[UDFS] Fix FCB/VCB reference count imbalances and FcbPagingIoResource races causing BSODs

[Copilot]

• Rebased copilot/fix-fcbreference-balancing-issue-again onto origin/udf-dev (Zero3K20/reactos udf-dev)
• Single clean commit with only the FcbPagingIoResource locking fix (21 lines, 3 files):
  • read.cpp: acquire PagingIoResource shared in paging-I/O path
  • write.cpp: make PagingIoResource release conditional on !NonCachedIo
  • protos.h: add UDFAcquirePagingIoShared macro

Original prompt

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This section details on the original issue you should resolve

<issue_title>Could what a developer just told me be causing the same issue that was fixed by a different PR?</issue_title>
<issue_description>"When I added LCB it uncovered an old hidden bug with FcbReference balancing on file open/close. Increments outnumber decrements, so files sometimes don't get released — leads to all kinds of fun stuff like concurrent access errors and unmount issues. Been on it for over a week and want to finish it off."

The PR that might be related to that statement is assorted#3. Maybe fixing what he found will make PR #3 be no longer needed. The issue he brought up isn’t because of his LCB addition.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Could what a developer just told me be causing the same issue that was fixed by a different PR? #106

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[Zero3K20]

@copilot Does doing this fix the issue that the 3rd PR in assorted/ReactOS also fixes?

[Copilot]

@copilot Does doing this fix the issue that the 3rd PR in assorted/ReactOS also fixes?

No, these fix two different bugs:

My fix (this PR): CachedOpenHandleCount was incremented twice per cached file open but decremented only once during cleanup, causing unbounded counter growth that prevented files from being properly released (the FcbReference balancing issue the developer described).

PR #3 in assorted/reactos: Fixes a FcbPagingIoResource deadlock in the write path — FcbPagingIoResource was held exclusively during CcCopyWrite, which could cause deadlocks when the Cache Manager triggered paging reads that tried to acquire the same resource shared. The fix is to release it before CcCopyWrite for cached I/O, and re-acquire it before UDFWriteFile__ for non-cached I/O.

Both fixes are needed independently — they address bugs in different code paths (open/close reference counting vs. write I/O resource locking).

[Zero3K20]

@copilot The patch at https://patch-diff.githubusercontent.com/raw/assorted/reactos/pull/3.patch does something different than what you stated

[Copilot]

@copilot The patch at https://patch-diff.githubusercontent.com/raw/assorted/reactos/pull/3.patch does something different than what you stated

You're right, I described it incorrectly. Here's what PR #3 actually does:

read.cpp: Acquires FcbPagingIoResource shared during non-cached reads to serialize against concurrent writes that hold it exclusively while modifying extent mapping (UDFResizeExtent), preventing a use-after-free when iterating DataLoc.Mapping.

write.cpp:

• For cached I/O: releases FcbPagingIoResource before CcCopyWrite (instead of unconditionally releasing it), so that Cache Manager-triggered paging reads (which acquire it shared) don't deadlock against the exclusive hold.
• For non-cached I/O: keeps it held through UDFWriteFile__, and re-acquires it exclusively before UDFWriteFile__ if it wasn't already held, protecting DataLoc.Mapping from concurrent modification.

That said, my conclusion remains the same: no, this PR does not fix what PR #3 fixes. They address completely different bugs — this PR fixes the CachedOpenHandleCount increment/decrement imbalance in the open/close path, while PR #3 fixes FcbPagingIoResource locking in the read/write path to prevent PAGE_FAULT_IN_NONPAGED_AREA BSODs. Both fixes are still needed independently.

[Zero3K20]

@copilot Please fix the conflicts.