the change risky/not risky? (and alternatives if risky): Currently-unused code (preparing platform for a feature not yet exposed in the printing UI).
• String changes made/needed:

Comment on attachment 9172659 [details]
Bug 1659856 - followup - Also handle duplex field in nsPrintSettingsWin::InitWithInitializer. r=bobowen

(See comment 16 for uplift form.)

https://hg.mozilla.org/releases/mozilla-beta/rev/b72628def781
https://hg.mozilla.org/releases/mozilla-beta/rev/64addc77c426
https://hg.mozilla.org/releases/mozilla-beta/rev/c0c7074bc9dc
https://hg.mozilla.org/releases/mozilla-beta/rev/04db4148284c

Comment on attachment 9171543 [details]
Bug 1659856 - patch 1 - Add IDL constants for duplexing options to nsIPrintSettings.

Approved for 81.0b4.

https://hg.mozilla.org/mozilla-central/rev/0312e30104cc