Refuse logging in if key is empty

Before the previous commit, an empty key file would be created if key
wasn't specified, and after the previous commit, the key file would not
be created if the key wasn't specified and stay empty if it was empty.

Now the log command checks the key file if a key is not specified and
exits with an error message if either the key file couldn't be opened or
is empty. If a key is specified, the key file is just created with it as
before.

src/command/client/sync/login.rs

@@ -1,8 +1,11 @@
 use std::io;
 
 use clap::Parser;
-use eyre::Result;
-use tokio::{fs::File, io::AsyncWriteExt};
+use eyre::{bail, Result};
+use tokio::{
+    fs::File,
+    io::{AsyncReadExt, AsyncWriteExt},
+};
 
 use atuin_client::{api_client, settings::Settings};
 use atuin_common::api::LoginRequest;
@@ -42,6 +45,23 @@ impl Cmd {
         let username = or_user_input(&self.username, "username");
         let key = or_user_input(&self.key, "encryption key [blank to use existing key file]");
         let password = self.password.clone().unwrap_or_else(read_user_password);
+
+        let key_path = settings.key_path.as_str();
+        if key.is_empty() {
+            let mut file = match File::open(key_path).await {
+                Err(_e) => bail!("key file couldn't be opened, key must be specified"),
+                Ok(f) => f,
+            };
+            let mut bytes = Vec::new();
+            file.read_to_end(&mut bytes).await?;
+            if bytes.is_empty() {
+                bail!("key file empty, key must be specified");
+            }
+        } else {
+            let mut file = File::create(key_path).await?;
+            file.write_all(key.as_bytes()).await?;
+        }
+
         let session = api_client::login(
             settings.sync_address.as_str(),
             LoginRequest { username, password },
@@ -52,12 +72,6 @@ impl Cmd {
         let mut file = File::create(session_path).await?;
         file.write_all(session.session.as_bytes()).await?;
 
-        if !key.is_empty() {
-            let key_path = settings.key_path.as_str();
-            let mut file = File::create(key_path).await?;
-            file.write_all(key.as_bytes()).await?;
-        }
-
         println!("Logged in!");
 
         Ok(())