Fix a deadlock in channels, again.

This deadlock was caused when the channel was closed at just the right time, so
the extra `self.cnt.fetch_add` actually should have preserved the DISCONNECTED
state of the channel. by modifying this the channel entered a state such that
the port would never succeed in dropping.

This also moves the increment of self.steals until after the MAX_STEALS block.
The reason for this is that in 'fn recv()' the steals variable is decremented
immediately after the try_recv(), which could in theory set steals to -1 if it
was previously set to 0 in try_recv().

Closes #12340

src/libstd/comm/shared.rs

@@ -305,7 +305,6 @@ pub fn try_recv(&mut self) -> Result<T, Failure> {
             // See the discussion in the stream implementation for why we we
             // might decrement steals.
             Some(data) => {
-                self.steals += 1;
                 if self.steals > MAX_STEALS {
                     match self.cnt.swap(0, atomics::SeqCst) {
                         DISCONNECTED => {
@@ -314,11 +313,12 @@ pub fn try_recv(&mut self) -> Result<T, Failure> {
                         n => {
                             let m = cmp::min(n, self.steals);
                             self.steals -= m;
-                            self.cnt.fetch_add(n - m, atomics::SeqCst);
+                            self.bump(n - m);
                         }
                     }
                     assert!(self.steals >= 0);
                 }
+                self.steals += 1;
                 Ok(data)
             }
 

src/libstd/comm/stream.rs

@@ -213,7 +213,6 @@ pub fn try_recv(&mut self) -> Result<T, Failure<T>> {
             // down as much as possible (without going negative), and then
             // adding back in whatever we couldn't factor into steals.
             Some(data) => {
-                self.steals += 1;
                 if self.steals > MAX_STEALS {
                     match self.cnt.swap(0, atomics::SeqCst) {
                         DISCONNECTED => {
@@ -222,11 +221,12 @@ pub fn try_recv(&mut self) -> Result<T, Failure<T>> {
                         n => {
                             let m = cmp::min(n, self.steals);
                             self.steals -= m;
-                            self.cnt.fetch_add(n - m, atomics::SeqCst);
+                            self.bump(n - m);
                         }
                     }
                     assert!(self.steals >= 0);
                 }
+                self.steals += 1;
                 match data {
                     Data(t) => Ok(t),
                     GoUp(up) => Err(Upgraded(up)),