Remove several usages of 'unsafe' (#968)

* Replace UnsafeCell in DateServiceInner with Cell

The previous API was extremely dangerous - calling `get_ref()`
followed by `reset()` would trigger instant UB, without requiring
any `unsafe` blocks in the caller.

By making DateInner `Copy`, we can use a normal `Cell` instead
of an `UnsafeCell`. This makes it impossible to cause UB (or even panic)
with the API.

* Split unsafe block HttpServiceHandlerResponse

Also add explanation of the safety of the usage of `unsafe`

* Replace UnsafeCell with RefCell in PayloadRef

This ensures that a mistake in the usage of 'get_mut' will cause
a panic, not undefined behavior.

actix-http/src/config.rs

@@ -1,4 +1,4 @@
-use std::cell::UnsafeCell;
+use std::cell::Cell;
 use std::fmt;
 use std::fmt::Write;
 use std::rc::Rc;
@@ -172,6 +172,7 @@ impl ServiceConfig {
     }
 }
 
+#[derive(Copy, Clone)]
 struct Date {
     bytes: [u8; DATE_VALUE_LENGTH],
     pos: usize,
@@ -205,28 +206,28 @@ impl fmt::Write for Date {
 struct DateService(Rc<DateServiceInner>);
 
 struct DateServiceInner {
-    current: UnsafeCell<Option<(Date, Instant)>>,
+    current: Cell<Option<(Date, Instant)>>,
 }
 
 impl DateServiceInner {
     fn new() -> Self {
         DateServiceInner {
-            current: UnsafeCell::new(None),
+            current: Cell::new(None),
         }
     }
 
-    fn get_ref(&self) -> &Option<(Date, Instant)> {
-        unsafe { &*self.current.get() }
+    fn get(&self) -> Option<(Date, Instant)> {
+        self.current.get()
     }
 
     fn reset(&self) {
-        unsafe { (&mut *self.current.get()).take() };
+        self.current.set(None);
     }
 
     fn update(&self) {
         let now = Instant::now();
         let date = Date::new();
-        *(unsafe { &mut *self.current.get() }) = Some((date, now));
+        self.current.set(Some((date, now)));
     }
 }
 
@@ -236,7 +237,7 @@ impl DateService {
     }
 
     fn check_date(&self) {
-        if self.0.get_ref().is_none() {
+        if self.0.get().is_none() {
             self.0.update();
 
             // periodic date update
@@ -252,14 +253,13 @@ impl DateService {
 
     fn now(&self) -> Instant {
         self.check_date();
-        self.0.get_ref().as_ref().unwrap().1
+        self.0.get().unwrap().1
     }
 
-    fn date(&self) -> &Date {
+    fn date(&self) -> Date {
         self.check_date();
 
-        let item = self.0.get_ref().as_ref().unwrap();
-        &item.0
+        self.0.get().unwrap().0
     }
 }
 

actix-http/src/service.rs

@@ -466,18 +466,20 @@ where
             State::Unknown(ref mut data) => {
                 if let Some(ref mut item) = data {
                     loop {
-                        unsafe {
-                            let b = item.1.bytes_mut();
+                        // Safety - we only write to the returned slice.
+                        let b = unsafe { item.1.bytes_mut() };
                         let n = try_ready!(item.0.poll_read(b));
                         if n == 0 {
                             return Ok(Async::Ready(()));
                         }
-                            item.1.advance_mut(n);
+                        // Safety - we know that 'n' bytes have
+                        // been initialized via the contract of
+                        // 'poll_read'
+                        unsafe { item.1.advance_mut(n) };
                         if item.1.len() >= HTTP2_PREFACE.len() {
                             break;
                         }
                     }
-                    }
                 } else {
                     panic!()
                 }

