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• DVCS with a Windows central repository

  - by Mikko Rantanen

  We are currently using VSS for version control. Quite few of our developers are interested in a distributed model (And want to get rid of VSS). Our network is full of Windows machines and while our IT department has experience maintaining Linux machines they would prefer not to. What DVCS systems can host their central repository on Windows while providing.. Push access to the repository. Basic authentication. Mostly just a way to allow or deny access to the whole repository. No need for fine grained access. Server process so users don't need write right to the repository reducing the risk of accidentally messing with it. On the client side a GUI such as Tortoise would be more or less a requirement (Sorry, Windows shell sucks. :|). Ease of installation would be a huge plus as our IT department is already quite low on resources. And using windows credentials for authentication would be an advantage but not a requirement as long as the client is able to store the credentials. I have had a (really) quick look at Git, Mercurial and Bazaar. Git seemed to use ssh or simple WebDAV for repository access, requiring write permission for the users. Mercurial had a built in http server, but this seemed to be only for pull purposes. Update: Mercurial supports push as well. Bazaar Seemed to use sftp for repository access, again requiring a write permission for the users. Are there windows server processes for any DVCS systems and has anyone managed to set one up in a Windows land? And apologies if this is a duplicate question. I couldn't find one. Update Got Mercurial working for push purposes! Detailed list what was required can be found as an answer below.

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• Correct way to generate order numbers in SQL Server

  - by Anton Gogolev

  This question certainly applies to a much broader scope, but here it is. I have a basic ecommerce app, where users can, naturally enough, place orders. Said orders need to have a unique number, which I'm trying to generate right now. Each order is Vendor-specific. Basically, I have an OrderNumberInfo (VendorID, OrderNumber) table. Now whenever a customer places an order I need to increment OrderNumber for a particuar Vendor and return that value. Naturally, I don't want other processes to interfere with me, so I need to exclusively lock this row somehow: begin tranaction declare @n int select @n = OrderNumber from OrderNumberInfo where VendorID = @vendorID update OrderNumberInfo set OrderNumber = @n + 1 where OrderNumber = @n and VendorID = @vendorID commit transaction Now, I've read about select ... with (updlock rowlock), pessimistic locking, etc., but just cannot fit all this in a coherent picture: How do these hints play with SQL Server 2008s' snapshot isolation? Do they perform row-level, page-level or even table-level locks? How does this tolerate multiple users trying to generate numbers for a single Vendor? What isolation levels are appropriate here? And generally - what is the way to do such things?

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• Throttling CPU/Memory usage of a Thread in Java?

  - by Nalandial

  I'm writing an application that will have multiple threads running, and want to throttle the CPU/memory usage of those threads. There is a similar question for C++, but I want to try and avoid using C++ and JNI if possible. I realize this might not be possible using a higher level language, but I'm curious to see if anyone has any ideas. EDIT: Added a bounty; I'd like some really good, well thought out ideas on this. EDIT 2: The situation I need this for is executing other people's code on my server. Basically it is completely arbitrary code, with the only guarantee being that there will be a main method on the class file. Currently, multiple completely disparate classes, which are loaded in at runtime, are executing concurrently as separate threads. I inherited this code (the original author is gone). The way it's written, it would be a pain to refactor to create separate processes for each class that gets executed. If that's the only good way to limit memory usage via the VM arguments, then so be it. But I'd like to know if there's a way to do it with threads. Even as a separate process, I'd like to be able to somehow limit its CPU usage, since as I mentioned earlier, several of these will be executing at once. I don't want an infinite loop to hog up all the resources. EDIT 3: An easy way to approximate object size is with java's Instrumentation classes; specifically, the getObjectSize method. Note that there is some special setup needed to use this tool.

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• Is it possible to store pointers in shared memory without using offsets?

