sync: Add rwlocks (half-done) and test cases

Author: bblum

src/libcore/sync.rs

@@ -8,6 +8,7 @@
 export condvar;
 export semaphore, new_semaphore;
 export mutex, new_mutex;
+export rwlock;
 
 // FIXME (#3119) This shouldn't be a thing exported from core.
 import arc::exclusive;
@@ -58,6 +59,17 @@ enum sem<Q: send> = exclusive<{
     blocked:   Q,
 }>;
 
+fn new_sem<Q: send>(count: int, +q: Q) -> sem<Q> {
+    let (wait_tail, wait_head)  = pipes::stream();
+    sem(exclusive({ mut count: count,
+                    waiters: { head: wait_head, tail: wait_tail },
+                    blocked: q }))
+}
+fn new_sem_and_signal(count: int) -> sem<waitqueue> {
+    let (block_tail, block_head) = pipes::stream();
+    new_sem(count, { head: block_head, tail: block_tail })
+}
+
 impl<Q: send> &sem<Q> {
     fn acquire() {
         let mut waiter_nobe = none;
@@ -217,12 +229,7 @@ impl &sem<waitqueue> {
 enum semaphore = sem<()>;
 
 /// Create a new semaphore with the specified count.
-fn new_semaphore(count: int) -> semaphore {
-    let (wait_tail, wait_head)  = pipes::stream();
-    semaphore(sem(exclusive({ mut count: count,
-                              waiters: { head: wait_head, tail: wait_tail },
-                              blocked: () })))
-}
+fn new_semaphore(count: int) -> semaphore { semaphore(new_sem(count, ())) }
 
 impl &semaphore {
     /// Create a new handle to the semaphore.
@@ -257,13 +264,7 @@ impl &semaphore {
 enum mutex = sem<waitqueue>;
 
 /// Create a new mutex.
-fn new_mutex() -> mutex {
-    let (wait_tail,  wait_head)  = pipes::stream();
-    let (block_tail, block_head) = pipes::stream();
-    mutex(sem(exclusive({ mut count: 1,
-                          waiters: { head: wait_head,  tail: wait_tail  },
-                          blocked: { head: block_head, tail: block_tail } })))
-}
+fn new_mutex() -> mutex { mutex(new_sem_and_signal(1)) }
 
 impl &mutex {
     /// Create a new handle to the mutex.
@@ -282,7 +283,86 @@ impl &mutex {
  * Reader-writer locks
  ****************************************************************************/
 
-// FIXME(#3145) implement
+// NB: Uncyclopedia - Readers-writers_problem#The_third_readers-writers_problem
+
+struct rwlock_inner {
+    read_mode:  bool;
+    read_count: uint;
+}
+
+/// A blocking, no-starvation, reader-writer lock with an associated condvar.
+struct rwlock {
+    order_lock:  semaphore;
+    access_lock: sem<waitqueue>;
+    state:       arc::exclusive<rwlock_inner>;
+}
+
+fn rwlock() -> rwlock {
+    rwlock { order_lock: new_semaphore(1), access_lock: new_sem_and_signal(1),
+             state: arc::exclusive(rwlock_inner { read_mode:  false,
+                                                  read_count: 0 }) }
+}
+
+impl &rwlock {
+    // Create a new handle to the rwlock.
+    fn clone() -> rwlock {
+        rwlock { order_lock:  (&(self.order_lock)).clone(),
+                 access_lock: sem((*self.access_lock).clone()),
+                 state:       self.state.clone() }
+    }
+
+    /**
+     * Run a function with the rwlock in read mode. Calls to 'read' from other
+     * tasks may run concurrently with this one.
+     */
+    fn read<U>(blk: fn() -> U) -> U {
+        do (&self.order_lock).access {
+            let mut first_reader = false;
+            do self.state.with |state| {
+                state.read_mode = true;
+                first_reader = (state.read_count == 0);
+                state.read_count += 1;
+            }
+            if first_reader {
+                (&self.access_lock).acquire();
+            }
+        }
+        let result = blk();
+        let mut last_reader = false;
+        do self.state.with |state| {
+            assert state.read_mode;
+            state.read_count -= 1;
+            last_reader = (state.read_count == 0);
+        }
+        if last_reader {
+            (&self.access_lock).release();
+        }
+        result
+    }
+
+    /**
+     * Run a function with the rwlock in write mode. No calls to 'read' or
+     * 'write' from other tasks will run concurrently with this one.
+     */
+    fn write<U>(blk: fn() -> U) -> U {
+        (&self.order_lock).acquire();
+        do (&self.access_lock).access {
+            (&self.order_lock).release();
+            blk()
+        }
+    }
+
+    /**
+     * As write(), but also with a handle to a condvar. Waiting on this
+     * condvar will allow readers and writers alike to take the rwlock before
+     * the waiting task is signalled.
+     */
+    fn write_cond<U>(_blk: fn(condvar) -> U) -> U {
+        fail ~"Need implement lock order lock before access lock";
+    }
+
+    // to-do implement downgrade
+}
 
