Cope with missing 'tenantid' in '.zenith/config' file.

We generate the initial tenantid and store it in the file, so it shouldn't
be missing. But let's cope with it. (This comes handy with the bigger
changes I'm working on at https://github.com/zenithdb/zenith/pull/788)

control_plane/src/local_env.rs

@@ -39,9 +39,11 @@ pub struct LocalEnv {
     // Path to pageserver binary.
     pub zenith_distrib_dir: PathBuf,
 
-    // keeping tenant id in config to reduce copy paste when running zenith locally with single tenant
-    #[serde(with = "hex")]
-    pub tenantid: ZTenantId,
+    // Default tenant ID to use with the 'zenith' command line utility, when
+    // --tenantid is not explicitly specified.
+    #[serde(with = "opt_tenantid_serde")]
+    #[serde(default)]
+    pub default_tenantid: Option<ZTenantId>,
 
     // jwt auth token used for communication with pageserver
     pub auth_token: String,
@@ -173,7 +175,7 @@ pub fn init(
         pg_distrib_dir,
         zenith_distrib_dir,
         base_data_dir: base_path,
-        tenantid,
+        default_tenantid: Some(tenantid),
         auth_token,
         auth_type,
         private_key_path,
@@ -204,3 +206,30 @@ pub fn load_config() -> Result<LocalEnv> {
     let config = fs::read_to_string(repopath.join("config"))?;
     toml::from_str(config.as_str()).map_err(|e| e.into())
 }
+
+/// Serde routines for Option<ZTenantId>. The serialized form is a hex string.
+mod opt_tenantid_serde {
+    use serde::{Deserialize, Deserializer, Serialize, Serializer};
+    use std::str::FromStr;
+    use zenith_utils::zid::ZTenantId;
+
+    pub fn serialize<S>(tenantid: &Option<ZTenantId>, ser: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        tenantid.map(|t| t.to_string()).serialize(ser)
+    }
+
+    pub fn deserialize<'de, D>(des: D) -> Result<Option<ZTenantId>, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        let s: Option<String> = Option::deserialize(des)?;
+        if let Some(s) = s {
+            return Ok(Some(
+                ZTenantId::from_str(&s).map_err(serde::de::Error::custom)?,
+            ));
+        }
+        Ok(None)
+    }
+}