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• ActiveRecord Associations Question

  - by Mutuelinvestor

  I'm new to rails and have volunteered to help out the local High School Track team with a simple database that tracks the runners performances. For the moment, I have three models: Runners, Race_Data and Races. I have the following associations. Runners have_many Race_Data Races have_many Race_Data I also want create the association Runners Have_Many Races Through Race_Data, but as my look at the diagram I have drawn, there is already a many to one relationship from Race_data to Races. Does the combination of Runners having many Race_Data and Race_Data having one Race imply a Many_to_Many relationship between Runners and Races?

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• windbg/cdb hangs when bp hit

  - by aaron

  I have a problem where cdb or windbg hangs frequently, but not all the time, when I'm debugging with it and I attach to a specific application on my machine. I found this article: http://www.nynaeve.net/?p=164 which talks about a symbol loading race condition being the problem, but I can force load the symbols, actually have a breakpoint in-app work, and still have it hang elsewhere. Here is the stack from cdb itself when I attach to it with another debugger: ntdll!NtReadFile kernel32!ReadFile cdb!ReadNonConLine cdb!ConIn cdb!MainLoop cdb!main !analyze reports that APPLICATION_HANG_BusyHang is the problem bucket, and 'ReadNonConLine' is the offending function. as far as the stack goes: ffffffff`fffffffe 00000000`00000000 00000001`3f641498 00000000`0014ea50 : kernel32!ReadFile+0x86 00000000`000002a4 00000000`0014ebb0 00000000`00001000 00000000`00000000 : cdb!ReadNonConLine+0x6d ReadNonConLine has the string "g" at 0014ebb0 passed as a param, which may be part of the command I had at the hanging breakpoint (it was something like bp foo "dt a; g") ReadFile takes a handle as its first parameter. I'm surprised by the value -2, though, that doesn't look valid. Any help is appreciated. Thanks! Aaron

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• NSURLConnection still calls delegate AFTER cancel method has been called

  - by Shizam

  Hi All, Having a problem with NSURLConnection, if I create a NSURLConnection and call [connection connectionWithRequest] let it load a little then call [connection cancel] most of the time that works fine. However occasionally even after I call [connection cancel] the connection's delegate still gets called (which crashes the app). Googling around it looks like the problem here is a race condition in the runloop, I cancel the connection and release the delegate but before the runloop cycles it calls the delegate functions - crash. Is there a way for me to, after I call [connection cancel] confirm the connection has actually canceled? Even a crappy while() loop will do :(

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• Fastest way to list all primes below N in python

  - by jbochi

  This is the best algorithm I could come up with after struggling with a couple of Project Euler's questions. def get_primes(n): numbers = set(range(n, 1, -1)) primes = [] while numbers: p = numbers.pop() primes.append(p) numbers.difference_update(set(range(p*2, n+1, p))) return primes >>> timeit.Timer(stmt='get_primes.get_primes(1000000)', setup='import get_primes').timeit(1) 1.1499958793645562 Can it be made even faster? EDIT: This code has a flaw: Since numbers is an unordered set, there is no guarantee that numbers.pop() will remove the lowest number from the set. Nevertheless, it works (at least for me) for some input numbers: >>> sum(get_primes(2000000)) 142913828922L #That's the correct sum of all numbers below 2 million >>> 529 in get_primes(1000) False >>> 529 in get_primes(530) True EDIT: The rank so far (pure python, no external sources, all primes below 1 million): Sundaram's Sieve implementation by myself: 327ms Daniel's Sieve: 435ms Alex's recipe from Cookbok: 710ms EDIT: ~unutbu is leading the race.

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• How is thread synchronization implemented, at the assembly language level?

  - by Martin

  While I'm familiar with concurrent programming concepts such as mutexes and semaphores, I have never understood how they are implemented at the assembly language level. I imagine there being a set of memory "flags" saying: lock A is held by thread 1 lock B is held by thread 3 lock C is not held by any thread etc But how is access to these flags synchronized between threads? Something like this naive example would only create a race condition: mov edx, [myThreadId] wait: cmp [lock], 0 jne wait mov [lock], edx ; I wanted an exclusive lock but the above ; three instructions are not an atomic operation :(

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• Need some tips on my SQL script?