 /****************************************************************************
  * Tests
@@ -443,9 +523,8 @@ mod tests {
         }
         let _ = port.recv(); // Wait until child wakes up
     }
-    #[test]
-    fn test_mutex_cond_broadcast() {
-        let num_waiters: uint = 12;
+    #[cfg(test)]
+    fn test_mutex_cond_broadcast_helper(num_waiters: uint) {
         let m = ~new_mutex();
         let mut ports = ~[];
 
@@ -471,6 +550,25 @@ mod tests {
         // wait until all children wake up
         for ports.each |port| { let _ = port.recv(); }
     }
+    #[test]
+    fn test_mutex_cond_broadcast() {
+        test_mutex_cond_broadcast_helper(12);
+    }
+    #[test]
+    fn test_mutex_cond_broadcast_none() {
+        test_mutex_cond_broadcast_helper(0);
+    }
+    #[test]
+    fn test_mutex_cond_no_waiter() {
+        let m = ~new_mutex();
+        let m2 = ~m.clone();
+        do task::try {
+            do m.lock_cond |_x| { }
+        };
+        do m2.lock_cond |cond| {
+            assert !cond.signal();
+        }
+    }
     #[test] #[ignore(cfg(windows))]
     fn test_mutex_killed_simple() {
         // Mutex must get automatically unlocked if failed/killed within.
@@ -508,11 +606,72 @@ mod tests {
         assert result.is_err();
         // child task must have finished by the time try returns
         do m.lock_cond |cond| {
-            let woken = cond.signal();
+            let _woken = cond.signal();
             // FIXME(#3145) - The semantics of pipes are not quite what I want
             // here - the pipe doesn't get 'terminated' if the child was
             // punted awake during failure.
             // assert !woken;
         }
     }
+    #[cfg(test)]
+    fn test_rwlock_exclusion(reader1: bool, reader2: bool) {
+        // Test mutual exclusion between readers and writers. Just like the
+        // mutex mutual exclusion test, a ways above.
+        let (c,p) = pipes::stream();
+        let m = ~rwlock();
+        let m2 = ~m.clone();
+        let sharedstate = ~0;
+        let ptr = ptr::addr_of(*sharedstate);
+        do task::spawn {
+            let sharedstate = unsafe { unsafe::reinterpret_cast(ptr) };
+            access_shared(sharedstate, m2, reader1, 10);
+            c.send(());
+        }
+        access_shared(sharedstate, m, reader2, 10);
+        let _ = p.recv();
+
+        assert *sharedstate == 20;
+
+        fn access_shared(sharedstate: &mut int, m: &rwlock, reader: bool,
+                         n: uint) {
+            let lock_fn = fn@(m: &rwlock, blk: fn()) {
+                if reader { m.read(blk); } else { m.write(blk); }
+            };
+            for n.times {
+                do lock_fn(m) {
+                    let oldval = *sharedstate;
+                    task::yield();
+                    *sharedstate = oldval + 1;
+                }
+            }
+        }
+    }
+    #[test]
+    fn test_rwlock_readers_wont_modify_the_data() {
+        test_rwlock_exclusion(true, false);
+        test_rwlock_exclusion(false, true);
+    }
+    #[test]
+    fn test_rwlock_writers_and_writers() {
+        test_rwlock_exclusion(false, false);
+    }
+    #[test]
+    fn test_rwlock_readers_and_readers() {
+        // Much like sem_multi_resource.
+        let x = ~rwlock();
+        let x2 = ~x.clone();
+        let (c1,p1) = pipes::stream();
+        let (c2,p2) = pipes::stream();
+        do task::spawn {
+            do x2.read {
+                let _ = p2.recv();
+                c1.send(());
+            }
+        }
+        do x.read {
+            c2.send(());
+            let _ = p1.recv();
+        }
+
+    }
 }