backport: Merge bitcoin#27254

[thepastaclaw]

Issue being fixed or feature implemented

Backports bitcoin#27254, the earliest remaining prerequisite in the bitcoin#29034 backport chain.

This PR previously bundled bitcoin#27254 and bitcoin#27419. It has been split back to one prerequisite at a time; bitcoin#27419 will follow separately.

What was done?

• Moved filesystem helpers from util/system into util/fs and util/fs_helpers.
• Updated direct includes for the moved filesystem APIs, including Dash-specific callers.
• Completed omitted platform-specific extraction hunks and the direct node/chainstate.cpp include.
• Preserved Dash's existing descriptive FileCommit() errors and RenameOver() behavior.
• Removed an unrelated util/bitdeque.h include copied from later history.

The final change is scoped to bitcoin#27254 only: 95 files changed, 468 insertions, and 393 deletions.

How Has This Been Tested?

On macOS:

git diff --check
python3 test/lint/lint-whitespace.py
python3 test/lint/lint-includes.py
python3 test/lint/lint-include-guards.py
python3 test/lint/lint-format-strings.py
python3 test/lint/lint-circular-dependencies.py
make -C src -j6 CCACHE_DIR=/private/tmp/dash-ccache test/test_dash
src/test/test_dash --run_test=fs_tests,util_tests,argsman_tests,getarg_tests,settings_tests

The targeted unit-test run completed 83 test cases with no errors.

Local pre-push backport prerequisite gate: passed at 8b4b8ad02fca18eec8dc226dba6fa50f0cbadef8 with no findings.

Breaking Changes

None expected. This is a filesystem-helper extraction with Dash-specific behavior preserved.

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have made corresponding changes to the documentation
• I have assigned this pull request to a milestone

[thepastaclaw]

Code Review

Source: exact-SHA automated review for 173bd17f8f3d023626255ece0b48ed50916afb57; dash-core-commit-history and backport-reviewer specialists ran. Opus/Claude reviewer and verifier lanes failed due quota (resets Jul 10, 8am America/Chicago); Codex gpt-5.5 reviewer lanes and Codex verifier completed. No existing GitHub review-thread context was present for this PR.

Note: posted as a COMMENT review because GitHub does not allow formal approval/request-changes on my own PR.

🔴 1 blocking

• Missing prerequisite: bitcoin#27254 (src/common/args.cpp:39-41)

  Upstream bitcoin#27419 starts after bitcoin#27254, where src/util/fs.h and src/util/fs_helpers.h exist and common/args.cpp includes them directly. At this exact Dash SHA, the full Dash merge f5dec3bce1a named in the PR description is not an ancestor; the branch only contains the older partial 27254 compatibility commit.

  The current backport works around that by forward-declaring RenameOver and GetSpecialFolderPath in src/common/args.cpp, while src/common/args.h still includes the old <fs.h> and exports GetDefaultDataDir instead of getting it from util/fs_helpers.h. That may be an intentional Dash adaptation, but then the PR/commit description needs to explicitly say Dash is keeping the partial 27254 layout and explain these deviations. Otherwise, backport the full bitcoin#27254 prerequisite first.

  Policy gate (backport-prereq-restore): For full upstream backport PRs, a missing prerequisite is blocking unless explicitly allowlisted, for example with intentional_exclusion / policy_override evidence.

🤖 Prompt for all review comments with AI agents

These findings are from an automated code review. Verify each finding against the current code and only fix it if needed.

In `src/common/args.cpp`:
- [BLOCKING] src/common/args.cpp:39-41: Missing prerequisite: bitcoin#27254
  Upstream bitcoin#27419 starts after bitcoin#27254, where `src/util/fs.h` and `src/util/fs_helpers.h` exist and `common/args.cpp` includes them directly. At this exact Dash SHA, the full Dash merge `f5dec3bce1a` named in the PR description is not an ancestor; the branch only contains the older partial 27254 compatibility commit. Either backport the full bitcoin#27254 prerequisite first or explicitly document that Dash is intentionally keeping the partial 27254 layout and that these declarations/header placements are deliberate.

[thepastaclaw]

✅ Review complete (commit e86f8b3)

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