What happens with outstanding IRPs when a process terminates.

When Windows kernel terminates a process it inserts APC in each thread, in turn this APC calls PspExitThread that calls IoCancelThreadIo to cancel if possible all outstanding IRPs by calling IoCancelIrp and waits for IRP cancelation or completion. A thread waits only for IRPs that have been associated with a thread by calling IopQueueThreadIrp that adds IRP in a list of IRPs associated with a thread, the list head is IrpList field of the ETHREAD structure.

A thread will be blocked until any of the two conditions takes place

 - IrpList becomes empty, that means all outstanding IRPs completed in a normal way or were cancelled

 - 5 minutes timeout expired, in that case IopDisassociateThreadIrp is called to perform IRPs disassociation by removing them from IrpList and setting IRP->Tail.Overlay.Thread to NULL

Below is a call stack for a terminating thread with four outstanding IRPs ( marked yellow ).

        THREAD 870788e8  Cid 1100.0f5c  Teb: 7ffab000 Win32Thread: 00000000 WAIT: (DelayExecution) KernelMode Non-Alertable
            86d29460  SynchronizationEvent
        IRP List:
            87305dc8: (0006,0100) Flags: 00060a00  Mdl: 00000000
            86ecd6f8: (0006,0100) Flags: 00060a00  Mdl: 00000000
            862fd7b8: (0006,0100) Flags: 00060a00  Mdl: 00000000
            87221d80: (0006,0100) Flags: 00060a00  Mdl: 00000000
        Not impersonating
        DeviceMap                 975b0820
        Owning Process            8514a030       Image:         explorer.exe
        Attached Process          N/A            Image:         N/A
        Wait Start TickCount      22339          Ticks: 1 (0:00:00:00.015)
        Context Switch Count      2734           IdealProcessor: 1          
        UserTime                  00:00:00.000
        KernelTime                00:00:00.031
        Win32 Start Address 0x769842ed
        Stack Init a86bfed0 Current a86bfa38 Base a86c0000 Limit a86bd000 Call 0
        Priority 10 BasePriority 8 UnusualBoost 0 ForegroundBoost 2 IoPriority 2 PagePriority 2
        ChildEBP RetAddr
        a86bfa50 82ad269d nt!KiSwapContext+0x26 (FPO: [Uses EBP] [0,0,4])
        a86bfa88 82ad14f7 nt!KiSwapThread+0x266
        a86bfab0 82ad11d5 nt!KiCommitThreadWait+0x1df
        a86bfb0c 82cb9171 nt!KeDelayExecutionThread+0x2aa
        a86bfb40 82cbe519 nt!IoCancelThreadIo+0x70
        a86bfbb4 82cd2051 nt!PspExitThread+0x48e
        a86bfbcc 82b058c0 nt!PsExitSpecialApc+0x22
        a86bfc1c 82a922a4 nt!KiDeliverApc+0x28b
        a86bfc1c 778770b4 nt!KiServiceExit+0x64 (FPO: [0,3] TrapFrame @ a86bfc34)
        074fe3e4 00000000 ntdll!KiFastSystemCallRet (FPO: [0,0,0])

A main process thread waits for child threads termination.

        THREAD 86e307f0  Cid 1100.1104  Teb: 7ffdf000 Win32Thread: fe9c8a88 WAIT: (Executive) KernelMode Non-Alertable
            870788e8  Thread
        Not impersonating
        DeviceMap                 975b0820
        Owning Process            8514a030       Image:         explorer.exe
        Attached Process          N/A            Image:         N/A
        Wait Start TickCount      21665          Ticks: 675 (0:00:00:10.530)
        Context Switch Count      15565          IdealProcessor: 1          
        UserTime                  00:00:00.249
        KernelTime                00:00:00.530
        Win32 Start Address 0x00e50efa
        Stack Init a87b5ed0 Current a87b5a38 Base a87b6000 Limit a87b3000 Call 4f0
        Priority 12 BasePriority 8 UnusualBoost 0 ForegroundBoost 2 IoPriority 2 PagePriority 5

        ChildEBP RetAddr
        a87b5a50 82ad269d nt!KiSwapContext+0x26 (FPO: [Uses EBP] [0,0,4])
        a87b5a88 82ad14f7 nt!KiSwapThread+0x266
        a87b5ab0 82acb0cf nt!KiCommitThreadWait+0x1df
        a87b5b2c 82cbe28e nt!KeWaitForSingleObject+0x393
        a87b5bb4 82cd2051 nt!PspExitThread+0x203
        a87b5bcc 82b058c0 nt!PsExitSpecialApc+0x22
        a87b5c1c 82a922a4 nt!KiDeliverApc+0x28b
        a87b5c1c 77876fc0 nt!KiServiceExit+0x64 (FPO: [0,3] TrapFrame @ a87b5c34)
        000ffb18 00000000 ntdll!KiUserCallbackDispatcher (FPO: [0,0,0])