Add support for error transformation

src/error.rs

@@ -31,7 +31,7 @@ use actix_web::{
 
 #[derive(Debug, thiserror::Error)]
 pub enum Error {
-    #[error("Error in file function, {0}")]
+    #[error("{0}")]
     FileFn(#[from] actix_web::Error),
     #[error("Error parsing payload, {0}")]
     Payload(#[from] PayloadError),

src/middleware.rs

@@ -101,7 +101,16 @@ where
         let fut = self.service.call(req);
 
         Box::pin(async move {
-            let uploaded = handle_multipart(multipart, form).await?;
+            let uploaded = match handle_multipart(multipart, form.clone()).await {
+                Ok(uploaded) => uploaded,
+                Err(e) => {
+                    if let Some(f) = form.transform_error.clone() {
+                        return Err((f)(e));
+                    } else {
+                        return Err(e.into());
+                    }
+                }
+            };
             let _ = tx.send(uploaded);
             fut.await
         })

src/types.rs

@@ -468,10 +468,11 @@ impl Map {
 /// ```
 #[derive(Clone)]
 pub struct Form {
-    pub max_fields: u32,
-    pub max_field_size: usize,
-    pub max_files: u32,
-    pub max_file_size: usize,
+    pub(crate) max_fields: u32,
+    pub(crate) max_field_size: usize,
+    pub(crate) max_files: u32,
+    pub(crate) max_file_size: usize,
+    pub(crate) transform_error: Option<Arc<dyn Fn(crate::error::Error) -> actix_web::Error>>,
     inner: Map,
 }
 
@@ -479,16 +480,33 @@ impl Form {
     /// Create a new form
     ///
     /// If you wish to provide your own executor, use the `with_executor` method.
+    ///
+    /// Default values are as follows
+    /// - max_fields: 100
+    /// - max_field_size: 10_000 bytes
+    /// - max_files: 20
+    /// - max_files_size: 10_000_000 bytes
     pub fn new() -> Self {
         Form {
             max_fields: 100,
             max_field_size: 10_000,
             max_files: 20,
             max_file_size: 10_000_000,
+            transform_error: None,
             inner: Map::new(),
         }
     }
 
+    /// Add an optional error handler to transform errors produced by the middleware
+    pub fn transform_error(
+        mut self,
+        f: impl Fn(crate::error::Error) -> actix_web::Error + 'static,
+    ) -> Self {
+        self.transform_error = Some(Arc::new(f));
+
+        self
+    }
+
     /// Set the maximum number of fields allowed in the upload
     ///
     /// The upload will error if too many fields are provided.