  - by Joseph Garvin

  When using shared memory, each process may mmap the shared region into a different area of their address space. This means that when storing pointers within the shared region, you need to store them as offsets of the start of the shared region. Unfortunately, this complicates use of atomic instructions (e.g. if you're trying to write a lock free algorithm). For example, say you have a bunch of reference counted nodes in shared memory, created by a single writer. The writer periodically atomically updates a pointer 'p' to point to a valid node with positive reference count. Readers want to atomically write to 'p' because it points to the beginning of a node (a struct) whose first element is a reference count. Since p always points to a valid node, incrementing the ref count is safe, and makes it safe to dereference 'p' and access other members. However, this all only works when everything is in the same address space. If the nodes and the 'p' pointer are stored in shared memory, then clients suffer a race condition: x = read p y = x + offset Increment refcount at y During step 2, p may change and x may no longer point to a valid node. The only workaround I can think of is somehow forcing all processes to agree on where to map the shared memory, so that real pointers rather than offsets can be stored in the mmap'd region. Is there any way to do that? I see MAP_FIXED in the mmap documentation, but I don't know how I could pick an address that would be safe.

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• Python access an object byref / Need tagging

  - by Aaron C. de Bruyn

  I need to suck data from stdin and create a object. The incoming data is between 5 and 10 lines long. Each line has a process number and either an IP address or a hash. For example: pid=123 ip=192.168.0.1 - some data pid=123 hash=ABCDEF0123 - more data hash=ABCDEF123 - More data ip=192.168.0.1 - even more data I need to put this data into a class like: class MyData(): pid = None hash = None ip = None lines = [] I need to be able to look up the object by IP, HASH, or PID. The tough part is that there are multiple streams of data intermixed coming from stdin. (There could be hundreds or thousands of processes writing data at the same time.) I have regular expressions pulling out the PID, IP, and HASH that I need, but how can I access the object by any of those values? My thought was to do something like this: myarray = {} for each line in sys.stdin.readlines(): if pid and ip: #If we can get a PID out of the line myarray[pid] = MyData().pid = pid #Create a new MyData object, assign the PID, and stick it in myarray accessible by PID. myarray[pid].ip = ip #Add the IP address to the new object myarray[pid].lines.append(data) #Append the data myarray[ip] = myarray[pid] #Take the object by PID and create a key from the IP. <snip>do something similar for pid and hash, hash and ip, etc...</snip> This gives my an array with two keys (a PID and an IP) and they both point to the same object. But on the next iteration of the loop, if I find (for example) an IP and HASH and do: myarray[hash] = myarray[ip] The following is False: myarray[hash] == myarray[ip] Hopefully that was clear. I hate to admit that waaay back in the VB days, I remember being able handle objects byref instead of byval. Is there something similar in Python? Or am I just approaching this wrong?

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• Load Spikes on a Apache MySQL Server with Wordpress MU

  - by Vikram Goyal

  Hi there, I am trying to investigate the reasons for some mysterious load spikes on a Linux Apache server (2.2.14) running PHP 5.2.9 on a dedicated server with enough processing power and memory. My primary web application is a Wordpress MU (2.9.2) installation. I have investigated and ruled out DOS attack, MySQL or Apache configuration issues. The log files don't give me anything of interest, except to tell me that there is severe load. The load (which can go up to 100) just seems to come and go. It helps that I have a script that checks every 3 minutes for the load, and restarts Apache. Restarting it helps, and the server comes back, till it happens again. There seems to be no set time frame, or visitor numbers on the site that can trigger this. Even a low number of concurrent visitors (20) can trigger it. I am almost convinced that there is a rewrite loop somewhere that is causing Apache to go mad. Apache is trying to serve something that is causing it to spawn more and more processes till it keels over. My question is: Given that I am convinced that this is a rewrite issue or something similar, how can I try and figure out what the issue is? What should I monitor? Apache logs are voluminous, and not very helpful. Of course, if this is not the issue, then at least knowing what to look for will help me eliminate this as an issue and look for something else. Thanks! Vikram

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• How can I validate/secure/authenticate a JavaScript-based POST request?

