[cli] return, not exit, from get_migrations (#335)

Previously, the cli-internal method `get_migrations` would print an
error and call `std::process::exit` if no migrations directory was
found, despite the fact that it has a `Result` return type.

This caused unintended consequences in #100, which unconditionally
calls `get_migrations` on process start. It is expected that no
migrations directory would exist prior to invoking `butane init`, but
this invocation causes `butane init` to fail.

It's both less-surprising and more idiomatic rust for `get_migrations`
to return an error rather than failing.

Fixes #331, however this issue also highlights that we have a large
test gap with butane-cli. This change does not close that gap.

Author: Electron100

Cargo.lock

@@ -53,6 +53,7 @@ serde_json = "1.0"
 sqlparser = "0.55"
 syn = { version = "2", features = ["extra-traits", "full"] }
 tempfile = "3.10"
+thiserror = "2.0"
 tokio = { version = "1"}
 tokio-postgres = "0.7"
 tokio-test = { version = "0.4"}

Cargo.toml

@@ -36,3 +36,4 @@ nonempty.workspace = true
 quote = { workspace = true }
 serde = { workspace = true }
 serde_json = { workspace = true }
+thiserror = { workspace = true }

butane_cli/Cargo.toml

@@ -666,11 +666,11 @@ pub fn clean(base_dir: &Path) -> Result<()> {
 
 pub fn get_migrations(base_dir: &Path) -> Result<FsMigrations> {
     let root = base_dir.join("migrations");
-    if !root.is_dir() {
-        eprintln!("No butane migrations directory found. Add at least one model to your project and build.");
-        std::process::exit(1);
+    if root.is_dir() {
+        Ok(migrations::from_root(root))
+    } else {
+        Err(anyhow::Error::new(CliError::NoButaneMigrationsDir))
     }
-    Ok(migrations::from_root(root))
 }
 
 pub fn working_dir_path() -> PathBuf {
@@ -739,9 +739,20 @@ pub fn base_dir() -> PathBuf {
     current_directory
 }
 
+#[derive(thiserror::Error, Debug)]
+pub enum CliError {
+    #[error(
+        "No butane migrations directory found. Add at least one model to your project and build."
+    )]
+    NoButaneMigrationsDir,
+}
+
 pub fn handle_error(r: Result<()>) {
     if let Err(e) = r {
-        eprintln!("Encountered unexpected error: {e}");
+        match e.downcast_ref::<CliError>() {
+            Some(e2) => eprintln!("{e2}"),
+            None => eprintln!("Encountered unexpected error: {e}"),
+        }
         std::process::exit(1);
     }
 }

butane_cli/src/lib.rs

@@ -54,7 +54,7 @@ serde = { features = ["derive"], workspace = true }
 serde_json = { workspace = true }
 sqlparser = { workspace = true }
 syn = { workspace = true }
-thiserror = "2.0"
+thiserror = { workspace = true }
 uuid = { workspace = true, optional = true }
 
 [dev-dependencies]