NSDictionary Enumeration Cause "malloc: double free" Error - ios

Question

I get a double free error when using [nsdictionary enumerateKeysAndObjectsUsingBlock:]
CnFExhibition_0821(74624,0x114853000) malloc: * error for object 0x7fe972814fa0: double free
This happen mostly when using appendFormat: in enumeration block, not always happen but quite often.
I finally prevent it by not using enumerateKeysAndObjectsUsingBlock:, but still wondering why?
This happen when calling this line, which "newData.list[0]" is a NSDictionary,
[database updateRow:#"01" inTable:#"Exhibition" setting:newData.list[0] error:nil];
SQLiteHelper.m
-(BOOL)updateRow:(id)rowID inTable:(NSString*)tablename setting:(NSDictionary*)setting error:(NSError *__autoreleasing *)err{
if ([self checkTableName:tablename error:err]){
if ([self checkUpdateSetting:setting error:err]) {
NSMutableString * updateCmd = [sqlCmdUpdateFromTable(tablename) mutableCopy];
[updateCmd appendString:sqlCmdSetRow(setting)];
if (rowID) {
[updateCmd appendString:sqlCmdWhereCondition(#{[self pkOfTable:tablename]:rowID})];
}
return [self execCmdStr:updateCmd error:err];
}else{
return NO;
}
}else{
return NO;
}
}
NSString * sqlCmdSetRow(NSDictionary*setting){
if (setting && setting.count){
NSMutableString * setCmd = [NSMutableString stringWithString: #" SET "];
[[setting copy] enumerateKeysAndObjectsWithOptions:NSEnumerationConcurrent usingBlock:^(id key, id obj, BOOL *stop){
if ([obj isKindOfClass:[NSString class]]){
//*Mostly crush at this line*
[setCmd appendFormat:#"%#='%#', ",key,obj]];
}
else{
[setCmd appendFormat:#"%#=%#, ",key,obj];
}
}];
[setCmd deleteCharactersInRange:NSMakeRange(setCmd.length-2, 2)];
return setCmd;
}
else{
return nil;
}
}
Replacing enumeration in "sqlCmdSetRow" with code below and never happen again
NSArray * a = [setting allKeys];
for (NSString * s in a) {
if ([setting[s] isKindOfClass:[NSString class]]){
[setCmd appendString:[NSString stringWithFormat:#"%#='%#', ",s,setting[s]]];
}else{
[setCmd appendFormat:#"%#=%#, ",s,setting[s]];
}
}

Answer 1

By using -[NSDictionary enumerateKeysAndObjectsWithOptions:usingBlock: with the NSEnumerationConcurrent option, you are effectively calling appendString: and appendFormat: on the same NSMutableString object (setCmd) on multiple threads simultaneously. The documentation of those methods doesn't say anything about thread safety, so they probably aren't thread-safe. Your random crashes back that up.
You did the right thing by changing to a for-in loop. Now that you are only touching setCmd on a single thread, there is no thread safety issue and the crashes went away.