  - by Bungle

  A product I'm helping to develop will basically work like this: A Web publisher creates a new page on their site that includes a <script> from our server. When a visitor reaches that new page, that <script> gathers the text content of the page and sends it to our server via a POST request (cross-domain, using a <form> inside of an <iframe>). Our server processes the text content and returns a response (via JSONP) that includes an HTML fragment listing links to related content around the Web. This response is cached and served to subsequent visitors until we receive another POST request with text content from the same URL, at which point we regenerate a "fresh" response. These POSTs only happen when our cached TTL expires, at which point the server signifies that and prompts the <script> on the page to gather and POST the text content again. The problem is that this system seems inherently insecure. In theory, anyone could spoof the HTTP POST request (including the referer header, so we couldn't just check for that) that sends a page's content to our server. This could include any text content, which we would then use to generate the related content links for that page. The primary difficulty in making this secure is that our JavaScript is publicly visible. We can't use any kind of private key or other cryptic identifier or pattern because that won't be secret. Ideally, we need a method that somehow verifies that a POST request corresponding to a particular Web page is authentic. We can't just scrape the Web page and compare the content with what's been POSTed, since the purpose of having JavaScript submit the content is that it may be behind a login system. Any ideas? I hope I've explained the problem well enough. Thanks in advance for any suggestions.

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• How to store a file on a server(web container) through a Java EE web application?

  - by Yatendra Goel

  I have developed a Java EE web application. This application allows a user to upload a file with the help of a browser. Once the user has uploaded his file, this application first stores the uploaded file on the server (on which it is running) and then processes it. At present, I am storing the file on the server as follows: try { FormFile formFile = programForm.getTheFile(); // formFile represents the uploaded file String path = getServlet().getServletContext().getRealPath("") + "/" + formFile.getFileName(); System.out.println(path); file = new File(path); outputStream = new FileOutputStream(file); outputStream.write(formFile.getFileData()); } where, the formFile represents the uploaded file. Now, the problem is that it is running fine on some servers but on some servers the getServlet().getServletContext().getRealPath("") is returning null so the final path that I am getting is null/filename and the file doesn't store on the server. When I checked the API for ServletContext.getRealPath() method, I found the following: public java.lang.String getRealPath(java.lang.String path) Returns a String containing the real path for a given virtual path. For example, the path "/index.html" returns the absolute file path on the server's filesystem would be served by a request for "http://host/contextPath/index.html", where contextPath is the context path of this ServletContext. The real path returned will be in a form appropriate to the computer and operating system on which the servlet container is running, including the proper path separators. This method returns null if the servlet container cannot translate the virtual path to a real path for any reason (such as when the content is being made available from a .war archive). So, Is there any other way by which I can store files on those servers also which is returning null for getServlet().getServletContext().getRealPath("")

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• is there a Universal Model for languages?

  - by Smandoli

  Many programming languages share generic and even fairly universal features. For example, if you compared Java, VB6, .NET, PHP, Python, then you would find common functions such as control structures, numeric and string manipulation, etc. What has been done to define these features at a meta-language (or language-agnostic) level? UML offers a descriptive reference of software in every aspect, but the real-world focus seems to be data processes. Is UML relevant? I'm not asking "Why we don't have a single language that replaces the current plethora." We need many different tools (at least in this eon). I'm not asking that all languages fit a template -- assembly vs. compiled languages are different enough to make that unfeasible (and some folks call HTML a language, though I wouldn't). Any attempt would start with a properly narrow scope. In line with this, I wouldn't expect the model to cover even a small selection with full validity. I would expect however that such a model could be used to transpose from one language to another (with limited goals -- think jist translation).

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• Is it legal to have different SOAP namespaces/versions between the request and response?

