validate --data-dir is not a file before startup

zingo-cli/Cargo.toml

@@ -26,3 +26,6 @@ shellwords = { workspace = true }
 tokio = { workspace = true }
 tracing-subscriber = { workspace = true }
 zip32 = { workspace = true }
+
+[dev-dependencies]
+tempfile = "3"

zingo-cli/src/lib.rs

@@ -398,7 +398,16 @@
         };
 
         let data_dir = if let Some(dir) = matches.get_one::<String>("data-dir") {
-            PathBuf::from(dir.clone())
+            let path = PathBuf::from(dir.clone());
+            if path.is_file() {
+                return Err(format!(
+                    "--data-dir must be a directory, not a file.\n\
+                     You provided: {dir}\n\
+                     Hint: use the parent directory instead, e.g. --data-dir {}",
+                    path.parent().map(|p| p.display().to_string()).unwrap_or_else(|| ".".into())
+                ));
+            }
+            path
         } else {
             PathBuf::from("wallets")
         };

zingo-cli/src/tests.rs

@@ -500,6 +500,14 @@ mod config_template {
             let err = fill(&[examples::BIN_NAME, "--server", "https://example.com"]).unwrap_err();
             assert!(err.contains("scheme"));
         }
+
+        #[test]
+        fn data_dir_is_a_file() {
+            let tmp = tempfile::NamedTempFile::new().unwrap();
+            let file_path = tmp.path().to_str().unwrap();
+            let err = fill(&[examples::BIN_NAME, "--data-dir", file_path]).unwrap_err();
+            assert!(err.contains("must be a directory, not a file"));
+        }
     }
 
     mod zingo_config {