test: refuse to test fs_errors on root environments (#364)

* fix(test): skip fs_errors test when running as root

Root (uid=0) bypasses Unix discretionary access controls, so removing
read permissions with chmod has no effect. This causes the fs_errors
test to fail because read_dir() succeeds instead of returning
Permission denied. Skip the test with an explanatory message when
euid is 0.

https://claude.ai/code/session_01Ki7wbrgQXWs2GXf8h2hybQ

* refactor(test): probe permissions instead of checking euid

Replace the `unsafe { libc::geteuid() }` check with a direct probe:
after chmod -r, try read_dir and skip if it still succeeds. This
removes the `unsafe` block and the `libc` dev-dependency while being
more semantically correct — it tests the actual condition we care
about rather than a proxy.

https://claude.ai/code/session_01Ki7wbrgQXWs2GXf8h2hybQ

* Revert "refactor(test): probe permissions instead of checking euid"

This reverts commit bb8e8cc.

* fix(test): fail loudly on root with cfg escape hatch for fs_errors

Instead of silently skipping, panic with an actionable error message
when running as root. The test can be excluded entirely via
`RUSTFLAGS='--cfg pdu_test_skip_fs_errors'` for environments like
Claude Code Web where root is unavoidable.

https://claude.ai/code/session_01Ki7wbrgQXWs2GXf8h2hybQ

* docs: improve error message

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* fix(test): allow unexpected_cfgs for pdu_test_skip_fs_errors

Suppress the -D unexpected-cfgs error by adding #[allow(unexpected_cfgs)]
before the custom cfg gate.

https://claude.ai/code/session_01Ki7wbrgQXWs2GXf8h2hybQ

* fix(test): register pdu_test_skip_fs_errors in check-cfg

The #[allow(unexpected_cfgs)] approach doesn't override -D warnings
used in CI release builds. Register the custom cfg in Cargo.toml's
[lints.rust] section instead, which properly declares it as a known
cfg to the compiler.

https://claude.ai/code/session_01Ki7wbrgQXWs2GXf8h2hybQ

* style(cargo): move [lints.rust] section after [dev-dependencies]

https://claude.ai/code/session_01Ki7wbrgQXWs2GXf8h2hybQ

* fix(test): gate fs_errors imports and helper with pdu_test_skip_fs_errors

Gate the Unix-specific imports and fs_permission helper with
cfg(all(unix, not(pdu_test_skip_fs_errors))) so they don't produce
dead code warnings when the test is skipped.

https://claude.ai/code/session_01Ki7wbrgQXWs2GXf8h2hybQ

* style(test): use separate cfg attributes instead of cfg(all(...))

https://claude.ai/code/session_01Ki7wbrgQXWs2GXf8h2hybQ

* fix(cargo): pin libc to exact patch version for consistency

https://claude.ai/code/session_01Ki7wbrgQXWs2GXf8h2hybQ

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: 3 people

Cargo.lock

@@ -86,7 +86,11 @@ zero-copy-pads = "0.2.0"
 [dev-dependencies]
 build-fs-tree = "0.8.1"
 command-extra = "1.0.0"
+libc = "0.2.182"
 maplit = "1.0.2"
 normalize-path = "0.2.1"
 pretty_assertions = "1.4.1"
 rand = "0.10.0"
+
+[lints.rust]
+unexpected_cfgs = { level = "warn", check-cfg = ['cfg(pdu_test_skip_fs_errors)'] }

Cargo.toml

@@ -10,8 +10,10 @@ use std::process::{Command, Output, Stdio};
 use text_block_macros::text_block;
 
 #[cfg(unix)]
+#[cfg(not(pdu_test_skip_fs_errors))]
 use maplit::btreeset;
 #[cfg(unix)]
+#[cfg(not(pdu_test_skip_fs_errors))]
 use parallel_disk_usage::{
     bytes_format::BytesFormat,
     data_tree::DataTree,
@@ -23,6 +25,7 @@ use parallel_disk_usage::{
     visualizer::{BarAlignment, ColumnWidthDistribution, Direction, Visualizer},
 };
 #[cfg(unix)]
+#[cfg(not(pdu_test_skip_fs_errors))]
 use std::{collections::BTreeSet, path::Path};
 
 fn stdio(command: Command) -> Command {
@@ -33,6 +36,7 @@ fn stdio(command: Command) -> Command {
 }
 
 #[cfg(unix)]
+#[cfg(not(pdu_test_skip_fs_errors))]
 fn fs_permission(path: impl AsRef<Path>, permission: &'static str, recursive: bool) {
     let Output { status, stderr, .. } = Command::new("chmod")
         .pipe(|cmd| if recursive { cmd.with_arg("-R") } else { cmd })
@@ -106,8 +110,17 @@ fn max_depth_0() {
 }
 
 #[cfg(unix)]
+#[cfg(not(pdu_test_skip_fs_errors))]
 #[test]
 fn fs_errors() {
+    if unsafe { libc::geteuid() } == 0 {
+        panic!(
+            "{}\n{}",
+            "error: This test must not be run as root because running with elevated privileges would affect its accuracy.",
+            "hint: Either run this test as a non-root user or set `RUSTFLAGS='--cfg pdu_test_skip_fs_errors'` to skip this test.",
+        );
+    }
+
     let workspace = SampleWorkspace::default();
     fs_permission(workspace.join("empty-dir"), "-r", false);
     fs_permission(workspace.join("nested").join("0"), "-r", false);