Merge bitcoin/bitcoin#28076: util: Replace std::filesystem with util/fs.h

bbbbdb0cd57d75a06357d2811363d30a498f4499 ci: Add filesystem lint check (MarcoFalke)
fada2f91108a56cc5c447bd6b6fac411e4d5cdca refactor: Replace <filesystem> with <util/fs.h> (MarcoFalke)

Pull request description:

  Using `std::filesystem` is problematic:

  * There is a `fs` namespace wrapper for it. So having two ways to achieve the same is confusing.
  * Not using the `fs` wrapper is dangerous and buggy, because it disables known bugs by deleting problematic functions.

  Fix all issues by removing use of it and adding a linter to avoid using it again in the future.

ACKs for top commit:
  TheCharlatan:
    ACK  bbbbdb0cd57d75a06357d2811363d30a498f4499
  fanquake:
    ACK bbbbdb0cd57d75a06357d2811363d30a498f4499 🦀

Tree-SHA512: 0e2d49742b08eb2635e6fce41485277cb9c40fe20b81017c391d3472a43787db1278a236825714ca1e41c9d2f59913865cfb0c649e3c8ab1fb598c849f80c660

4 files changed, 104 insertions, 0 deletions

diff --git a/test/lint/README.md b/test/lint/README.md
index c0889b59af..484008298b 100644
--- a/test/lint/README.md
+++ b/test/lint/README.md
@@ -13,6 +13,14 @@ DOCKER_BUILDKIT=1 docker build -t bitcoin-linter --file "./ci/lint_imagefile" ./
 Building the container can be done every time, because it is fast when the
 result is cached and it prevents issues when the image changes.
 
+test runner
+===========
+
+To run the checks in the test runner outside the docker, use:
+
+```sh
+( cd ./test/lint/test_runner/ && cargo fmt && cargo clippy && cargo run )
+```
 
 check-doc.py
 ============
diff --git a/test/lint/test_runner/Cargo.lock b/test/lint/test_runner/Cargo.lock
new file mode 100644
index 0000000000..ca83aa9331
--- /dev/null
+++ b/test/lint/test_runner/Cargo.lock
@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "test_runner"
+version = "0.1.0"
diff --git a/test/lint/test_runner/Cargo.toml b/test/lint/test_runner/Cargo.toml
new file mode 100644
index 0000000000..053ce43d6c
--- /dev/null
+++ b/test/lint/test_runner/Cargo.toml
@@ -0,0 +1,12 @@
+# Copyright (c) The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or https://opensource.org/license/mit/.
+
+[package]
+name = "test_runner"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
diff --git a/test/lint/test_runner/src/main.rs b/test/lint/test_runner/src/main.rs
new file mode 100644
index 0000000000..b7ec9ee3b2
--- /dev/null
+++ b/test/lint/test_runner/src/main.rs
@@ -0,0 +1,77 @@
+// Copyright (c) The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or https://opensource.org/license/mit/.
+
+use std::env;
+use std::path::PathBuf;
+use std::process::Command;
+use std::process::ExitCode;
+
+use String as LintError;
+
+/// Return the git command
+fn git() -> Command {
+    Command::new("git")
+}
+
+/// Return stdout
+fn check_output(cmd: &mut std::process::Command) -> Result<String, LintError> {
+    let out = cmd.output().expect("command error");
+    if !out.status.success() {
+        return Err(String::from_utf8_lossy(&out.stderr).to_string());
+    }
+    Ok(String::from_utf8(out.stdout)
+        .map_err(|e| format!("{e}"))?
+        .trim()
+        .to_string())
+}
+
+/// Return the git root as utf8, or panic
+fn get_git_root() -> String {
+    check_output(git().args(["rev-parse", "--show-toplevel"])).unwrap()
+}
+
+fn lint_std_filesystem() -> Result<(), LintError> {
+    let found = git()
+        .args([
+            "grep",
+            "std::filesystem",
+            "--",
+            "./src/",
+            ":(exclude)src/util/fs.h",
+        ])
+        .status()
+        .expect("command error")
+        .success();
+    if found {
+        Err(r#"
+^^^
+Direct use of std::filesystem may be dangerous and buggy. Please include <util/fs.h> and use the
+fs:: namespace, which has unsafe filesystem functions marked as deleted.
+            "#
+        .to_string())
+    } else {
+        Ok(())
+    }
+}
+
+fn main() -> ExitCode {
+    let test_list = [("std::filesystem check", lint_std_filesystem)];
+
+    let git_root = PathBuf::from(get_git_root());
+
+    let mut test_failed = false;
+    for (lint_name, lint_fn) in test_list {
+        // chdir to root before each lint test
+        env::set_current_dir(&git_root).unwrap();
+        if let Err(err) = lint_fn() {
+            println!("{err}\n^---- Failure generated from {lint_name}!");
+            test_failed = true;
+        }
+    }
+    if test_failed {
+        ExitCode::FAILURE
+    } else {
+        ExitCode::SUCCESS
+    }
+}