  - by Lord Torgamus

  THIRD EDIT: I now believe that this problem is due to a SOAP version mismatch (1.1 request, 1.2 response) masquerading as a namespace problem. Is it illegal to mix versions, or just bad style? Am I completely out of luck if I can't change my SOAP version or the service's? SECOND EDIT: Clarified error message, and tried to reduce "tl;dr"-ness. EDIT: [Link deleted, not related] Using soapUI, I'm sending a request that starts with: <soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" ... and getting a response that starts with: <soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope" ... I know the service is getting the info, because processes down the line are working. However, my soapUI teststep fails. It has two active assertions: "SOAP Response" and "Not SOAP Fault." The failure marker is next to "SOAP Response," with the following message: line -1: Element Envelope@http://www.w3.org/2003/05/soap-envelope is not a valid Envelope@http://schemas.xmlsoap.org/soap/envelope/ document or a valid substitution. I have tried mixing and matching the namespace prefixes and schema URLs. Changing prefixes seems to have no effect; changing URLs causes a VersionMismatch error. I have also tried to use a substitution group, but that doesn't seem to be legal.

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• Resume Activity with back button

  - by Hallik

  I have an application I am creating with a DashboardActivity & a SettingsActivity. On the dashboard, I have one object displayed, but when I go into settings, I want to be able to select/deselect X options. Once the user clicks the back button, I save that data locally and to the server. Once the phone receives a success message from the server that it was stored properly, I want to reload the dashboard. I thought I would do this with the onPause and onResume, but they are called when the DashboardActivity is first created. What would be the best way to reload the dashboard by calling my web service after the settings were saved to the server? Here is what I am doing when the back button is hit @Override public boolean onKeyDown(int keyCode, KeyEvent event) { //Save data to the server once the user hits the back button if ((keyCode == KeyEvent.KEYCODE_BACK)) { SchoolSearchActivity.this.divisionProxy = new DivisionProxy(SchoolSearchActivity.this.saveUserDivisionHandler); SchoolSearchActivity.this.divisionProxy.submitUserDivisions(SchoolSearchActivity.this.userDivisions, SchoolSearchActivity.this.user_id); //SchoolSearchActivity.this.finish(); //Toast.makeText(SchoolSearchActivity.this, "Divisions Saved",Toast.LENGTH_LONG).show(); } return true; } The above opens an HTTP connection, and when the response is received, the handler processes the response. I am going to: 1) throw up a progress dialog box letting the user know it's being submitted to the server, and once a response is received, 2) go back to the previous activity and call the "refresh" webservice again. What's the best way to accomplish 1 & 2?

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• Copy a multi-dimentional array by Value (not by reference) in PHP.

  - by Simon R

  Language: PHP I have a form which asks users for their educational details, course details and technical details. When the form is submitted the page goes to a different page to run processes on the information and save parts to a database. HOWEVER(!) I then need to return the page back to the original page, where having access to the original post information is needed. I thought it would be simple to copy (by value) the multi-dimensional (md) $_POST array to $_SESSION['post'] session_start(); $_SESSION['post'] = $_POST; However this only appears to place the top level of the array into $_SESSION['post'] not doing anything with the children/sub-arrays. An abbreviated form of the md $_POST array is as follows: Array ( [formid] = 2 [edu] = Array ( ['id'] = Array ( [1] = new_1 [2] = new_2 ) ['nameOfInstitution'] = Array ( [1] = 1 [2] = 2 ) ['qualification'] = Array ( [1] = blah [2] = blah ) ['grade'] = Array ( [1] = blah [2] = blah ) ) [vID] = 61 [Submit] = Save and Continue ) If I echo $_SESSION['post']['formid'] it writes "2", and if I echo $_SESSION['post']['edu'] it returns "Array". If I check that edu is an array (is_array($_SESSION['post']['edu])) it returns true. If I echo $_SESSION['post']['edu']['id'] it returns array, but when checked (is_array($_SESSION['post']['edu]['id'])) it returns false and I cannot echo out any of the elements. How do I successfully copy (by value, not by reference) the whole array (including its children) to an new array?

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• How to migrate primary key generation from "increment" to "hi-lo"?

