Add 'TempFile::open()' to stream its data.

Resolves #2296.

core/lib/src/fs/temp_file.rs

@@ -10,7 +10,7 @@ use crate::fs::FileName;
 
 use tokio::task;
 use tokio::fs::{self, File};
-use tokio::io::AsyncWriteExt;
+use tokio::io::{AsyncWriteExt, AsyncBufRead, BufReader};
 use tempfile::{NamedTempFile, TempPath};
 use either::Either;
 
@@ -110,7 +110,7 @@ pub enum TempFile<'v> {
     },
     #[doc(hidden)]
     Buffered {
-        content: &'v str,
+        content: &'v [u8],
     }
 }
 
@@ -160,7 +160,7 @@ impl<'v> TempFile<'v> {
     ///
     ///     Ok(())
     /// }
-    /// # let file = TempFile::Buffered { content: "hi".into() };
+    /// # let file = TempFile::Buffered { content: "hi".as_bytes() };
     /// # rocket::async_test(handle(file)).unwrap();
     /// ```
     pub async fn persist_to<P>(&mut self, path: P) -> io::Result<()>
@@ -190,7 +190,7 @@ impl<'v> TempFile<'v> {
             }
             TempFile::Buffered { content } => {
                 let mut file = File::create(&new_path).await?;
-                file.write_all(content.as_bytes()).await?;
+                file.write_all(content).await?;
                 *self = TempFile::File {
                     file_name: None,
                     content_type: None,
@@ -231,7 +231,7 @@ impl<'v> TempFile<'v> {
     ///
     ///     Ok(())
     /// }
-    /// # let file = TempFile::Buffered { content: "hi".into() };
+    /// # let file = TempFile::Buffered { content: "hi".as_bytes() };
     /// # rocket::async_test(handle(file)).unwrap();
     /// ```
     pub async fn copy_to<P>(&mut self, path: P) -> io::Result<()>
@@ -258,7 +258,7 @@ impl<'v> TempFile<'v> {
             TempFile::Buffered { content } => {
                 let path = path.as_ref();
                 let mut file = File::create(path).await?;
-                file.write_all(content.as_bytes()).await?;
+                file.write_all(content).await?;
                 *self = TempFile::File {
                     file_name: None,
                     content_type: None,
@@ -296,7 +296,7 @@ impl<'v> TempFile<'v> {
     ///
     ///     Ok(())
     /// }
-    /// # let file = TempFile::Buffered { content: "hi".into() };
+    /// # let file = TempFile::Buffered { content: "hi".as_bytes() };
     /// # rocket::async_test(handle(file)).unwrap();
     /// ```
     pub async fn move_copy_to<P>(&mut self, path: P) -> io::Result<()>
@@ -313,6 +313,49 @@ impl<'v> TempFile<'v> {
         Ok(())
     }
 
+    /// Open the file for reading, returning an `async` stream of the file.
+    ///
+    /// This method should be used sparingly. `TempFile` is intended to be used
+    /// when the incoming data is destined to be stored on disk. If the incoming
+    /// data is intended to be streamed elsewhere, prefer to implement a custom
+    /// form guard via [`FromFormField`] that directly streams the incoming data
+    /// to the ultimate destination.
+    ///
+    /// # Example
+    ///
+    /// ```rust
+    /// # #[macro_use] extern crate rocket;
+    /// use rocket::fs::TempFile;
+    /// use rocket::tokio::io;
+    ///
+    /// #[post("/", data = "<file>")]
+    /// async fn handle(file: TempFile<'_>) -> std::io::Result<()> {
+    ///     let mut stream = file.open().await?;
+    ///     io::copy(&mut stream, &mut io::stdout()).await?;
+    ///     Ok(())
+    /// }
+    /// # let file = TempFile::Buffered { content: "hi".as_bytes() };
+    /// # rocket::async_test(handle(file)).unwrap();
+    /// ```
+    pub async fn open(&self) -> io::Result<impl AsyncBufRead + '_> {
+        use tokio_util::either::Either;
+
+        match self {
+            TempFile::File { path, .. } => {
+                let path = match path {
+                    either::Either::Left(p) => p.as_ref(),
+                    either::Either::Right(p) => p.as_path(),
+                };
+
+                let reader = BufReader::new(File::open(path).await?);
+                Ok(Either::Left(reader))
+            },
+            TempFile::Buffered { content } => {
+                Ok(Either::Right(*content))
+            },
+        }
+    }
+
     /// Returns the size, in bytes, of the file.
     ///
     /// This method does not perform any system calls.
@@ -353,7 +396,7 @@ impl<'v> TempFile<'v> {
     ///
     ///     Ok(())
     /// }
-    /// # let file = TempFile::Buffered { content: "hi".into() };
+    /// # let file = TempFile::Buffered { content: "hi".as_bytes() };
     /// # rocket::async_test(handle(file)).unwrap();
     /// ```
     pub fn path(&self) -> Option<&Path> {
@@ -474,7 +517,7 @@ impl<'v> TempFile<'v> {
 impl<'v> FromFormField<'v> for Capped<TempFile<'v>> {
     fn from_value(field: ValueField<'v>) -> Result<Self, Errors<'v>> {
         let n = N { written: field.value.len() as u64, complete: true  };
-        Ok(Capped::new(TempFile::Buffered { content: field.value }, n))
+        Ok(Capped::new(TempFile::Buffered { content: field.value.as_bytes() }, n))
     }
 
     async fn from_data(