Fix Multipart consuming payload before header checks

What
--
Split up logic in the constructor into two functions:

- **from_boundary:** build Multipart from boundary and stream
- **from_error:** build Multipart for MultipartError

Also we make the `boundary`, `from_boundary`, `from_error`  methods public within the crate so that we can use them in the extractor.

The extractor is then able to perform header checks and only consume the
payload if the checks pass.

actix-multipart/CHANGES.md

@@ -1,6 +1,7 @@
 # Changes
 
 ## Unreleased - 2020-xx-xx
+* Fix multipart consuming payload before header checks #1513
 
 
 ## 3.0.0 - 2020-09-11

actix-multipart/src/extractor.rs

@@ -36,6 +36,9 @@ impl FromRequest for Multipart {
 
     #[inline]
     fn from_request(req: &HttpRequest, payload: &mut Payload) -> Self::Future {
-        ok(Multipart::new(req.headers(), payload.take()))
+        ok(match Multipart::boundary(req.headers()) {
+            Ok(boundary) => Multipart::from_boundary(boundary, payload.take()),
+            Err(err) => Multipart::from_error(err),
+        })
     }
 }

actix-multipart/src/server.rs

@@ -64,26 +64,13 @@ impl Multipart {
         S: Stream<Item = Result<Bytes, PayloadError>> + Unpin + 'static,
     {
         match Self::boundary(headers) {
-            Ok(boundary) => Multipart {
+            Ok(boundary) => Multipart::from_boundary(boundary, stream),
-                error: None,
+            Err(err) => Multipart::from_error(err),
-                safety: Safety::new(),
-                inner: Some(Rc::new(RefCell::new(InnerMultipart {
-                    boundary,
-                    payload: PayloadRef::new(PayloadBuffer::new(Box::new(stream))),
-                    state: InnerState::FirstBoundary,
-                    item: InnerMultipartItem::None,
-                }))),
-            },
-            Err(err) => Multipart {
-                error: Some(err),
-                safety: Safety::new(),
-                inner: None,
-            },
         }
     }
 
     /// Extract boundary info from headers.
-    fn boundary(headers: &HeaderMap) -> Result<String, MultipartError> {
+    pub(crate) fn boundary(headers: &HeaderMap) -> Result<String, MultipartError> {
         if let Some(content_type) = headers.get(&header::CONTENT_TYPE) {
             if let Ok(content_type) = content_type.to_str() {
                 if let Ok(ct) = content_type.parse::<mime::Mime>() {
@@ -102,6 +89,32 @@ impl Multipart {
             Err(MultipartError::NoContentType)
         }
     }
+
+    /// Create multipart instance for given boundary and stream
+    pub(crate) fn from_boundary<S>(boundary: String, stream: S) -> Multipart
+    where
+        S: Stream<Item = Result<Bytes, PayloadError>> + Unpin + 'static,
+    {
+        Multipart {
+            error: None,
+            safety: Safety::new(),
+            inner: Some(Rc::new(RefCell::new(InnerMultipart {
+                boundary,
+                payload: PayloadRef::new(PayloadBuffer::new(Box::new(stream))),
+                state: InnerState::FirstBoundary,
+                item: InnerMultipartItem::None,
+            }))),
+        }
+    }
+
+    /// Create Multipart instance from MultipartError
+    pub(crate) fn from_error(err: MultipartError) -> Multipart {
+        Multipart {
+            error: Some(err),
+            safety: Safety::new(),
+            inner: None,
+        }
+    }
 }
 
 impl Stream for Multipart {