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Switch to atomic for setting autocreated objects.

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- Update semaphore comment to new scope.
- Use an atomic swap to avoid needing to use the semaphore.

This means the semaphore is create only when extension are auto created (less
memory usage).
pull/7988/head
Thomas Van Lenten 4 years ago
parent de5d1b98c2
commit 2123ed5df7
3 changed files with 77 additions and 61 deletions
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  1. 125
      objectivec/GPBMessage.m
  2. 9
      objectivec/GPBUtilities.m
  3. 4
      objectivec/GPBUtilities_PackagePrivate.h

125
objectivec/GPBMessage.m
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@ -71,6 +71,8 @@ static NSString *const kGPBDataCoderKey = @"GPBData";
@package
GPBUnknownFieldSet *unknownFields_;
NSMutableDictionary *extensionMap_;
// Readonly access to autocreatedExtensionMap_ is protected via
// readOnlySemaphore_.
NSMutableDictionary *autocreatedExtensionMap_;
// If the object was autocreated, we remember the creator so that if we get
@ -79,10 +81,10 @@ static NSString *const kGPBDataCoderKey = @"GPBData";
GPBFieldDescriptor *autocreatorField_;
GPBExtensionDescriptor *autocreatorExtension_;
// A lock to provide mutual exclusion from internal data that can be modified
// by *read* operations such as getters (autocreation of message fields and
// message extensions, not setting of values). Used to guarantee thread safety
// for concurrent reads on the message.
// Message can only be mutated from one thread. But some *readonly* operations
// modifify internal state because they autocreate things. The
// autocreatedExtensionMap_ is one such structure. Access during readonly
// operations is protected via this semaphore.
// NOTE: OSSpinLock may seem like a good fit here but Apple engineers have
// pointed out that they are vulnerable to live locking on iOS in cases of
// priority inversion:
@ -583,19 +585,30 @@ static id GetOrCreateArrayIvarWithField(GPBMessage *self,
// This is like GPBGetObjectIvarWithField(), but for arrays, it should
// only be used to wire the method into the class.
static id GetArrayIvarWithField(GPBMessage *self, GPBFieldDescriptor *field) {
id array = GPBGetObjectIvarWithFieldNoAutocreate(self, field);
if (!array) {
// Check again after getting the lock.
GPBPrepareReadOnlySemaphore(self);
dispatch_semaphore_wait(self->readOnlySemaphore_, DISPATCH_TIME_FOREVER);
array = GPBGetObjectIvarWithFieldNoAutocreate(self, field);
if (!array) {
array = CreateArrayForField(field, self);
GPBSetAutocreatedRetainedObjectIvarWithField(self, field, array);
}
dispatch_semaphore_signal(self->readOnlySemaphore_);
uint8_t *storage = (uint8_t *)self->messageStorage_;
_Atomic(id) *typePtr = (_Atomic(id) *)&storage[field->description_->offset];
id array = atomic_load(typePtr);
if (array) {
return array;
}
return array;
id expected = nil;
id autocreated = CreateArrayForField(field, self);
if (atomic_compare_exchange_strong(typePtr, &expected, autocreated)) {
// Value was set, return it.
return autocreated;
}
// Some other thread set it, release the one created and return what got set.
if (GPBFieldDataTypeIsObject(field)) {
GPBAutocreatedArray *autoArray = autocreated;
autoArray->_autocreator = nil;
} else {
GPBInt32Array *gpbArray = autocreated;
gpbArray->_autocreator = nil;
}
[autocreated release];
return expected;
}
static id GetOrCreateMapIvarWithField(GPBMessage *self,
@ -613,19 +626,31 @@ static id GetOrCreateMapIvarWithField(GPBMessage *self,
// This is like GPBGetObjectIvarWithField(), but for maps, it should
// only be used to wire the method into the class.
static id GetMapIvarWithField(GPBMessage *self, GPBFieldDescriptor *field) {
id dict = GPBGetObjectIvarWithFieldNoAutocreate(self, field);
if (!dict) {
// Check again after getting the lock.
GPBPrepareReadOnlySemaphore(self);
dispatch_semaphore_wait(self->readOnlySemaphore_, DISPATCH_TIME_FOREVER);
dict = GPBGetObjectIvarWithFieldNoAutocreate(self, field);
if (!dict) {
dict = CreateMapForField(field, self);
GPBSetAutocreatedRetainedObjectIvarWithField(self, field, dict);
}
dispatch_semaphore_signal(self->readOnlySemaphore_);
uint8_t *storage = (uint8_t *)self->messageStorage_;
_Atomic(id) *typePtr = (_Atomic(id) *)&storage[field->description_->offset];
id dict = atomic_load(typePtr);
if (dict) {
return dict;
}
return dict;
id expected = nil;
id autocreated = CreateMapForField(field, self);
if (atomic_compare_exchange_strong(typePtr, &expected, autocreated)) {
// Value was set, return it.
return autocreated;
}
// Some other thread set it, release the one created and return what got set.
if ((field.mapKeyDataType == GPBDataTypeString) &&
GPBFieldDataTypeIsObject(field)) {
GPBAutocreatedDictionary *autoDict = autocreated;
autoDict->_autocreator = nil;
} else {
GPBInt32Int32Dictionary *gpbDict = autocreated;
gpbDict->_autocreator = nil;
}
[autocreated release];
return expected;
}
#endif // !defined(__clang_analyzer__)
@ -3337,30 +3362,34 @@ id GPBGetMessageMapField(GPBMessage *self, GPBFieldDescriptor *field) {
id GPBGetObjectIvarWithField(GPBMessage *self, GPBFieldDescriptor *field) {
NSCAssert(!GPBFieldIsMapOrArray(field), @"Shouldn't get here");
if (GPBGetHasIvarField(self, field)) {
uint8_t *storage = (uint8_t *)self->messageStorage_;
id *typePtr = (id *)&storage[field->description_->offset];
return *typePtr;
}
// Not set...
// Non messages (string/data), get their default.
if (!GPBFieldDataTypeIsMessage(field)) {
if (GPBGetHasIvarField(self, field)) {
uint8_t *storage = (uint8_t *)self->messageStorage_;
id *typePtr = (id *)&storage[field->description_->offset];
return *typePtr;
}
// Not set...non messages (string/data), get their default.
return field.defaultValue.valueMessage;
}
GPBPrepareReadOnlySemaphore(self);
dispatch_semaphore_wait(self->readOnlySemaphore_, DISPATCH_TIME_FOREVER);
GPBMessage *result = GPBGetObjectIvarWithFieldNoAutocreate(self, field);
if (!result) {
// For non repeated messages, create the object, set it and return it.
// This object will not initially be visible via GPBGetHasIvar, so
// we save its creator so it can become visible if it's mutated later.
result = GPBCreateMessageWithAutocreator(field.msgClass, self, field);
GPBSetAutocreatedRetainedObjectIvarWithField(self, field, result);
}
dispatch_semaphore_signal(self->readOnlySemaphore_);
return result;
uint8_t *storage = (uint8_t *)self->messageStorage_;
_Atomic(id) *typePtr = (_Atomic(id) *)&storage[field->description_->offset];
id msg = atomic_load(typePtr);
if (msg) {
return msg;
}
id expected = nil;
id autocreated = GPBCreateMessageWithAutocreator(field.msgClass, self, field);
if (atomic_compare_exchange_strong(typePtr, &expected, autocreated)) {
// Value was set, return it.
return autocreated;
}
// Some other thread set it, release the one created and return what got set.
GPBClearMessageAutocreator(autocreated);
[autocreated release];
return expected;
}
#pragma clang diagnostic pop