  - by Bevan

  I'm working with a moderate sized SQL Server 2008 database (around 120 tables, backups are around 4GB compressed) where all the table primary keys are declared as simple int columns. At present, primary key values are generated by NHibernate with the increment identity generator, which has worked well thus far, but precludes moving to a multiprocessing environment. Load on the system is growing, so I'm evaluating the work required to allow the use of multiple servers accessing a common database backend. Transitioning to the hi-lo generator seems to be the best way forward, but I can't find a lot of detail about how such a migration would work. Will NHibernate automatically create rows in the hi-lo table for me, or do I need to script these manually? If NHibernate does insert rows automatically, does it properly take account of existing key values? If NHibernate does take care of thing automatically, that's great. If not, are there any tools to help? Update NHibernate's increment identifier generator works entirely in-memory. It's seeded by selecting the maximum value of used identifiers from the table, but from that point on allocates new values by a simple increment, without reference back to the underlying database table. If any other process adds rows to the table, you end up with primary key collisions. You can run multiple threads within the one process just fine, but you can't run multiple processes. For comparison, the NHibernate identity generator works by configuring the database tables with identity columns, putting control over primary key generation in the hands of the database. This works well, but compromises the unit of work pattern. The hi-lo algorithm sits inbetween these - generation of primary keys is coordinated through the database, allowing for multiprocessing, but actual allocation can occur entirely in memory, avoiding problems with the unit of work pattern.

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• Efficient Method for Preventing Hotlinking via .htaccess

  - by Michael Robinson

  I need to confirm something before I go accuse someone of ... well I'd rather not say. The problem: We allow users to upload images and embed them within text on our site. In the past we allowed users to hotlink to our images as well, but due to server load we unfortunately had to stop this. Current "solution": The method the programmer used to solve our "too many connections" issue was to rename the file that receives and processes image requests (image_request.php) to image_request2.php, and replace the contents of the original with <?php header("HTTP/1.1 500 Internal Server Error") ; ?> Obviously this has caused all images with their src attribute pointing to the original image_request.php to be broken, and is also the wrong code to be sending in this case. Proposed solution: I feel a more elegant solution would be: In .htaccess If the request is for image_request.php Check referrer If referrer is not our site, send the appropriate header If referrer is our site, proceed to image_request.php and process image request What I would like to know is: Compared to simply returning a 500 for each request to image_request.php: How much more load would be incurred if we were to use my proposed alternative solution outlined above? Is there a better way to do this? Our main concern is that the site stays up. I am not willing to agree that breaking all internally linked images is the best / only way to solve this. I refuse to tell our users that because of something WE changed they must now manually change the embed code in all their previously uploaded content.

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• Suggestions for a Cron like scheduler in Python?

  - by jamesh

  I'm looking for a library in Python which will provide at and cron like functionality. I'd quite like have a pure Python solution, rather than relying on tools installed on the box; this way I run on machines with no cron. For those unfamiliar with cron: you can schedule tasks based upon an expression like: 0 2 * * 7 /usr/bin/run-backup # run the backups at 0200 on Every Sunday 0 9-17/2 * * 1-5 /usr/bin/purge-temps # run the purge temps command, every 2 hours between 9am and 5pm on Mondays to Fridays. The cron time expression syntax is less important, but I would like to have something with this sort of flexibility. If there isn't something that does this for me out-the-box, any suggestions for the building blocks to make something like this would be gratefully received. Edit I'm not interested in launching processes, just "jobs" also written in Python - python functions. By necessity I think this would be a different thread, but not in a different process. To this end, I'm looking for the expressivity of the cron time expression, but in Python. Cron has been around for years, but I'm trying to be as portable as possible. I cannot rely on its presence.

