enroll: spool a token minted outside the server, adopted on first use

A second process cannot append a token to the config: the server holds
its config in memory and writes it back whole, so the append loses the
race with the next save, silently. spool_pending writes one file per
token, named by the hash, into a private directory; take_pending lets
the running server move it into its own config the first time the phone
presents it, and sweeps anything older than an hour unused. This is what
lets a tool -- Dev Updater -- ask for an enrolment link without being at
the terminal the QR is printed on.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

server/src/certs.rs

@@ -155,10 +155,7 @@ mod tests {
     }
 
     fn addresses() -> Vec<IpAddr> {
-        vec![
-            "192.168.1.5".parse().unwrap(),
-            "127.0.0.1".parse().unwrap(),
-        ]
+        vec!["192.168.1.5".parse().unwrap(), "127.0.0.1".parse().unwrap()]
     }
 
     #[test]

server/src/enroll.rs

@@ -24,7 +24,10 @@
 //! middleware, which stays in each project because it is generic over
 //! that project's state.
 
+use std::fs;
 use std::net::IpAddr;
+use std::path::Path;
+use std::time::Duration;
 
 use anyhow::{Context, Result};
 use base64::Engine;
@@ -97,6 +100,74 @@ pub fn print_enrollment(scheme: &str, host: IpAddr, port: u16, token: &str) -> R
     Ok(())
 }
 
+/// How long a spooled enrolment stays valid unused. The link is meant to be
+/// opened straight away, by the tool that asked for it; one that was never
+/// opened should not stay a valid credential on disk.
+pub const PENDING_TTL: Duration = Duration::from_secs(60 * 60);
+
+/// Records a token minted by another process for the running server to
+/// adopt on first use -- see [`take_pending`].
+///
+/// Why a spool rather than writing the config: the server holds its config
+/// in memory and writes it back whole, so a second process appending a
+/// token to the file loses the race with the next save, silently. Here the
+/// other process writes only into `dir` (created private to the user), one
+/// file per token, named by the hash and holding the device name; the
+/// server owns the config as before and moves the entry across itself.
+/// Only the hash touches disk, as with every stored token.
+pub fn spool_pending(dir: &Path, name: &str, token: &str) -> Result<()> {
+    fs::create_dir_all(dir).with_context(|| format!("create {}", dir.display()))?;
+    #[cfg(unix)]
+    {
+        use std::os::unix::fs::PermissionsExt;
+        fs::set_permissions(dir, fs::Permissions::from_mode(0o700))
+            .with_context(|| format!("restrict {}", dir.display()))?;
+    }
+    let path = dir.join(token_hash_hex(token));
+    fs::write(&path, name).with_context(|| format!("write {}", path.display()))?;
+    Ok(())
+}
+
+/// Adopts a spooled token if `presented` is one: returns the device name
+/// it was spooled under and removes the entry, so a spooled token is
+/// consumed exactly once and belongs to the config from then on. Anything
+/// older than [`PENDING_TTL`] is removed rather than honoured.
+///
+/// A missing directory is the common case -- nothing has ever been
+/// spooled -- and answers `None` like an empty one.
+pub fn take_pending(dir: &Path, presented: &str) -> Result<Option<String>> {
+    let entries = match fs::read_dir(dir) {
+        Ok(entries) => entries,
+        Err(error) if error.kind() == std::io::ErrorKind::NotFound => return Ok(None),
+        Err(error) => return Err(error).with_context(|| format!("read {}", dir.display())),
+    };
+    let wanted = token_hash_hex(presented);
+    let mut found = None;
+    for entry in entries {
+        let entry = entry.with_context(|| format!("read {}", dir.display()))?;
+        let path = entry.path();
+        let fresh = entry
+            .metadata()
+            .and_then(|meta| meta.modified())
+            .ok()
+            .and_then(|modified| modified.elapsed().ok())
+            .is_some_and(|age| age < PENDING_TTL);
+        if !fresh {
+            let _ = fs::remove_file(&path);
+            continue;
+        }
+        let name = entry.file_name();
+        let name = name.to_string_lossy();
+        if bool::from(name.as_bytes().ct_eq(wanted.as_bytes())) {
+            found = Some(path);
+        }
+    }
+    let Some(path) = found else { return Ok(None) };
+    let device = fs::read_to_string(&path).with_context(|| format!("read {}", path.display()))?;
+    fs::remove_file(&path).with_context(|| format!("remove {}", path.display()))?;
+    Ok(Some(device))
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -143,4 +214,56 @@ mod tests {
         let other = enrollment_uri("aiapp", "10.66.0.1".parse().unwrap(), 8443, "tok");
         assert!(other.starts_with("aiapp://enroll?"));
     }
+
+    fn scratch_dir() -> std::path::PathBuf {
+        let dir = std::env::temp_dir().join(format!("wg-app-link-enroll-{}", generate_token()));
+        fs::create_dir_all(&dir).unwrap();
+        dir
+    }
+
+    /// A spooled token is adopted once, under the name it was spooled with,
+    /// and by nothing but that token.
+    #[test]
+    fn a_spooled_token_is_taken_exactly_once() {
+        let dir = scratch_dir();
+        let token = generate_token();
+        spool_pending(&dir, "tablet", &token).unwrap();
+        assert_eq!(take_pending(&dir, "wrong").unwrap(), None);
+        assert_eq!(
+            take_pending(&dir, &token).unwrap().as_deref(),
+            Some("tablet")
+        );
+        assert_eq!(take_pending(&dir, &token).unwrap(), None, "consumed");
+        assert!(
+            fs::read_dir(&dir).unwrap().next().is_none(),
+            "nothing left behind"
+        );
+        fs::remove_dir_all(dir).unwrap();
+    }
+
+    /// Nothing spooled -- not even the directory -- is an ordinary miss.
+    #[test]
+    fn no_spool_is_a_miss() {
+        let dir = scratch_dir().join("never-made");
+        assert_eq!(take_pending(&dir, "anything").unwrap(), None);
+    }
+
+    /// An entry past its age is swept rather than honoured.
+    #[test]
+    fn a_stale_entry_is_swept_not_honoured() {
+        let dir = scratch_dir();
+        let token = generate_token();
+        spool_pending(&dir, "old", &token).unwrap();
+        let path = dir.join(token_hash_hex(&token));
+        let past = std::time::SystemTime::now() - PENDING_TTL - Duration::from_secs(1);
+        fs::File::options()
+            .write(true)
+            .open(&path)
+            .unwrap()
+            .set_modified(past)
+            .unwrap();
+        assert_eq!(take_pending(&dir, &token).unwrap(), None);
+        assert!(!path.exists(), "swept");
+        fs::remove_dir_all(dir).unwrap();
+    }
 }