  - by Nano HE

  Hi I plan to create a tale to store the race result like this, Place RaceNumber Gender Name Result 12 0112 Male Mike Lee 1:32:40 16 0117 Female Rose Marry 2:20:40 I confused at the items type definiation. Q1.I am not sure the result can be set to varchar(32) or other type? Q2. and for racenumber, between int(11) and varchar(11), which one is better? Q3. Can I use `UNIQUE KEY` like my way? Q4. Do I need split name to firstName and lastName in my DB table? DROP TABLE IF EXISTS `race_result`; CREATE TABLE IF NOT EXISTS `race_result` ( `id` int(11) NOT NULL auto_increment, `racenumber` int(11) NOT NULL, `gender` enum('male','female') NOT NULL, `name` varchar(16) NOT NULL, `result` varchar(32) NOT NULL, PRIMARY KEY (`id`), UNIQUE KEY `racenumber` (`racenumber`,`id`) ) ENGINE=MyISAM AUTO_INCREMENT=3 DEFAULT CHARSET=utf8 AUTO_INCREMENT=3;

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• Can I inject a SessionBean into a JEE AroundInvoke-Interceptor?

  - by Michael Locher

  I have an EAR with modules: foo-api.jar foo-impl.jar interceptor.jar In foo-api there is: @Local FooService // (interface of a local stateless session bean) In foo-impl there is: @Stateless FooServiceImpl implements FooService //(implementation of the foo service) In interceptor.jar I want public class BazInterceptor { @EJB private FooService foo; @AroundInvoke public Object intercept( final InvocationContext i) throws Exception { // do someting with foo service return i.proceed(); } The question is: Will a Java EE 5 compliant application server (e.g. JBoss 5) inject into the interceptor? If no, what is good strategy for accessing the session bean? To consider: Deployment ordering / race conditions

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• ZooKeeper and RabbitMQ/Qpid together - overkill or a good combination?

  - by Chris Sears

  Greetings, I'm evaluating some components for a multi-data center distributed system. We're going to be using message queues (via either RabbitMQ or Qpid) so agents can make asynchronous requests to other agents without worrying about addressing, routing, load balancing or retransmission. In many cases, the agents will be interacting with components that were not designed for highly concurrent access, so locking and cross-agent coordination will be needed to avoid race conditions. Also, we'd like the system to automatically respond to agent or data center failures. With the above use cases in mind, ZooKeeper seemed like it might be a good fit. But I'm wondering if trying to use both ZK and message queuing is overkill. It seems like what Zookeeper does could be accomplished by my own cluster manager using AMQP messaging, but that would be hard to get really right. On the other hand, I've seen some examples where ZooKeeper was used to implement message queuing, but I think RabbitMQ/Qpid are a more natural fit for that. Has anyone out there used a combination like this? Thanks in advance, -Chris

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• Binding Dropdownlist using jquery

  - by Geetha

  Hi All, I am try to bind the dropdowmlist using jquery. But is showing some error. Code: $.ajax({ type: "POST", contentType: "application/json; charset=utf-8", data: "{product: '" + product + "'}", url: "Search.aspx/FetchCategory", dataType: "json", success: function(data) { $.each(data.d, function() { $("#ddlCategory").append($("<option></option>").val(this['ID']).html(this['Category'])); }); } }); values in the data: [{"Category":"All","ID":"%"},"Category":"Action,"ID":"4"},"Category":"Race,"ID":"5"},"Category":"Sports,"ID":"6"}] Error: $("#ddlCategory").append($("").val(this['ID']).html(this['Category'])); Microsoft JScript runtime error: Object doesn't support this property or method Geetha

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• sql query question

  - by bu0489

  hey guys, just having a bit of difficulty with a query, i'm trying to figure out how to show the most popular naturopath that has been visited in a centre. My tables look as follows; Patient(patientId, name, gender, DoB, address, state,postcode, homePhone, businessPhone, maritalStatus, occupation, duration,unit, race, registrationDate , GPNo, NaturopathNo) and Naturopath (NaturopathNo, name, contactNo, officeStartTime, officeEndTime, emailAddress) now to query this i've come up with SELECT count(*), naturopathno FROM dbf10.patient WHERE naturopathno != 'NULL' GROUP BY naturopathno; which results in; COUNT(*) NATUROPATH 2 NP5 1 NP6 3 NP2 1 NP1 2 NP3 1 NP7 2 NP8 My question is, how would I go about selecting the highest count from this list, and printing that value with the naturopaths name? Any suggestions are very welcome, Brad

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• Producer Consumer Issue with Core Data

  - by Mugunth Kumar

  I've a Core Data application. In the producer thread, I pull data from a web service and store it in my object and call save. My consumer object is a table view controller that displays the same. However, the app crashes and I get NSFetchedResultsController Error: expected to find object (entity: FeedEntry; id: 0xf46f40 ; data: ) in section (null) for deletion on the console. When I debug it, everything works fine. So I understood that it's like a race issue. How is these kind of problem solved? What's the best way to design a producer-consumer app with core-data?

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• What static analysis tools are available for C#?

