Am 11.11.2012 19:46, schrieb Alex Rønne Petersen: > Something needs to be done about shared. I don't know what, but the > current situation is -- and I'm really not exaggerating here -- > laughable. I think we either need to just make it perfectly clear that > shared is for documentation purposes and nothing else, or, figure out an > alternative system to shared, because I don't see shared actually being > useful for real world work no matter what we do with it. >
After reading Walter's comment, it suddenly seemed obvious that we are currently using 'shared' the wrong way. Shared is just not meant to be used on objects at all (or only in some special cases like synchronization primitives). I just experimented a bit with a statically checked library based solution and a nice way to use shared is to only use it for disabling access to non-shared members while its monitor is not locked. A ScopedLock proxy and a lock() function can be used for this: --- class MyClass { void method(); } void main() { auto inst = new shared(MyClass); //inst.method(); // forbidden { ScopedLock!MyClass l = lock(inst); l.method(); // now allowed as long as 'l' is in scope } // can also be called like this: inst.lock().method(); } --- ScopedLock is non-copyable and handles the dirty details of locking and casting away 'shared' when its safe to do so. No tagging of the class with 'synchronized' or 'shared' needs to be done and everything works nicely without casts. This comes with a restriction, though. Doing all this is only safe as long as the instance is known to not contain any unisolated aliasing*. So use would be restricted to types that contain only immutable or unique/isolated references. So I also implemented an Isolated!(T) type that is recognized by ScopedLock, as well as functions such as spawn(). The resulting usage can be seen in the example at the bottom. It doesn't provide all the flexibility that a built-in 'isolated' type would do, but the possible use cases at least look interesting. There are still some details to be worked out, such as writing a spawn() function that correctly moves Isolated!() parameters instead of copying or the forward reference error mentioned in the example. I'll now try and see if some of my earlier multi-threading designs fit into this system. --- import std.stdio; import std.typecons; import std.traits; import stdx.typecons; class SomeClass { } class Test { private { string m_test1 = "test 1"; Isolated!SomeClass m_isolatedReference; // currently causes a size forward reference error: //Isolated!Test m_next; } this() { //m_next = ...; } void test1() const { writefln(m_test1); } void test2() const { writefln("test 2"); } } void main() { writefln("Shared locking"); // create a shared instance of Test - no members will // be accessible auto t = new shared(Test); { // temporarily lock t to make all non-shared members // safely available // lock() words only for objects with no unisolated // aliasing. ScopedLock!Test l = lock(t); l.test1(); l.test2(); } // passing a shared object to a different thread works as usual writefln("Shared spawn"); spawn(&myThreadFunc1, t); // create an isolated instance of Test // currently, Test may not contain unisolated aliasing, but // this requirement may get lifted, // as long as only pure methods are called Isolated!Test u = makeIsolated!Test(); // move ownership to a different function and recover writefln("Moving unique"); Isolated!Test v = myThreadFunc2(u.move()); // moving to a different thread also works writefln("Moving unique spawn"); spawn(&myThreadFunc2, v.move()); // another possibility is to convert to immutable auto w = makeIsolated!Test(); writefln("Convert to immutable spawn"); spawn(&myThreadFunc3, w.freeze()); // or just loose the isolation and act on the base type writefln("Convert to mutable"); auto x = makeIsolated!Test(); Test xm = x.extract(); xm.test1(); xm.test2(); } void myThreadFunc1(shared(Test) t) { // call non-shared method on shared object t.lock().test1(); t.lock().test2(); } Isolated!Test myThreadFunc2(Isolated!Test t) { // call methods as usual on an isolated object t.test1(); t.test2(); return t.move(); } void myThreadFunc3(immutable(Test) t) { t.test1(); t.test2(); } // fake spawn function just to test the type constraints void spawn(R, ARGS...)(R function(ARGS) func, ARGS args) { foreach( i, T; ARGS ) static assert(!hasUnisolatedAliasing!T || !hasUnsharedAliasing!T, "Parameter "~to!string(i)~" of type" ~T.stringof~" has unshared or unisolated aliasing. Cannot safely be passed to a different thread."); // TODO: do this in a different thread... // TODO: don't cheat with the 1-parameter move detection static if(__traits(compiles, func(args[0])) ) func(args); else func(args[0].move()); } --- * shared aliasing would also be OK, but this is not yet handled by the implementation.