After POC, I think the best way to do this is to move the files on creation in another directory.
The name of the file is saved in NSUserDefault.
On recovery, the crashdump is deleted.

On next startup, if there is no newer crash report in the breakpad directory, the crash report is uploaded. Then the uploading is restarted.
This check must be done before Breakpad starts uploading (otherwhise, we can have a false negative).
Q1: the safest way is to do the check on main thread before breakpad starts uploading. It will make a blocking call on main thread to list the current files in a directory.
Is that acceptable?

Q2: As we now only upload on real UTE, we may relax some of the privacy guards (only one report per session and switch to create reports every k ms to update the stack.
WDYT?

Q1: "blocking call on main thread" sounds scary. But you are only checking the existence of *1* file with name already saved in NSUserDefault, right? If so, even though the call is synchronous, it is still finite and more or less constant.

Q2: SGTM

Q1. Not really. We need to check if another report has been created after we generated our freeze report. This basically means iterating over all reports and check creation date. If a user only uses cell data, this can be a medium-sized list.

The alternatives could be to store the number of reports present in the crash directory and only upload if there are no new crash report. Checking the number of files in directory should be fast. We would have to update this number regularly to avoid the scenario were an old report is uploaded during the freeze, and a new is generated, we miss the change of report numbers.

Other alternative would be to make this check in the watchdog thread, and only start uploading reports once this is done.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e188d664530362d3f5c494e3811d94574decc682

commit e188d664530362d3f5c494e3811d94574decc682
Author: Olivier Robin <olivierrobin@chromium.org>
Date: Mon Jan 14 13:50:29 2019

Address privacy recommendations for UTE crash reports

Crash reports are only sent if the previous session did not
end with a crash.

Bug:  918862 
Change-Id: I9af14c619a7eaf10e4d27f209c550483396dd71e
Reviewed-on: https://chromium-review.googlesource.com/c/1405508
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Peter Lee <pkl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622442}
[modify] https://crrev.com/e188d664530362d3f5c494e3811d94574decc682/ios/chrome/browser/crash_report/BUILD.gn
[modify] https://crrev.com/e188d664530362d3f5c494e3811d94574decc682/ios/chrome/browser/crash_report/breakpad_helper.mm
[modify] https://crrev.com/e188d664530362d3f5c494e3811d94574decc682/ios/chrome/browser/crash_report/breakpad_helper_unittest.mm
[modify] https://crrev.com/e188d664530362d3f5c494e3811d94574decc682/ios/chrome/browser/crash_report/main_thread_freeze_detector.h
[modify] https://crrev.com/e188d664530362d3f5c494e3811d94574decc682/ios/chrome/browser/crash_report/main_thread_freeze_detector.mm