hive-c0re: deliver a matrix credential through hive-priv, not by writing it

`deliver` read the value out of the secret store and wrote it itself, as the
`hive-core` user, at 0600. The file lands in a directory owned by the agent,
so it arrived owned by `hive-core` — the agent's matrix daemon woke on it
appearing and could not read its own credential. `priv_client::write_agent_
matrix_token` already existed and already had two callers; this was the one
path that did not use it.

hive-priv now owns the filename too, so the name the daemon's path unit globs
for is decided in one place instead of being built identically in two.

That move exposed a disagreement worth fixing rather than routing around.
The secret store accepts `[A-Za-z0-9_-]` for an account name; hive-priv's
`validate_name_chars` accepts lowercase, digits and hyphen only. An account is
an attribute name in `hyperhive.matrixAccounts`, typed `attrsOf` with no
charset constraint, so `Ops_Relay9` is a key an operator can already have
written — and it would have read out of the store and then failed to land.

So hive-priv grows `validate_account_name` rather than widening the existing
one: an agent name is an `Ident` and lowercase by design, an account name is an
attrset key, and one validator serving two name domains is what let them drift.

The test that caught this came from `credential.rs`, which used to build the
path. It moves to hive-priv with both of its controls intact, because the
controls are the point — they assert which names must be ACCEPTED, and a
validator narrower than the store's passes every rejection case. A second
moved test pins the `matrix-token` prefix where the name is now built; the
old one would have kept passing while asserting a function that no longer
decided anything.

Refs #3726

hive-priv/src/main.rs

@@ -546,13 +546,10 @@ fn write_matrix_token(
     homeserver: Option<&str>,
 ) -> Result<(String, String)> {
     validate_agent_name(agent_name)?;
-    let filename = match account {
-        None => "matrix-token".to_owned(),
-        Some(a) => {
-            validate_name_chars(a)?;
-            format!("matrix-token-{a}")
-        }
-    };
+    if let Some(a) = account {
+        validate_account_name(a)?;
+    }
+    let filename = matrix_token_filename(account);
     let res = write_agent_state_file(agent_name, &filename, &format!("{token}\n"))?;
     if let (Some(a), Some(hs)) = (account, homeserver) {
         let meta = serde_json::to_string(&MatrixAccountSidecar { homeserver: hs })
@@ -1364,6 +1361,20 @@ fn ensure_plain_filename(who: &str, filename: &str) -> Result<()> {
     Ok(())
 }
 
+/// Basename an agent's matrix token is written under — `matrix-token` for the
+/// hive's own account, `matrix-token-<account>` for an extra one.
+///
+/// Half of an agreement whose other half is a `systemd.paths` glob in
+/// `nix/agent-modules/matrix.nix`, which no test here can reach: a name that
+/// stopped matching `matrix-token*` would land a credential the agent's daemon
+/// never wakes for — no error, just a token that silently never arrives.
+fn matrix_token_filename(account: Option<&str>) -> String {
+    match account {
+        None => "matrix-token".to_owned(),
+        Some(a) => format!("matrix-token-{a}"),
+    }
+}
+
 /// Name the content is written under before being renamed onto `filename`.
 /// The leading dot is load-bearing rather than tidy: `nix/agent-modules/
 /// matrix.nix` starts the agent's matrix daemon on the glob `matrix-token*`,
@@ -2797,6 +2808,29 @@ fn validate_name_chars(name: &str) -> Result<()> {
     Ok(())
 }
 
+/// Validate a matrix account name, which is a wider charset than
+/// [`validate_name_chars`] allows on purpose.
+///
+/// An agent name is an `Ident` and lowercase by design. An account name is an
+/// attribute name in `hyperhive.matrixAccounts`, typed `attrsOf` with no
+/// charset constraint, so `Ops_Relay9` is a key an operator may already have
+/// written. `swarm_secret_client::path::checked_segment` accepts exactly this
+/// set for the same name in the secret store — the two must agree, or a
+/// credential reads out of the store and then fails to land on disk.
+///
+/// Still a single plain component: no `/`, no `.`, no whitespace, so it cannot
+/// climb out of the agent's state dir or name the dir itself.
+fn validate_account_name(name: &str) -> Result<()> {
+    if name.is_empty()
+        || !name
+            .bytes()
+            .all(|b| b.is_ascii_alphanumeric() || b == b'-' || b == b'_')
+    {
+        bail!("invalid account name {name:?}: must be non-empty [A-Za-z0-9_-]");
+    }
+    Ok(())
+}
+
 /// Validate a bind-mount path: must be absolute, non-empty, and contain
 /// no newlines, null bytes, double-quotes, or colons.
 ///
@@ -3061,8 +3095,9 @@ mod tests {
     use super::{
         BindMount, OwnedFd, PAUSED_MARKER_FILE, PrivRequest, check_fd_agreement,
         clear_runner_credentials, contains_secret_shaped_run, git_overlay_flags,
-        limits_dropin_body, partial_name, redact_secret_line, remove_marker_in, single_output_path,
-        toplevel_attr, write_agent_dir_file, write_state_file_nofollow,
+        limits_dropin_body, matrix_token_filename, partial_name, redact_secret_line,
+        remove_marker_in, single_output_path, toplevel_attr, validate_account_name,
+        write_agent_dir_file, write_state_file_nofollow,
     };
     use std::path::PathBuf;
     use std::sync::atomic::{AtomicU32, Ordering};
@@ -3403,6 +3438,39 @@ mod tests {
         std::fs::remove_dir_all(&dir).ok();
     }
 
+    /// Ported from `hive-c0re`'s `credential.rs`, which used to build this path
+    /// itself. The controls are the load-bearing half: an account name is an
+    /// attrset key in `hyperhive.matrixAccounts`, so uppercase and underscore
+    /// are names an operator can already have written, and the secret store
+    /// accepts exactly this set for the same name. A validator narrower than
+    /// the store's reads a credential out and then refuses to land it.
+    #[test]
+    fn an_account_name_is_checked_against_the_same_charset_the_store_uses() {
+        for bad in ["../argus", "a/b", "a b", "a.b", ""] {
+            assert!(
+                validate_account_name(bad).is_err(),
+                "account {bad:?} must be refused"
+            );
+        }
+        assert!(validate_account_name("ops-relay").is_ok());
+        assert!(validate_account_name("Ops_Relay9").is_ok());
+    }
+
+    /// Pinned here because here is where the name is decided. `hive-c0re` used
+    /// to assert this against a path helper of its own, which stopped deciding
+    /// anything the moment credential delivery started routing through this
+    /// process — a test that would have kept passing while the real filename
+    /// drifted.
+    #[test]
+    fn a_matrix_token_is_named_for_the_glob_the_daemon_watches() {
+        assert_eq!(matrix_token_filename(None), "matrix-token");
+        assert_eq!(matrix_token_filename(Some("ccc")), "matrix-token-ccc");
+        // And the temp the publish goes through must not match that same glob,
+        // which is only checkable now that both names are built in one place.
+        let tmp = partial_name(&matrix_token_filename(Some("ccc")));
+        assert!(!tmp.starts_with("matrix-token"), "got {tmp}");
+    }
+
     /// The temp name is the whole reason the rename is safe to watch: a
     /// `systemd.path` unit globbing `matrix-token*` would fire on a temp that
     /// merely suffixed the real name, on exactly the empty file the rename