/*

    File:       AESendThreadSafe.c

    Contains:   Code to send Apple events in a thread-safe manner.

    Written by: DTS

    Copyright:  Copyright (c) 2007 Apple Inc. All Rights Reserved.

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                The Apple Software is provided by Apple on an "AS IS" basis. 

                APPLE MAKES NO WARRANTIES, EXPRESS OR IMPLIED, INCLUDING

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    Change History (most recent first):

$Log: AESendThreadSafe.c,v $

Revision 1.3  2007/02/27 10:45:15         

In the destructor, add an assert that the thread-local storage has been set to NULL. Also fixed a comment typo.

Revision 1.2  2007/02/12 11:59:09         

Added a type cast for the malloc result.

Revision 1.1  2007/02/09 10:55:24         

First checked in.

*/

/////////////////////////////////////////////////////////////////

#include "AESendThreadSafe.h"

#include <pthread.h>

#include <mach/mach.h>

/////////////////////////////////////////////////////////////////

/*

    How It Works

    ------------

    The basic idea behind this module is that it uses per-thread storage to keep 

    track of an Apple event reply for any given thread.  The first time that the 

    thread calls AESendMessageThreadSafeSynchronous, the per-thread storage will 

    not be initialised and the code will grab an Apple event reply port and 

    assign it to the per-thread storage.  Subsequent calls to AESendMessageThreadSafeSynchronous 

    will continue to use that port.  When the thread dies, pthreads will automatically 

    call the destructor for the per-thread storage, and that will clean up the port.

    Because we can't dispose of the reply port (without triggering the Apple 

    Event Manager bug that's the reason we wrote this code in the first place), 

    the destructor doesn't actually dispose of the port.  Rather, it adds the 

    port to a pool of ports that are available for reuse.  The next time a thread 

    needs to allocate a port, it will grab it from the pool rather than allocating 

    it from scratch.

    This technique means that the code still 'leaks' Apple event reply ports, but 

    the size of the leak is limited to the maximum number of threads that you run 

    simultaneously.  This isn't a problem in practice.

*/

/////////////////////////////////////////////////////////////////

// The PerThreadStorage structure is a trivial wrapper around the Mach port.  

// I added this because I need to attach this structure a thread using 

// per-thread storage.  The API for that (<x-man-page://3/pthread_setspecific>) 

// is pointer based.  I could've just cast the Mach port to a (void *), but 

// that's ugly because of a) pointer size issues (are pointers always bigger than 

// ints?), and b) because it implies an equivalent between NULL and MACH_PORT_NULL.

// Given this, I simply decided to create a structure to wrap the Mach port.

enum {

    kPerThreadStorageMagic = 'PTSm'

};

struct PerThreadStorage {

    OSType          magic;          // must be kPerThreadStorageMagic

    mach_port_t     port;

};

typedef struct PerThreadStorage PerThreadStorage;

// The following static variables manage the per-thread storage key 

// (sPerThreadStorageKey) and the pool of Mach ports (wrapped in 

// PerThreadStorage structures) that are not currently attached to a thread.

static pthread_once_t       sInited = PTHREAD_ONCE_INIT;    // covers initialisation of all of the 

                                                            // following static variables

static OSStatus             sPerThreadStorageKeyInitErrNum; // latches result of initialisation

static pthread_key_t        sPerThreadStorageKey = 0;       // key for our per-thread storage

static pthread_mutex_t      sPoolMutex;                     // protects sPool

static CFMutableArrayRef    sPool;                          // array of (PerThreadStorage *), holds 

                                                            // the ports that aren't currently bound to 

                                                            // a thread

static void PerThreadStorageDestructor(void *keyValue);     // forward declaration

static void InitRoutine(void)

    // Call once (via pthread_once) to initialise various static variables.

{

    OSStatus    err;

    // Create the per-thread storage key.  Note that we assign a destructor to this key; 

    // pthreads call the destructor to clean up that item of per-thread storage whenever 

    // a thread terminates.

    err = (OSStatus) pthread_key_create(&sPerThreadStorageKey, PerThreadStorageDestructor);

    // Create the pool of Mach ports that aren't bound to any thread, and its associated 

    // lock.  The pool starts out empty.

    if (err == noErr) {

        err = (OSStatus) pthread_mutex_init(&sPoolMutex, NULL);

    }

    if (err == noErr) {

        sPool = CFArrayCreateMutable(NULL, 0, NULL);

        if (sPool == NULL) {

            err = coreFoundationUnknownErr;

        }

    }

    assert(err == 0);

    sPerThreadStorageKeyInitErrNum = err;

}

static OSStatus AllocatePortFromPool(PerThreadStorage **storagePtr)

    // Grab a Mach port from sPool; if sPool is empty, create one.

{

    OSStatus            err;

    OSStatus            junk;

    PerThreadStorage *  storage;

    assert( storagePtr != NULL);

    assert(*storagePtr == NULL);

    storage = NULL;

    // First try to get an entry from pool.  We try to grab the last one because 

    // that minimises the amount of copying that CFArrayRemoveValueAtIndex has to 

    // do.

    err = (OSStatus) pthread_mutex_lock(&sPoolMutex);

    if (err == noErr) {

        CFIndex             poolCount;

        poolCount = CFArrayGetCount(sPool);

        if (poolCount > 0) {

            storage = (PerThreadStorage *) CFArrayGetValueAtIndex(sPool, poolCount - 1);

            CFArrayRemoveValueAtIndex(sPool, poolCount - 1);

        }

        junk = (OSStatus) pthread_mutex_unlock(&sPoolMutex);

        assert(junk == noErr);

    }

    // If we failed to find an entry in the pool, create a new one.

    if ( (err == noErr) && (storage == NULL) ) {

        storage = (PerThreadStorage *) malloc(sizeof(*storage));

        if (storage == NULL) {

            err = memFullErr;

