refactor(cli): Simplify namespace update-limit command code

influxdb_iox/src/commands/namespace/mod.rs

@@ -16,9 +16,6 @@ pub enum Error {
 
     #[error("Client error: {0}")]
     ClientError(#[from] influxdb_iox_client::error::Error),
-
-    #[error("No valid limit was provided")]
-    InvalidLimit,
 }
 
 pub type Result<T, E = Error> = std::result::Result<T, E>;

influxdb_iox/src/commands/namespace/update_limit.rs

@@ -1,7 +1,7 @@
 use influxdb_iox_client::connection::Connection;
 use influxdb_iox_client::namespace::generated_types::LimitUpdate;
 
-use crate::commands::namespace::{Error, Result};
+use crate::commands::namespace::Result;
 
 #[derive(Debug, clap::Parser)]
 pub struct Config {
@@ -15,6 +15,12 @@ pub struct Config {
 
 #[derive(Debug, clap::Args)]
 #[clap(group(
+            // This arg group "limit" links the members of the below struct 
+            // named "max_tables" and "max_columns_per_table" together as 
+            // mutually exclusive flags. As we specify all flags & commands
+            // using clap-derive rather than the imperative builder, v3 only
+            // properly supports this kind of behaviour in a macro code block.
+            // NOTE: It takes the variable names and not the flag long names.
             clap::ArgGroup::new("limit")
                 .required(true)
                 .args(&["max_tables", "max_columns_per_table"])
@@ -29,22 +35,20 @@ struct Args {
     max_columns_per_table: Option<i32>,
 }
 
-impl TryFrom<Args> for LimitUpdate {
-    type Error = Error;
-    fn try_from(args: Args) -> Result<Self> {
+impl From<Args> for LimitUpdate {
+    fn from(args: Args) -> Self {
         let Args {
             max_tables,
             max_columns_per_table,
         } = args;
 
         if let Some(n) = max_tables {
-            return Ok(Self::MaxTables(n));
+            return Self::MaxTables(n);
         }
         if let Some(n) = max_columns_per_table {
-            return Ok(Self::MaxColumnsPerTable(n));
+            return Self::MaxColumnsPerTable(n);
         }
-
-        Err(Error::InvalidLimit)
+        unreachable!();
     }
 }
 
@@ -54,7 +58,7 @@ pub async fn command(connection: Connection, config: Config) -> Result<()> {
     let namespace = client
         .update_namespace_service_protection_limit(
             &config.namespace,
-            LimitUpdate::try_from(config.args)?,
+            LimitUpdate::from(config.args),
         )
         .await?;
     println!("{}", serde_json::to_string_pretty(&namespace)?);