feat: Add only_up config field to prevent reversing migrations (#205)

README.md

@@ -65,6 +65,12 @@ migrations_dir = "migrations"
 #
 # Default: (unset) (use the embedded default migration templates)
 templates_dir = ".squill/templates"
+
+# Whether only up migrations should be allowed. This can be used to avoid
+# accidental data loss in shared environments.
+#
+# Default: false (allow down migrations)
+only_up = true
 ```
 
 Then, generate the first migration that sets up Squill's requirements:

squill-cli/src/main.rs

@@ -135,10 +135,13 @@ fn extract(fig: Figment) -> anyhow::Result<Config> {
         None
     };
 
+    let only_up: bool = extract_inner_or_default(&fig, "only_up")?;
+
     Ok(Config {
         database_connect_options,
         migrations_dir: migrations_dir.relative(),
         templates_dir: templates_dir.map(|dir| dir.relative()),
+        only_up,
     })
 }
 
@@ -410,7 +413,7 @@ async fn undo(config: &Config) -> anyhow::Result<()> {
     let mut conn = config.connect().await?;
 
     println!("Running down migration: {}", migration);
-    migration.down(&mut conn).await?;
+    migration.down(&mut conn, config.only_up).await?;
 
     Ok(())
 }
@@ -434,7 +437,7 @@ pub async fn redo(config: &Config) -> anyhow::Result<()> {
     let mut conn = config.connect().await?;
 
     println!("Running down migration: {}", migration);
-    migration.down(&mut conn).await?;
+    migration.down(&mut conn, config.only_up).await?;
 
     println!("Running up migration: {}", migration);
     migration.up(&mut conn).await?;

squill/src/config.rs

@@ -8,6 +8,9 @@ pub struct Config {
 
     pub migrations_dir: PathBuf,
     pub templates_dir: Option<PathBuf>,
+
+    /// Only allow up migrations to run.
+    pub only_up: bool,
 }
 
 impl Config {

squill/src/lib.rs

@@ -336,8 +336,18 @@ mod tests {
         let last = status.applied.last().unwrap();
         let last = status.available.get(last.id).unwrap();
 
+        // Pretend that only_up was set by default.
         let mut conn = config.connect().await.unwrap();
-        last.down(&mut conn).await.unwrap();
+        match last.down(&mut conn, true).await {
+            Err(MigrateError::OnlyUp) => (),
+
+            Err(err) => panic!("Unexpected error: {:?}", err),
+            Ok(_) => panic!("Unexpected success"),
+        }
+
+        // Now unset only_up to allow the reversal.
+        let mut conn = config.connect().await.unwrap();
+        last.down(&mut conn, false).await.unwrap();
 
         // Make sure the right tables exist
         let mut conn = config.connect().await.unwrap();

squill/src/migrate.rs

@@ -185,8 +185,11 @@ impl MigrationDirectory {
         Ok(())
     }
 
-    // TODO: Add some sort of "forward-only" flag that prevents down migrations.
-    pub async fn down(&self, conn: &mut PgConnection) -> Result<(), MigrateError> {
+    pub async fn down(&self, conn: &mut PgConnection, only_up: bool) -> Result<(), MigrateError> {
+        if only_up {
+            return Err(MigrateError::OnlyUp);
+        }
+
         let sql = std::fs::read_to_string(&self.down_path).map_err(|err| MigrateError::Read {
             path: self.down_path.to_path_buf(),
             err,
@@ -218,6 +221,9 @@ pub enum MigrateError {
 
     #[error("failed to execute migration: {0}")]
     Execute(sqlx::Error),
+
+    #[error("cannot execute down migration: not allowed with only_up")]
+    OnlyUp,
 }
 
 #[cfg(test)]

squill/src/testing.rs

@@ -47,6 +47,7 @@ impl TestEnv {
             database_connect_options: Some(self.database.connect_options.clone()),
             migrations_dir: self.migrations_dir.path().into(),
             templates_dir: Some(self.templates_dir.path().into()),
+            only_up: true,
         }
     }
 }