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  • Implementing a robust async stream reader

    - by Jon
    I recently provided an answer to this question: C# - Realtime console output redirection. As often happens, explaining stuff (here "stuff" was how I tackled a similar problem) leads you to greater understanding and/or, as is the case here, "oops" moments. I realized that my solution, as implemented, has a bug. The bug has little practical importance, but it has an extremely large importance to me as a developer: I can't rest easy knowing that my code has the potential to blow up. Squashing the bug is the purpose of this question. I apologize for the long intro, so let's get dirty. I wanted to build a class that allows me to receive input from a Stream in an event-based manner. The stream, in my scenario, is guaranteed to be a FileStream and there is also an associated StreamReader already present to leverage. The public interface of the class is this: public class MyStreamManager { public event EventHandler<ConsoleOutputReadEventArgs> StandardOutputRead; public void StartSendingEvents(); public void StopSendingEvents(); } Obviously this specific scenario has to do with a console's standard output, but that is a detail and does not play an important role. StartSendingEvents and StopSendingEvents do what they advertise; for the purposes of this discussion, we can assume that events are always being sent without loss of generality. The class uses these two fields internally: protected readonly StringBuilder inputAccumulator = new StringBuilder(); protected readonly byte[] buffer = new byte[256]; The functionality of the class is implemented in the methods below. To get the ball rolling: public void StartSendingEvents(); { this.stopAutomation = false; this.BeginReadAsync(); } To read data out of the Stream without blocking, and also without requiring a carriage return char, BeginRead is called: protected void BeginReadAsync() { if (!this.stopAutomation) { this.StandardOutput.BaseStream.BeginRead( this.buffer, 0, this.buffer.Length, this.ReadHappened, null); } } The challenging part: BeginRead requires using a buffer. This means that when reading from the stream, it is possible that the bytes available to read ("incoming chunk") are larger than the buffer. Since we are only handing off data from the stream to a consumer, and that consumer may well have inside knowledge about the size and/or format of these chunks, I want to call event subscribers exactly once for each chunk. Otherwise the abstraction breaks down and the subscribers have to buffer the incoming data and reconstruct the chunks themselves using said knowledge. This is much less convenient to the calling code, and detracts from the usefulness of my class. To this end, if the buffer is full after EndRead, we don't send its contents to subscribers immediately but instead append them to a StringBuilder. The contents of the StringBuilder are only sent back whenever there is no more to read from the stream (thus preserving the chunks). private void ReadHappened(IAsyncResult asyncResult) { var bytesRead = this.StandardOutput.BaseStream.EndRead(asyncResult); if (bytesRead == 0) { this.OnAutomationStopped(); return; } var input = this.StandardOutput.CurrentEncoding.GetString( this.buffer, 0, bytesRead); this.inputAccumulator.Append(input); if (bytesRead < this.buffer.Length) { this.OnInputRead(); // only send back if we 're sure we got it all } this.BeginReadAsync(); // continue "looping" with BeginRead } After any read which is not enough to fill the buffer, all accumulated data is sent to the subscribers: private void OnInputRead() { var handler = this.StandardOutputRead; if (handler == null) { return; } handler(this, new ConsoleOutputReadEventArgs(this.inputAccumulator.ToString())); this.inputAccumulator.Clear(); } (I know that as long as there are no subscribers the data gets accumulated forever. This is a deliberate decision). The good This scheme works almost perfectly: Async functionality without spawning any threads Very convenient to the calling code (just subscribe to an event) Maintains the "chunkiness" of the data; this allows the calling code to use inside knowledge of the data without doing any extra work Is almost agnostic to the buffer size (it will work correctly with any size buffer irrespective of the data being read) The bad That last almost is a very big one. Consider what happens when there is an incoming chunk with length exactly equal to the size of the buffer. The chunk will be read and buffered, but the event will not be triggered. This will be followed up by a BeginRead that expects to find more data belonging to the current chunk in order to send it back all in one piece, but... there will be no more data in the stream. In fact, as long as data is put into the stream in chunks with length exactly equal to the buffer size, the data will be buffered and the event will never be triggered. This scenario may be highly unlikely to occur in practice, especially since we can pick any number for the buffer size, but the problem is there. Solution? Unfortunately, after checking the available methods on FileStream and StreamReader, I can't find anything which lets me peek into the stream while also allowing async methods to be used on it. One "solution" would be to have a thread wait on a ManualResetEvent after the "buffer filled" condition is detected. If the event is not signaled (by the async callback) in a small amount of time, then more data from the stream will not be forthcoming and the data accumulated so far should be sent to subscribers. However, this introduces the need for another thread, requires thread synchronization, and is plain inelegant. Specifying a timeout for BeginRead would also suffice (call back into my code every now and then so I can check if there's data to be sent back; most of the time there will not be anything to do, so I expect the performance hit to be negligible). But it looks like timeouts are not supported in FileStream. Since I imagine that async calls with timeouts are an option in bare Win32, another approach might be to PInvoke the hell out of the problem. But this is also undesirable as it will introduce complexity and simply be a pain to code. Is there an elegant way to get around the problem? Thanks for being patient enough to read all of this.