        } else {

            storage->magic = kPerThreadStorageMagic;

            storage->port  = MACH_PORT_NULL;

            err = (OSStatus) mach_port_allocate(mach_task_self(), MACH_PORT_RIGHT_RECEIVE, &storage->port);

            if (err != noErr) {

                assert(storage->port == MACH_PORT_NULL);

                free(storage);

                storage = NULL;

            }

        }

    }

    if (err == noErr) {

        *storagePtr = storage;

    }

    assert( (err == noErr) == (*storagePtr != NULL) );

    assert( (*storagePtr == NULL) || ((*storagePtr)->magic == kPerThreadStorageMagic) );

    assert( (*storagePtr == NULL) || ((*storagePtr)->port  != MACH_PORT_NULL) );

    return err;

}

static void ReturnPortToPool(PerThreadStorage * storage)

    // Returns a port to sPool.

{

    OSStatus err;

    assert(storage != NULL);

    assert(storage->magic == kPerThreadStorageMagic);

    assert(storage->port  != MACH_PORT_NULL);

    err = (OSStatus) pthread_mutex_lock(&sPoolMutex);

    if (err == noErr) {

        CFArrayAppendValue(sPool, storage);

        err = (OSStatus) pthread_mutex_unlock(&sPoolMutex);

    }

    assert(err == noErr);

}

// Main Thread Notes

// -----------------

// There are two reasons why we don't assign a reply port to the main thread. 

// First, the main thread already has a reply port created for it by Apple 

// Event Manager.  Thus, we don't need a specific reply port.  Also, the 

// destructor for per-thread storage isn't called for the main thread, so 

// we wouldn't get a chance to clean up (although that's not really a problem 

// in practice).

static OSStatus BindReplyMachPortToThread(mach_port_t *replyPortPtr)

    // Get a reply port for this thread, remembering that we've done this 

    // in per-thread storage.

    // 

    // On success, *replyPortPtr is the port to use for this thread's reply 

    // port.  It will be MACH_PORT_NULL if you call it from the main thread.

{

    OSStatus    err;

    assert( replyPortPtr != NULL);

    assert(*replyPortPtr == MACH_PORT_NULL);

    // Initialise ourselves the first time that we're called.

    err = (OSStatus) pthread_once(&sInited, InitRoutine);

    // If something went wrong, return the latched error.

    if ( (err == noErr) && (sPerThreadStorageKeyInitErrNum != noErr) ) {

        err = sPerThreadStorageKeyInitErrNum;

    }

    // Now do the real work.

    if (err == noErr) {

        if ( pthread_main_np() ) {

            // This is the main thread, so do nothing; leave *replyPortPtr set 

            // to MACH_PORT_NULL.

            assert(*replyPortPtr == MACH_PORT_NULL);

        } else {

            PerThreadStorage *  storage;

            // Get the per-thread storage for this thread.

            storage = (PerThreadStorage *) pthread_getspecific(sPerThreadStorageKey);

            if (storage == NULL) {

                // The per-thread storage hasn't been allocated yet for this specific 

                // thread.  Let's go allocate it and attach it to this thread.

                err = AllocatePortFromPool(&storage);

                if (err == noErr) {

                    err = (OSStatus) pthread_setspecific(sPerThreadStorageKey, (void *) storage);

                    if (err != noErr) {

                        ReturnPortToPool(storage);

                        storage = NULL;

                    }

                }

            }

            assert( (err == noErr) == (storage != NULL) );

            // If all went well, copy the port out to our client.

            if (err == noErr) {

                assert(storage->magic == kPerThreadStorageMagic);

                assert(storage->port  != MACH_PORT_NULL);

                *replyPortPtr = storage->port;

            }

        }

    }

    // no error + MACH_PORT_NULL is a valid response if we're on the main 

    // thread.

    //

    // assert( (err == noErr) == (*replyPortPtr != MACH_PORT_NULL) );

    assert( (*replyPortPtr == MACH_PORT_NULL) || (err == noErr) );

    return err;

}

static void PerThreadStorageDestructor(void *keyValue)

    // Called by pthreads when a thread dies and it has a non-null value for our 

    // per-thread storage key.  We use this callback to return the thread's 

    // Apple event reply port to the pool.

{

    PerThreadStorage *  storage;

    storage = (PerThreadStorage *) keyValue;

    assert(storage != NULL);                    // pthread won't call us if it's NULL

    assert(storage->magic == kPerThreadStorageMagic);

    assert(storage->port  != MACH_PORT_NULL);

    // Return the port associated with this thread to the pool.

    ReturnPortToPool(storage);

    // pthreads has already set this thread's per-thread storage for our key to 

    // NULL before calling us. So we don't need to do anything to remove it.

    assert( pthread_getspecific(sPerThreadStorageKey) == NULL );

}

extern OSStatus AESendMessageThreadSafeSynchronous(

    AppleEvent *        eventPtr,

    AppleEvent *        replyPtr,

    long                timeOutInTicks

)

    // See comment in header.

{

    OSStatus        err;

    mach_port_t     replyPort;

    assert(eventPtr != NULL);

    assert(replyPtr != NULL);

    replyPort = MACH_PORT_NULL;

    // Set up the reply port if necessary.

    err = BindReplyMachPortToThread(&replyPort);

    if ( (err == noErr) && (replyPort != MACH_PORT_NULL) ) {

        err = AEPutAttributePtr(eventPtr, keyReplyPortAttr, typeMachPort, &replyPort, sizeof(replyPort));

    }

    // Call through to AESendMessage.

    if (err == noErr) {

        err = AESendMessage(eventPtr, replyPtr, kAEWaitReply, timeOutInTicks);

    }

    return err;

}