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• svnsync loses revision properties although hook installed

  - by roesslerj

  Hello all! I have a pretty weird problem. We have setup an SVN-Mirror via cronjob (because it needs to go from inside to outside of a firewall, so no post-commit-hook possible) and svnsync. We installed a pre-revprop-hook just as told. Everything seems to work fine, except that it doesn't. E.g. when manually executing the script. # svnsync --non-interactive sync file://<path-to-mirror> --source-username <usr> --source-password <pwd> Committed revision 19817. Copied properties for revision 19817. No error, no complaints. But if checking for the revision properties it says: # svnlook info <path-to-mirror> 0 # svn info -r HEAD file://<path-to-mirror> 2>&1 Path: <root-of-mirror> URL: file://<path-to-mirror> Repository Root: file://<path-to-mirror> Repository UUID: <uid> Revision: 19817 Node Kind: directory Last Changed Rev: 19817 So somehow the author and timestamp information gets lost. But we need that information for our internal processes. Since no error or warning is produced I have absolutely no idea even where to start to look. Everything is local (except for the remote master), so there are no server-logs to look at. I also tried to manually recopy via svnsync copy-revprops (http://chestofbooks.com/computers/revision-control/subversion-svn/svnsync-Copy-revprops-Ref-svnsync-C-Copy-revprops.html). It says Copied properties for revision 19885. But when I query them, it's just the same. Any ideas how I could approach that problem, or even better -- how to solve it? Any ideas appreciated.

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• Which apache worker to use with passenger and how?

  - by Millisami

  I've this config in my apache2.conf <IfModule mpm_prefork_module> StartServers 5 MinSpareServers 5 MaxSpareServers 10 MaxClients 150 MaxRequestsPerChild 0 </IfModule> # worker MPM # StartServers: initial number of server processes to start # MaxClients: maximum number of simultaneous client connections # MinSpareThreads: minimum number of worker threads which are kept spare # MaxSpareThreads: maximum number of worker threads which are kept spare # ThreadsPerChild: constant number of worker threads in each server process # MaxRequestsPerChild: maximum number of requests a server process serves <IfModule mpm_worker_module> StartServers 2 MaxClients 15 MinSpareThreads 4 MaxSpareThreads 5 ThreadsPerChild 15 MaxRequestsPerChild 50000 </IfModule> Now I'm confused here. Which module gets loaded on which conditions? The phusion guys have suggested to use the worker module. Since both are present in apache conf file, do I have to comment the mpm_prefork_module or leave it as it is? Following is my passenger conf file for apache: LoadModule passenger_module /usr/lib/ruby/gems/1.8/gems/passenger-2.2.4/ext/apache2/mod_passenger.so PassengerRoot /usr/lib/ruby/gems/1.8/gems/passenger-2.2.4 PassengerRuby /usr/bin/ruby1.8 PassengerMaxPoolSize 3 PassengerPoolIdleTime 999999 RailsFrameworkSpawnerIdleTime 0 RailsAppSpawnerIdleTime 0 I'm running just a single Rails 2.3.2 app on 256MB slice at slicehost. I'm not quite satisfied with the performance yet. Are the settings above are any good??

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• Hibernate database integrity with multiple java applications

  - by Austen

  We have 2 java web apps both are read/write and 3 standalone java read/write applications (one loads questions via email, one processes an xml feed, one sends email to subscribers) all use hibernate and share a common code base. The problem we have recently come across is that questions loaded via email sometimes overwrite questions created in one of the web apps. We originally thought this to be a caching issue. We've tried turning off the second level cache, but this doesn't make a difference. We are not explicitly opening and closing sessions, but rather let hibernate manage them via Util.getSessionFactory().getCurrentSession(), which thinking about it, may actually be the issue. We'd rather not setup a clustered 2nd level cache at this stage as this creates another layer of complexity and we're more than happy with the level of performance we get from the app as a whole. So does implementing a open-session-in-view pattern in the web apps and manually managing the sessions in the standalone apps sound like it would fix this? Or any other suggestions/ideas please? <property name="hibernate.transaction.factory_class">org.hibernate.transaction.JDBCTransactionFactory</property> <property name="hibernate.current_session_context_class">thread</property> <property name="hibernate.cache.use_second_level_cache">false</property>

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• Processing XML form input in ASP