actix-multipart/src/server.rs

@@ -1,5 +1,5 @@
 //! Multipart payload support
-use std::cell::{Cell, RefCell, UnsafeCell};
+use std::cell::{Cell, RefCell, RefMut};
 use std::marker::PhantomData;
 use std::rc::Rc;
 use std::{cmp, fmt};
@@ -112,7 +112,7 @@ impl Stream for Multipart {
             Err(err)
         } else if self.safety.current() {
             let mut inner = self.inner.as_mut().unwrap().borrow_mut();
-            if let Some(payload) = inner.payload.get_mut(&self.safety) {
+            if let Some(mut payload) = inner.payload.get_mut(&self.safety) {
                 payload.poll_stream()?;
             }
             inner.poll(&self.safety)
@@ -265,12 +265,12 @@ impl InnerMultipart {
                 }
             }
 
-            let headers = if let Some(payload) = self.payload.get_mut(safety) {
+            let headers = if let Some(mut payload) = self.payload.get_mut(safety) {
                 match self.state {
                     // read until first boundary
                     InnerState::FirstBoundary => {
                         match InnerMultipart::skip_until_boundary(
-                            payload,
+                            &mut *payload,
                             &self.boundary,
                         )? {
                             Some(eof) => {
@@ -286,7 +286,7 @@ impl InnerMultipart {
                     }
                     // read boundary
                     InnerState::Boundary => {
-                        match InnerMultipart::read_boundary(payload, &self.boundary)? {
+                        match InnerMultipart::read_boundary(&mut *payload, &self.boundary)? {
                             None => return Ok(Async::NotReady),
                             Some(eof) => {
                                 if eof {
@@ -303,7 +303,7 @@ impl InnerMultipart {
 
                 // read field headers for next field
                 if self.state == InnerState::Headers {
-                    if let Some(headers) = InnerMultipart::read_headers(payload)? {
+                    if let Some(headers) = InnerMultipart::read_headers(&mut *payload)? {
                         self.state = InnerState::Boundary;
                         headers
                     } else {
@@ -411,7 +411,7 @@ impl Stream for Field {
     fn poll(&mut self) -> Poll<Option<Self::Item>, Self::Error> {
         if self.safety.current() {
             let mut inner = self.inner.borrow_mut();
-            if let Some(payload) = inner.payload.as_ref().unwrap().get_mut(&self.safety)
+            if let Some(mut payload) = inner.payload.as_ref().unwrap().get_mut(&self.safety)
             {
                 payload.poll_stream()?;
             }
@@ -582,12 +582,12 @@ impl InnerField {
             return Ok(Async::Ready(None));
         }
 
-        let result = if let Some(payload) = self.payload.as_ref().unwrap().get_mut(s) {
+        let result = if let Some(mut payload) = self.payload.as_ref().unwrap().get_mut(s) {
             if !self.eof {
                 let res = if let Some(ref mut len) = self.length {
-                    InnerField::read_len(payload, len)?
+                    InnerField::read_len(&mut *payload, len)?
                 } else {
-                    InnerField::read_stream(payload, &self.boundary)?
+                    InnerField::read_stream(&mut *payload, &self.boundary)?
                 };
 
                 match res {
@@ -618,7 +618,7 @@ impl InnerField {
 }
 
 struct PayloadRef {
-    payload: Rc<UnsafeCell<PayloadBuffer>>,
+    payload: Rc<RefCell<PayloadBuffer>>,
 }
 
 impl PayloadRef {
@@ -628,15 +628,12 @@ impl PayloadRef {
         }
     }
 
-    fn get_mut<'a, 'b>(&'a self, s: &'b Safety) -> Option<&'a mut PayloadBuffer>
+    fn get_mut<'a, 'b>(&'a self, s: &'b Safety) -> Option<RefMut<'a, PayloadBuffer>>
     where
         'a: 'b,
     {
-        // Unsafe: Invariant is inforced by Safety Safety is used as ref counter,
-        // only top most ref can have mutable access to payload.
         if s.current() {
-            let payload: &mut PayloadBuffer = unsafe { &mut *self.payload.get() };
-            Some(payload)
+            Some(self.payload.borrow_mut())
         } else {
             None
         }