upki: add support for system-wide configuration/caching (alt)

[ctz]

This is an alternative of #106

[codspeed-hq]

Merging this PR will not alter performance

✅ 4 untouched benchmarks

---

_{Comparing jbp-system-dirs (1ca91cd) with main (cb0df8d)}

[djc]

Why should this be scoped to Unix only?

[ctz]

Why should this be scoped to Unix only?

Just trying to avoid looking at c:\etc\upki\config.toml on windows, which is -- at best -- not helpful.

[djc]

I think one of my PRs had a global path for Windows too.

[ctz]

Sorry, which one is that?

[djc]

Ah, maybe I never pushed it, but I found this in my git stash stack:

djc-2021 system-dirs upki $ git show stash@{1}
commit ed57832313a15096805019b55135426b3ad5bc70
Merge: 46f51ba e8f9072
Author: Dirkjan Ochtman <dirkjan@ochtman.nl>
Date:   Fri Apr 17 16:50:45 2026 +0200

    WIP on system-dirs: 46f51ba upki: take PathKind for Config construction

diff --cc upki/src/lib.rs
index 6c67473,6c67473..d011502
--- a/upki/src/lib.rs
+++ b/upki/src/lib.rs
@@@ -101,6 -101,6 +101,8 @@@ impl AsRef<Path> for ConfigPath 
  /// What kind of path is being determined.
  #[derive(Clone, Copy, Debug)]
  pub enum PathKind {
++    /// System-wide configuration and data.
++    System,
      /// User-relative configuration and data.
      User,
  }
@@@ -108,17 -108,17 +110,69 @@@
  impl PathKind {
      fn config_dir(self) -> Result<PathBuf, Error> {
          Ok(match self {
++            Self::System => platform::system_config_dir()?,
              Self::User => project_dirs()?.config_dir().to_owned(),
          })
      }
  
      fn cache_dir(self) -> Result<PathBuf, Error> {
          Ok(match self {
++            Self::System => platform::system_cache_dir()?,
              Self::User => project_dirs()?.cache_dir().to_owned(),
          })
      }
  }
  
++#[cfg(target_os = "macos")]
++mod platform {
++    use super::*;
++
++    pub(super) fn system_config_dir() -> Result<PathBuf, Error> {
++        Ok(PathBuf::from("/Library/Application Support").join(PREFIX))
++    }
++
++    pub(super) fn system_cache_dir() -> Result<PathBuf, Error> {
++        Ok(PathBuf::from("/Library/Caches").join(BUNDLE_ID))
++    }
++}
++
++#[cfg(target_os = "linux")]
++mod platform {
++    use super::*;
++
++    pub(super) fn system_config_dir() -> Result<PathBuf, Error> {
++        Ok(PathBuf::from("/etc").join(PREFIX))
++    }
++
++    pub(super) fn system_cache_dir() -> Result<PathBuf, Error> {
++        Ok(PathBuf::from("/var/cache").join(PREFIX))
++    }
++}
++
++#[cfg(target_os = "windows")]
++mod platform {
++    use super::*;
++
++    pub(super) fn system_config_dir() -> Result<PathBuf, Error> {
++        match env::var_os("ProgramData") {
++            Ok(base) => PathBuf::from(base)
++                .join(VENDOR)
++                .join(PREFIX),
++            Err(_) => return Err(Error::NoConfigDirectoryFound),
++        }
++    }
++
++    pub(super) fn system_cache_dir() -> Result<PathBuf, Error> {
++        match env::var_os("ProgramData") {
++            Ok(base) => PathBuf::from(base)
++                .join(VENDOR)
++                .join(PREFIX)
++                .join("cache"),
++            Err(_) => return Err(Error::NoCacheDirectoryFound),
++        }
++    }
++}
++
  /// Errors for the upki library API.
  #[non_exhaustive]
  #[derive(Debug)]
@@@ -181,5 -181,5 +235,8 @@@ fn project_dirs() -> Result<ProjectDirs
      ProjectDirs::from("dev", "rustls", PREFIX).ok_or(Error::NoValidHomeDirectory)
  }
  
++const BUNDLE_ID: &str = "dev.rustls.upki";
++#[cfg(target_os = "windows")]
++const VENDOR: &str = "rustls";
  const PREFIX: &str = "upki";
  const CONFIG_FILE: &str = "config.toml";