  - by Omar Kooheji

  I'm maintaining a legacy application which consists of some ASP.Net pages with c# code behinds and some asp pages. I need to change the way the application accepts it's input from reading a set of parameters from some form fields to reading in one form field which contains contains some XML and parsing to get the parameters out. I've written a C# class that takes an The NameValueCollection from the C# HttpRequest's Form Element. Like so NameValueCollection form = Request.Form; Dictionary<string, string> fieldDictionary = RequestDataExtractor.BuildFieldDictionary(form); The code in the class looks for a particular parameter and if it's there processes the XML and outputs a Dictionary, if its not there it just cycles through the Form parameters and puts them all into the dictionary (Allowing the old method to still work) How would I do this in ASP? Can I use my same class, or a modified version of it? or do I have to write some new code to get this working? If I have to write ASP code Whats the best way to process the XML in ASP? Sorry if this seems like a stupid question but I know next to nothing about ASP and VB.

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• Atomic int writes on file

  - by Waneck

  Hello! I'm writing an application that will have to be able to handle many concurrent accesses to it, either by threads as by processes. So no mutex'es or locks should be applied to this. To make the use of locks go down to a minimum, I'm designing for the file to be "append-only", so all data is first appended to disk, and then the address pointing to the info it has updated, is changed to refer to the new one. So I will need to implement a small lock system only to change this one int so it refers to the new address. How is the best way to do it? I was thinking about maybe putting a flag before the address, that when it's set, the readers will use a spin lock until it's released. But I'm afraid that it isn't at all atomic, is it? e.g. a reader reads the flag, and it is unset on the same time, a writer writes the flag and changes the value of the int the reader may read an inconsistent value! I'm looking for locking techniques but all I find is either for thread locking techniques, or to lock an entire file, not fields. Is it not possible to do this? How do append-only databases handle this? Thanks! Cauê

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• How do I use the 7-zip LZMA SDK 9.x to self-extract?

  - by Christopher

  I am writing a SFX for an installer. I have a number of good reasons for doing this, primarily: The installer is actually a large Python program which uses plugins. Using py2exe or pyinstaller makes doing plugins annoyingly complicated. I want to be able to pass command-line options directly to the Python installer script, as if it were getting run directly. Using the existing 7-zip SFX modules is clunky because I cannot pass command-line options directly into the processes I want to start. I need more flexibility than any of the existing SFX modules I have seen provide. I have already tried using the SDK to open the file, seek to the 7z archive signature, and run the decompression from there. That fails because the SzArEx_Open() call appears to assume that you are starting at a 0 offset in the file. I am using the File_Seek() call to perform the seeking. It seems like there must be a way to do this, since the 7z archive format itself supports multiple embedded streams. Any pointers to examples would be awesome, but narrative explanation is also quite welcome!

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• Naming multi-instance performance counters in .NET

  - by Roger Lipscombe

  Most multiple instance performance counters in Windows seem to automatically(?) have a #n on the end if there's more than one instance with the same name. For example: if, in Perfmon, you look under the Process category, you'll see: ... dwm explorer explorer#1 ... I have two explorer.exe processes, so the second counter has #1 appended to its name. When I attempt to do this in a .NET application: I can create the category, and register the instance (using the PerformanceCounterCategory.Create that takes a CounterCreationDataCollection). I can open the counter for write and write to it. When I open the counter a second time, it opens the same counter. This means that I have two applications fighting over the counters. The documentation for PerformanceCounter.InstanceName states that # is not allowed in the name. So: how do I have multiple-instance performance counters that are actually multiple instance? And where the second (and subsequent) instances get #n appended to the name? That is: I know that I can put the process ID (e.g.) on the instance name. This works, but has the unfortunate side effect that restarting the process results in a new PID, and Perfmon continues monitoring the old counter.