  - by Paul Mrozowski

  What tools are there available for static analysis against C# code? I know about FxCop and StyleCop. Are there others? I've run across NStatic before but it's been in development for what seems like forever - it's looking pretty slick from what little I've seen of it, so it would be nice if it would ever see the light of day. Along these same lines (this is primarily my interest for static analysis), tools for testing code for multithreading issues (deadlocks, race conditions, etc.) also seem a bit scarce. Typemock Racer just popped up so I'll be looking at that. Anything beyond this? Real-life opinions about tools you've used are appreciated.

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• UITableView via NSFetchedResultsControllerDelegate, select first record by default?

  - by deafgreatdane

  I have a UITableView that gets populated from CoreData via a controller that implements NSFetchedResultsControllerDelegate. How can I have it automatically select the first row (and fire the tableView:didSelectRowAtIndexPath message)? The tableview is used for a variety of predicate queries, so I'm suspicious of solutions that work on the UIViewController lifecycle (viewDidLoad, etc), but I'm new to the platform, so I'm open. I've tried a variety of things, but I'm not sure where in the call stack to put it. I've tried calling cell.selected = true inside tableView:cellForRowAtIndex: method, but that just ends up turning the cell black (and doesn't fire the selected callback method) A tagent question, with all the delegating and core data protocols, does it imply asynchronous data fetch (multiple threads)? Or is the NSFetchedResultsController calling all its related methods in the same thread? Maybe I'm just scared that if it is async, there would be race conditions that would be tough to troubleshoot later.

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• Managing EntityConnection lifetime

  - by kervin

  There have been many question on managing EntityContext lifetime, e.g. http://stackoverflow.com/questions/813457/instantiating-a-context-in-linq-to-entities I've come to the conclusion that the entity context should be considered a unit-of-work and therefore not reused. Great. But while doing some research for speeding up my database access, I ran into this blog post... Improving Entity Framework Performance The post argues that EFs poor performance compared to other frameworks is often due to the EntityConnection object being created each time a new EntityContext object is needed. To test this I manually created a static EntityConnection in Global.asax.cs Application_Start(). I then converted all my context using statements to using( MyObjContext currContext = new MyObjeContext(globalStaticEFConnection) { .... } This seems to have sped things up a bit without any errors so far as far as I can tell. But is this safe? Does using a applicationwide static EntityConnection introduce race conditions? Best regards, Kervin

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• Delete link to file without clearing readonly bit

  - by Joshua

  I have a set of files with multiple links to them. The files are owned by TFS source control but other links to them are made to them. How do I delete the additional links without clearing the readonly bit. It's safe to assume: The files have more than one link to them You are not deleting the name owned by TFS There are no potential race conditions You have ACL full control for the files The machine will not lose power, nor will your program be killed unless it takes way too long. It's not safe to assume: The readonly bit is set (don't set it if its not) You can leave the readonly bit clear if you encounter an error and it was initially set Do not migrate to superuser -- if migrated there the answer is impossible because no standard tool can do this.

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• Scala and the Java Memory Model

  - by Ben Lings

  The Java Memory Model (since 1.5) treats final fields differently to non-final fields. In particular, provided the this reference doesn't escape during construction, writes to final fields in the constructor are guaranteed to be visible on other threads even if the object is made available to the other thread via a data race. (Writes to non-final fields aren't guaranteed to be visible, so if you improperly publish them, another thread could see them in a partially constructed state.) Is there any documentation on how/if the Scala compiler creates final (rather than non-final) backing fields for classes? I've looked through the language specification and searched the web but can't find any definitive answers. (In comparison the @scala.volatile annotation is documented to mark a field as volatile)

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• localtime_r supposed to be thread safe, but causing errors in Valgrind DRD

  - by Nik

  I searched google as much as I could but I couldn't find any good answers to this. localtime_r is supposed to be a thread-safe function for getting the system time. However, when checking my application with Valgrind --tool=drd, it consistantly tells me that there is a data race condition on this function. Are the common search results lying to me, or am I just missing something? It doesn't seem efficient to surround each localtime_r call with a mutex, especially if it is supposed to by thread safe in the first place. here is how i'm using it: timeval handlerTime; gettimeofday(&handlerTime,NULL); tm handlerTm; localtime_r(&handlerTime.tv_sec,&handlerTm); Any ideas?

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• What threading analysis tools do you recommend?

  - by glutz78

  My primary IDE is Visual Studio 2005 and I have a large C/C++ project. I'm interested in what thread analysis tools are recommended. By that I mean, I want a tool, static or dynamic, to help find race conditions, deadlocks, and the like. So far I've casually researched the following: 1. Intel Thread Checker: I don't believe that it ties into VS 2005? 2. Valgrind/Helgrind: free. 3. Coverity: this is a costly tool if i understand correctly. Anyone have experience with any of these or other? I'd much appreciate any advice. Thank you.

