#34603 wallet: Fix detection of symlinks on Windows

Adds an `IsSymlink` filesystem helper that checks for `FILE_ATTRIBUTE_REPARSE_POINT` on Windows while delegating to `std::filesystem::is_symlink` on other platforms. It uses this helper in `ListDatabases` to prevent recursively traversing symlinked directories on Windows, and in `GetWalletPath` to reject symlinks passed as wallet files. It also re-enables symlink test coverage on Windows in `wallet_multiwallet.py`.

Problem: GCC's `libstdc++` on Windows does not implement symlink detection. Because of this, Windows builds fail to identify symlinks in wallet paths, allowing `ListDatabases` to follow directory symlinks unintentionally and causing `GetWalletPath` to misclassify file symlinks as regular files.

Evidence of importance:

• The PR references GCC's `libstdc++` source where Windows symlink detection remains a FIXME, causing `std::filesystem::is_symlink` and recursive directory iterators to fail on Windows cross-builds.
• Previously, `wallet_multiwallet.py` explicitly disabled symlink tests under Windows because symlink detection did not work as expected on cross-built Windows binaries.

Stale · Author silent 159d

No author activity for 159 days; a review may wait a long time for a response.

Waiting on (from the thread read):

• author: response to molnard's suggestion to add a nested-wallet junction test case 2026-09-22
• reviewer: review from hebasto after ping 2026-09-22

Strong

substantive support, no open objection (molnard)

Thread notes: Added molnard's Concept ACK with Windows 11 testing across MinGW, MSVC, and clang-cl. Noted pending review from hebasto following sedited's ping and author response to molnard's test suggestion.

Objections, suggestions, and questions:

Support:

• sedited (member) (ACK, 2026-06-07): The changes seem fine to me, but I would suggest adding a note in the description about hebasto's comment https://github.com/bitcoin/bitcoin/pull/34603#discussion_r2822782155 about why we're gating WI [not substantive]
• molnard (none) (Concept ACK, 2026-09-22) [tested: test/functional/wallet_multiwallet.py]: tested tip against base on Windows 11 across MinGW/GCC, MSVC, and clang-cl, confirming that recursive descent into junction subdirectories is resolved on MinGW while multiwallet tests pass

Participants:

• fanquake (member, question, 1 statements 2026-02-17 to 2026-02-17): asked whether symlinks work with LLVM/Clang on Windows
• hebasto (member, question, 3 statements 2026-02-18 to 2026-02-18): explained MSVC/clang-cl symlink and junction behavior, and asked if reparse point tags should be filtered
• luke-jr (contributor, objection, 5 statements 2026-02-18 to 2026-02-18): noted redundant IsSymlink calls and unnecessary directory checks, which were removed
• maflcko (member, neutral, 1 statements 2026-03-16 to 2026-03-16): spelling and capitalization nits
• sedited (member, support, 2 statements 2026-06-07 to 2026-09-22): utACKed and pinged hebasto for review
• molnard (none, support, 2 statements 2026-09-22 to 2026-09-22): cACK; tested on Windows 11 across MinGW, MSVC, and clang-cl, confirming the fix and suggesting an additional test case

66 lines under test/bench/ci.

• test/functional/wallet_multiwallet.py +27/-39
• src/util/fs_helpers.cpp +14/-0
• src/util/fs_helpers.h +7/-0
• src/wallet/db.cpp +6/-0
• src/wallet/wallet.cpp +3/-1

The short text the ranking pass compares across PRs.

Adds Windows symlink and reparse point detection via the Win32 API to work around GCC libstdc++'s lack of Windows symlink support. This fixes wallet directory traversal in ListDatabases so it avoids recursing into directory symlinks on Windows, and ensures GetWalletPath rejects symlinks to wallet files. It re-enables previously skipped symlink test assertions in the multiwallet functional test suite on Windows.

P3 · platform fix (ranking pass: Moved up: a supported-platform filesystem defect ranks above internal refactors.)

Adds a platform-specific filesystem primitive to correctly detect symlinks on Windows. While useful shared infrastructure, the utility function is straightforward and primarily created to serve wallet path checks.

Membership: Adds a new `IsSymlink` filesystem helper in `src/util/fs_helpers.cpp` and `src/util/fs_helpers.h` to work around MinGW/GCC `libstdc++` limitations on Windows.

Factors: security/stability 1, bug 1, performance 0, user value 1, leverage 0

P3 · bug fix

Fixes Windows wallet path resolution where missing symlink detection causes `ListDatabases` to recursively traverse symlinked directories and `GetWalletPath` to treat symlinks as regular files. This resolves an ongoing platform inconsistency that previously required disabling multiwallet symlink tests on Windows.

Membership: Carries the Wallet label, modifies `src/wallet/db.cpp` and `src/wallet/wallet.cpp`, and updates `test/functional/wallet_multiwallet.py`.

Factors: security/stability 1, bug 1, performance 0, user value 1, leverage 0