9
objectivec/GPBUtilities.m
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@ -504,15 +504,6 @@ static void GPBMaybeClearOneofPrivate(GPBMessage *self,
// Object types are handled slightly differently, they need to be released
// and retained.
void GPBSetAutocreatedRetainedObjectIvarWithField(
GPBMessage *self, GPBFieldDescriptor *field,
id __attribute__((ns_consumed)) value) {
uint8_t *storage = (uint8_t *)self->messageStorage_;
id *typePtr = (id *)&storage[field->description_->offset];
NSCAssert(*typePtr == NULL, @"Can't set autocreated object more than once.");
*typePtr = value;
}
void GPBClearAutocreatedMessageIvarWithField(GPBMessage *self,
GPBFieldDescriptor *field) {
if (GPBGetHasIvarField(self, field)) {

4
objectivec/GPBUtilities_PackagePrivate.h
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@ -289,10 +289,6 @@ void GPBSetRetainedObjectIvarWithFieldPrivate(GPBMessage *self,
id GPBGetObjectIvarWithFieldNoAutocreate(GPBMessage *self,
GPBFieldDescriptor *field);
void GPBSetAutocreatedRetainedObjectIvarWithField(
GPBMessage *self, GPBFieldDescriptor *field,
id __attribute__((ns_consumed)) value);
// Clears and releases the autocreated message ivar, if it's autocreated. If
// it's not set as autocreated, this method does nothing.
void GPBClearAutocreatedMessageIvarWithField(GPBMessage *self,

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