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• ActiveRecord exceptions not rescued

  - by zoopzoop

  I have the following code block: unless User.exist?(...) begin user = User.new(...) # Set more attributes of user user.save! rescue ActiveRecord::RecordInvalid, ActiveRecord::RecordNotUnique => e # Check if that user was created in the meantime user = User.exists?(...) raise e if user.nil? end end The reason is, as you can probably guess, that multiple processes might call this method at the same time to create the user (if it doesn't already exist), so while the first one enters the block and starts initializing a new user, setting the attributes and finally calling save!, the user might already be created. In that case I want to check again if the user exists and only raise the exception if it still doesn't (= if no other process has created it in the meantime). The problem is, that regularly ActiveRecord::RecordInvalid exceptions are raised from the save! and not rescued from the rescue block. Any ideas? EDIT: Alright, this is weird. I must be missing something. I refactored the code according to Simone's tip to look like this: unless User.find_by_email(...).present? # Here we know the user does not exist yet user = User.new(...) # Set more attributes of user unless user.save # User could not be saved for some reason, maybe created by another request? raise StandardError, "Could not create user for order #{self.id}." unless User.exists?(:email => ...) end end Now I got the following exception: ActiveRecord::RecordNotUnique: Mysql::DupEntry: Duplicate entry '[email protected]' for key 'index_users_on_email': INSERT INTO `users` ... thrown in the line where it says 'unless user.save'. How can that be? Rails thinks the user can be created because the email is unique but then the Mysql unique index prevents the insert? How likely is that? And how can it be avoided?

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• Culture Sensitive GetHashCode

  - by user114928

  Hi, I'm writing a c# application that will process some text and provide basic query functions. In order to ensure the best possible support for other languages, I am allowing the users of the application to specify the System.Globalization.CultureInfo (via the "en-GB" style code) and also the full range of collation options using the System.Globalization.CompareOptions flags enum. For regular string comparison I'm then using a combination of: a) String.Compare overload that accepts the culture and options b) For some bulk processes I'm caching the byte data (KeyData) from CompareInfo.GetSortKey (overload that accepts the options) and using a byte-by-byte comparison of the KeyData. This seemed fine (although please comment if you think these two methods shouldn't be mixed), but then I had reason to use the HashSet< class which only has an overload for IEqualityComparer<. MS documentation seems to suggest that I should use StringComparer (which implements both IEqualityComparer< and IComparer<), but this only seems to support the "IgnoreCase" option from CompareOptions and not "IgnoreKanaType", "IgnoreSymbols", "IgnoreWidth" etc. I'm assuming that a StringComparer that ignores these other options could produce different hashcodes for two strings that might be considered the same using my other comparison options. I'd therefore get incorrect results from my application. Only thought at the moment is to create my own IEqualityComparer< that generates a hashcode from the SortKey.KeyData and compares eqality be using the String.Compare overload. Any suggestions?

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• Wireshark Dissector: How to Identify Missing UDP Frames?

  - by John Dibling

  How do you identify missing UDP frames in a custom Wireshark dissector? I have written a custom dissector for the CQS feed (reference page). One of our servers gaps when receiving this feed. According to Wireshark, some UDP frames are never received. I know that the frames were sent because all of our other servers are gap-free. A CQS frame consists of multiple messages, each having its own sequence number. My custom dissector provides the following data to Wireshark: cqs.frame_gaps - the number of gaps within a UDP frame (always zero) cqs.frame_first_seq - the first sequence number in a UDP frame cqs.frame_expected_seq - the first sequence number expected in the next UDP frame cqs.frame_msg_count - the number of messages in this UDP frame And I am displaying each of these values in custom columns, as shown in this screenshot: I tried adding code to my dissector that simply saves the last-processed sequence number (as a local static), and flags gaps when the dissector processes a frame where current_sequence != (previous_sequence + 1). This did not work because the dissector can be called in random-access order, depending on where you click in the GUI. So you could process frame 10, then frame 15, then frame 11, etc. Is there any way for my dissector to know if the frame that came before it (or the frame that follows) is missing? The dissector is written in C. (See also a companion post on serverfault.com)

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