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• Can't change header logo in Magento?

  - by Jared

  Hello, I have created my own theme for magento, and I am trying to edit the logo and am having difficulty. I have created my own local/Mage/Page/etc/config.xml file, and specified within the config the following: <logo_src>images/Race-Ramps-logo-streaked-gradient.gif</logo_src> I created a custom layout in the file as well. I can see the custom layout file in the CMS, but my changes to the logo do absolutely nothing. I have even tried changing the original core config.xml, it still stays the same. On multiple websites, I have been told to go to System Configuration Design Header, but I do not have the header option? Help please!

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• How should I protect against hard link attacks?

  - by Thomas

  I want to append data to a file in /tmp. If the file doesn't exist I want to create it I don't care if someone else owns the file. The data is not secret. I do not want someone to be able to race-condition this into writing somewhere else, or to another file. What is the best way to do this? Here's my thought: fd = open("/tmp/some-benchmark-data.txt", O_APPEND | O_CREAT | O_NOFOLLOW | O_WRONLY, 0644); fstat(fd, &st); if (st.st_nlink != 1) { HARD LINK ATTACK! } What's the right way? Besides not using a world-writable directory.

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• How to implement blocking request-reply using Java concurrency primitives?

  - by Uri

  My system consists of a "proxy" class that receives "request" packets, marshals them and sends them over the network to a server, which unmarshals them, processes, and returns some "response packet". My "submit" method on the proxy side should block until a reply is received to the request (packets have ids for identification and referencing purposes) or until a timeout is reached. If I was building this in early versions of Java, I would likely implement in my proxy a collection of "pending messages ids", where I would submit a message, and wait() on the corresponding id (with a timeout). When a reply was received, the handling thread would notify() on the corresponding id. Is there a better way to achieve this using an existing library class, perhaps in java.util.concurrency? If I went with the solution described above, what is the correct way to deal with the potential race condition where a reply arrives before wait() is invoked?

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• Making only one task run at a time in celerybeat

  - by Noufal Ibrahim

  I have a task which I execute once a minute using celerybeat. It works fine. Sometimes though, the task takes a few seconds more than a minute to run because of which two instances of the task run. This leads to some race conditions that mess things up. I can (and probably should) fix my task to work properly but I wanted to know if celery has any builtin ways to ensure this. My cursory Google searches and RTFMs yielded no results.

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• How can I synchronize database access between a write-thread and a read-thread?

  - by Runcible

  My program has two threads: Main execution thread that handles user input and queues up database writes A utility thread that wakes up every second and flushes the writes to the database Inside the main thread, I occasionally need to make reads on the database. When this happens, performance is not important, but correctness is. (In a perfect world, I would be reading from a cache, not making a round-trip to the database - but let's put that aside for the sake of discussion.) How do I make sure that the main thread sees a correct / quiescent database? A standard mutex won't work, since I run the risk of having the main thread grab the mutex before the data gets flushed to the database. This would be a big race condition. What I really want is some sort of mutex that lets the main thread of execution proceed only AFTER the mutex has been grabbed and released once. Does such a thing exist? What's the best way to solve this problem?

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• How can I tell when changes to jquery html() have finished ?

  - by mike_t2e

  I'm using jQuery to change the HTML of a tag, and the new HTML can be a very long string. $("#divToChange").html(newHTML); I then want to select elements created in the new HTML, but if I put the code immediately following the above line it seems to create a race condition with a long string where the changes that html() is making may not necessarily be finished rendering. In that case, trying to select the new elements won't always work. What I want to know is, is there an event fired or some other way of being notified when changes to html() have finished rendering ? I came across the jQuery watch plugin, which works alright as workaround but it's not ideal. Is there a better way ?

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• Google App Engine - Dealing with concurrency issues of storing an object

  - by Spines

  My User object that I want to create and store in the datastore has an email, and a username. How do I make sure when creating my User object that another User object doesn't also have either the same email or the same username? If I just do a query to see if any other users have already used the username or the email, then there could be a race condition. UPDATE: The solution I'm currently considering is to use the MemCache to implement a locking mechanism. I would acquire 2 locks before trying to store the User object in the datastore. First a lock that locks based on email, then another that locks based on username. Since creating new User objects only happens at user registration time, and it's even rarer that two people try to use either the same username or the same email, I think it's okay to take the performance hit of locking. I'm thinking of using the MemCache locking code that is here: http://appengine-cookbook.appspot.com/recipe/mutex-using-memcache-api/ What do you guys think?

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