swarm-controller: read the queue client secret from the store, drop the file

The controller's OIDC client secret (client `swarm-controller`, used for
the queue connection, the auth-bridge bearer and the OTLP push) came from
an operator-placed file, `deploy.swarm-controller.queue.clientSecretFile`,
handed in by `LoadCredential=`.

Now `swarm-secret-publish`, which already copies authelia's minted OIDC
secrets into the store, also publishes this one, to
`swarm/controller/swarm-controller/oidc/client`. That path sits under
`controller/`, which no hive's policy reads. The controller reads it once
at start with its existing store certificate and holds it in memory, as
`swarm_queue_client::ClientSecret::Value`. If the store is down, it
retries for about a minute and then fails the start, so `Restart=` tries
again.

Policy delta: the controller gets `read` on that leaf, and the publisher
gets `create`/`update` on that leaf.

Removed: the `queue.clientSecretFile` option (both spellings, now removed
options with a message), its singleHostSwarm default, the credential and
placeholder, and the path watcher plus its restart oneshot. A controller
without a store identity is now an eval error, because it has no other
way to get the secret.

swarm-queue-client/src/lib.rs

@@ -347,12 +347,9 @@ pub struct QueueConfig {
     pub token_endpoint: String,
     /// The controller's own `OAuth2` client id.
     pub client_id: String,
-    /// File holding the client secret's PLAINTEXT.
-    ///
-    /// A path and not a value: the secret is minted on the authelia host and
-    /// read here, and putting it in the environment would publish it to
-    /// anything that can read `/proc/<pid>/environ`.
-    pub client_secret_file: PathBuf,
+    /// Where the client secret's plaintext comes from. Never the environment,
+    /// which would publish it to anything that can read `/proc/<pid>/environ`.
+    pub client_secret: ClientSecret,
     /// Extra trust anchor for the token endpoint and the queue itself, when
     /// they are not signed by a publicly-trusted CA.
     ///
@@ -368,6 +365,55 @@ pub struct QueueConfig {
     pub ca_file: Option<PathBuf>,
 }
 
+/// Where a [`QueueConfig`]'s client secret comes from.
+#[derive(Clone)]
+pub enum ClientSecret {
+    /// A file holding the plaintext, read at every token mint so a rotated
+    /// file takes effect without a restart.
+    File(PathBuf),
+    /// The plaintext itself, held only in this process's memory.
+    Value(String),
+}
+
+/// Redacts [`ClientSecret::Value`]: a [`QueueConfig`] is `Debug`, and printing
+/// one must not put the secret in a log.
+impl std::fmt::Debug for ClientSecret {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Self::File(path) => f.debug_tuple("File").field(path).finish(),
+            Self::Value(_) => f.write_str("Value(<redacted>)"),
+        }
+    }
+}
+
+impl ClientSecret {
+    async fn read(&self) -> Result<String, Error> {
+        match self {
+            Self::File(path) => {
+                tokio::fs::read_to_string(path)
+                    .await
+                    .map_err(|source| Error::ClientSecret {
+                        path: path.display().to_string(),
+                        source,
+                    })
+            }
+            Self::Value(value) => Ok(value.clone()),
+        }
+    }
+
+    fn read_blocking(&self) -> Result<String, Error> {
+        match self {
+            Self::File(path) => {
+                std::fs::read_to_string(path).map_err(|source| Error::ClientSecret {
+                    path: path.display().to_string(),
+                    source,
+                })
+            }
+            Self::Value(value) => Ok(value.clone()),
+        }
+    }
+}
+
 impl QueueConfig {
     /// Read the config from `<prefix>_NATS_URL`, `<prefix>_OIDC_TOKEN_ENDPOINT`,
     /// `<prefix>_OIDC_CLIENT_ID` and `<prefix>_OIDC_CLIENT_SECRET_FILE`, or
@@ -404,7 +450,7 @@ impl QueueConfig {
                 url,
                 token_endpoint,
                 client_id,
-                client_secret_file: PathBuf::from(secret),
+                client_secret: ClientSecret::File(PathBuf::from(secret)),
                 ca_file,
             })),
             // A partially-set environment is a deployment bug, and the failure
@@ -433,12 +479,7 @@ async fn mint_token(
 ) -> Result<CachedToken, Error> {
     // Read per call rather than caching: the file is small, and a cached
     // secret would survive a rotation that the operator believes took effect.
-    let secret = tokio::fs::read_to_string(&cfg.client_secret_file)
-        .await
-        .map_err(|source| Error::ClientSecret {
-            path: cfg.client_secret_file.display().to_string(),
-            source,
-        })?;
+    let secret = cfg.client_secret.read().await?;
 
     let response = token_request(http, cfg, secret.trim(), audience, scope)
         .send()
@@ -590,11 +631,7 @@ pub fn mint_token_for_blocking(
     // Read per call rather than caching — see `mint_token`'s identical
     // comment on the async path; the reasoning does not change with the
     // client type.
-    let secret =
-        std::fs::read_to_string(&cfg.client_secret_file).map_err(|source| Error::ClientSecret {
-            path: cfg.client_secret_file.display().to_string(),
-            source,
-        })?;
+    let secret = cfg.client_secret.read_blocking()?;
 
     let mut form = vec![("grant_type", "client_credentials")];
     if let Some(audience) = audience {
@@ -869,12 +906,32 @@ mod tests {
         }
     }
 
+    #[test]
+    fn a_held_secret_is_read_back_as_itself() {
+        let secret = ClientSecret::Value("s3cret".to_owned());
+        assert_eq!(
+            secret.read_blocking().expect("a value needs no I/O"),
+            "s3cret"
+        );
+    }
+
+    #[test]
+    fn a_debug_print_of_a_config_does_not_carry_a_held_secret() {
+        let cfg = QueueConfig {
+            client_secret: ClientSecret::Value("s3cret".to_owned()),
+            ..token_cfg()
+        };
+        let printed = format!("{cfg:?}");
+        assert!(!printed.contains("s3cret"), "{printed}");
+        assert!(printed.contains("<redacted>"), "{printed}");
+    }
+
     fn token_cfg() -> QueueConfig {
         QueueConfig {
             url: "nats://127.0.0.1:4222".to_owned(),
             token_endpoint: "https://auth.example.com/api/oidc/token".to_owned(),
             client_id: "hive-alpha".to_owned(),
-            client_secret_file: PathBuf::from("/nonexistent"),
+            client_secret: ClientSecret::File(PathBuf::from("/nonexistent")),
             ca_file: None,
         }
     }