I'm working on creating a mojo file service for use by Content Decryption Modules running in the utility process. This service calls AsyncFileUtil::CreateOrOpen() when opening the file. Normally the file returned in the CreateOrOpenCallback is simply passed to mojo to send to the client, as the service doesn't do any I/O. However, if mojo destroys the service (due to a connection error, for example), then the callback doesn't run due to a weak_ptr, and the base::File is cleaned up on the services thread, which doesn't allow for blocking calls.

Example crash (Linux):
[7095:7095:1025/162344.748076:FATAL:thread_restrictions.cc(29)] Check failed: !g_blocking_disallowed.Get().Get(). Function marked as blocking was called from a scope that disallows blocking! If this task is running inside the TaskScheduler, it needs to have MayBlock() in its TaskTraits. Otherwise, consider making this blocking work asynchronous or, as a last resort, you may use ScopedAllowBlocking in a narrow scope.
#0 0x0000056297a7 base::debug::StackTrace::StackTrace()
#1 0x000005647e41 logging::LogMessage::~LogMessage()
#2 0x0000056b6b60 base::AssertBlockingAllowed()
#3 0x000005638837 base::File::Close()
#4 0x00000562ef09 base::File::~File()
#5 0x0000078b2c2d storage::(anonymous namespace)::RunCreateOrOpenCallback()
#6 0x0000078b510c _ZN4base8internal7InvokerINS0_9BindStateIPFvPN7storage26FileSystemOperationContextERKNS_17RepeatingCallbackIFvNS_4FileERKNS6_IFvvEEEEEES7_EJNS0_12OwnedWrapperIS4_EESD_EEEFvS7_EE3RunEPNS0_13BindStateBaseEOS7_
#7 0x0000041c32b9 base::internal::ReplyAdapter<>()
#8 0x000003b2423b _ZN4base8internal7InvokerINS0_9BindStateIPFvNS_12OnceCallbackIFvN10extensions12_GLOBAL__N_112UnpackResultEEEEPS6_EJS8_NS0_12OwnedWrapperIS6_EEEEEFvvEE7RunOnceEPNS0_13BindStateBaseE
#9 0x0000056add3e base::(anonymous namespace)::PostTaskAndReplyRelay::RunReplyAndSelfDestruct()