feat(cfg): turn groupnames into gids at parse time

src/serdes/inner_helpers.rs

@@ -0,0 +1,103 @@
+use std::ffi::CString;
+
+use libc::gid_t;
+use serde::{Deserialize, Deserializer, Serialize, de::Error};
+
+/// Private module until its worth it to promote from size to a crate module.
+mod __libc_wrappers {
+    use std::{ffi::CStr, mem::MaybeUninit};
+
+    use libc::c_char;
+
+    #[derive(Debug, thiserror::Error)]
+    pub enum GetgrnamError {
+        #[error("no matching group record found")]
+        NoMatchingRecord,
+        /// ace variant for unexpected errors, error handling is shallow here
+        #[error("unexpected getgrnam_r error")]
+        Unexpected,
+    }
+
+    pub fn get_group_by_name(cname: &CStr) -> Result<libc::gid_t, GetgrnamError> {
+        use GetgrnamError as E;
+
+        let name = cname.as_ptr();
+        let mut grp = MaybeUninit::uninit();
+        let mut buf = [c_char::from(0); 1024];
+        let mut result: MaybeUninit<*mut libc::group> = MaybeUninit::uninit();
+        let ret = unsafe {
+            libc::getgrnam_r(
+                name,
+                grp.as_mut_ptr(),
+                buf.as_mut_ptr(),
+                buf.len(),
+                result.as_mut_ptr(),
+            )
+        };
+
+        // SAFETY: Pointers inside `grp` point may point to the buffer `buf` so they cannot be
+        // moved outside of this function.
+
+        // > On success, getgrnam_r() [..] return zero, and set `*result` to `grp`.
+        //
+        // > If no matching group record was found, these functions return 0 and store NULL in
+        // `*result`.
+        //
+        // > In case of error, an error number is returned, and NULL stored in `*result`.
+        //
+        // SAFETY: Either way, `result` is initialized to a nullable pointer.
+        let result_ptr = unsafe { result.assume_init() };
+        if ret == 0 && result_ptr == grp.as_mut_ptr() {
+            let grp_gid = unsafe { grp.assume_init() }.gr_gid;
+            Ok(grp_gid)
+        } else if ret == 0 && result_ptr.is_null() {
+            Err(E::NoMatchingRecord)
+        } else {
+            // ret should be != 0 and result NULL, but any other case is also unexpected and I'm
+            // not doing much FFI error handling rn
+            Err(E::Unexpected)
+        }
+    }
+}
+
+#[derive(Serialize, Deserialize)]
+#[serde(untagged)]
+enum EitherTyp {
+    Gid(gid_t),
+    Groupname(String),
+}
+
+/// Deserialize either a gid number or a groupname into a [`gid_t`]
+#[allow(clippy::missing_errors_doc)]
+pub fn deserialize_group<'de, D>(d: D) -> Result<gid_t, D::Error>
+where
+    D: Deserializer<'de>,
+{
+    let either = EitherTyp::deserialize(d)?;
+
+    match either {
+        EitherTyp::Gid(gid) => Ok(gid),
+        EitherTyp::Groupname(groupname) => {
+            use __libc_wrappers::GetgrnamError as E;
+            use serde::de::Unexpected;
+
+            let cname = CString::new(groupname).map_err(|nul_err| {
+                D::Error::invalid_value(
+                    Unexpected::Bytes(&nul_err.into_vec()),
+                    &"a string without null bytes",
+                )
+            })?;
+
+            let gid = __libc_wrappers::get_group_by_name(&cname).map_err(|err| match err {
+                E::NoMatchingRecord => D::Error::invalid_value(
+                    Unexpected::Other("non-existent group"),
+                    &"existent group",
+                ),
+                E::Unexpected => D::Error::custom("caught an unexpected getgrnam error"),
+            })?;
+
+            Ok(gid)
+        }
+    }
+}
+

src/serdes/mod.rs

@@ -0,0 +1,16 @@
+use std::fmt::Debug;
+
+use libc::gid_t;
+use serde::{Deserialize, Serialize};
+
+pub mod inner_helpers;
+
+#[repr(transparent)]
+#[derive(Serialize, Deserialize)]
+pub struct Group(#[serde(deserialize_with = "inner_helpers::deserialize_group")] pub gid_t);
+
+impl Debug for Group {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        Debug::fmt(&self.0, f)
+    }
+}