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  • Putting update logic in your migrations

    - by Daniel Abrahamsson
    A couple of times I've been in the situation where I've wanted to refactor the design of some model and have ended up putting update logic in migrations. However, as far as I've understood, this is not good practice (especially since you are encouraged to use your schema file for deployment, and not your migrations). How do you deal with these kind of problems? To clearify what I mean, say I have a User model. Since I thought there would only be two kinds of users, namely a "normal" user and an administrator, I chose to use a simple boolean field telling whether the user was an adminstrator or not. However, after I while I figured I needed some third kind of user, perhaps a moderator or something similar. In this case I add a UserType model (and the corresponding migration), and a second migration for removing the "admin" flag from the user table. And here comes the problem. In the "add_user_type_to_users" migration I have to map the admin flag value to a user type. Additionally, in order to do this, the user types have to exist, meaning I can not use the seeds file, but rather create the user types in the migration (also considered bad practice). Here comes some fictional code representing the situation: class CreateUserTypes < ActiveRecord::Migration def self.up create_table :user_types do |t| t.string :name, :nil => false, :unique => true end #Create basic types (can not put in seed, because of future migration dependency) UserType.create!(:name => "BASIC") UserType.create!(:name => "MODERATOR") UserType.create!(:name => "ADMINISTRATOR") end def self.down drop_table :user_types end end class AddTypeIdToUsers < ActiveRecord::Migration def self.up add_column :users, :type_id, :integer #Determine type via the admin flag basic = UserType.find_by_name("BASIC") admin = UserType.find_by_name("ADMINISTRATOR") User.all.each {|u| u.update_attribute(:type_id, (u.admin?) ? admin.id : basic.id)} #Remove the admin flag remove_column :users, :admin #Add foreign key execute "alter table users add constraint fk_user_type_id foreign key (type_id) references user_types (id)" end def self.down #Re-add the admin flag add_column :users, :admin, :boolean, :default => false #Reset the admin flag (this is the problematic update code) admin = UserType.find_by_name("ADMINISTRATOR") execute "update users set admin=true where type_id=#{admin.id}" #Remove foreign key constraint execute "alter table users drop foreign key fk_user_type_id" #Drop the type_id column remove_column :users, :type_id end end As you can see there are two problematic parts. First the row creation part in the first model, which is necessary if I would like to run all migrations in a row, then the "update" part in the second migration that maps the "admin" column to the "type_id" column. Any advice?

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  • Does MS Access 2010 have Tif image control display issue?

    - by ChuckB
    We run MS Access as the front end for most of our business. When we upgraded from Access 2000 to Access 2003 the image control we used to print out Tif images failed to load Tifs anymore. A bit of Googling showed that MS did this intentionally and I'm wondering if the problem still happens in Access 2010. Can someone with a downloaded copy drop an image control on an Access 2010 form or report and try to get it to display a Tif file?

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  • Apache tiles and body onload event. Known issue?

    - by Angus
    We're prototyping a new visual framework using Apache Tiles for markup templating. Often the document onload event (actually prototype's dom:loaded event) is getting fired before all DOM objects are actually loaded Is it possible that the tiles templates are loading in an asynchronous fashion and therefore doing an end run around the event handler? Has anyone else had this experience? Can anyone share a robust workaround? thanks in advance

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  • Reduce Multiple Errors logging in sysssislog

    - by Akshay
    Need help. I am trying to automate error notifications to be sent in mailers. For that I am querying the sysssislog table. I have pasted an "Execute SQl task" on the package event handler "On error". For testing purpose, I am deliberately trying to load duplicate keys in a table which consists of a Primary key column(so as to get an error). But instead of having just one error, "Violation of primary key constraint", SSIS records 3 in the table. PFA the screenshot as well. How can i restrict the tool to log only one error and not multiple ??? Package Structure. Package ("On error Event handler") - DFT - Oledb Source - Oledb Destination SSIS Error Code DTS_E_OLEDBERROR. An OLE DB error has occurred. Error code: 0x80004005. An OLE DB record is available. Source: "Microsoft SQL Server Native Client 10.0" Hresult: 0x80004005 Description: "The statement has been terminated.". An OLE DB record is available. Source: "Microsoft SQL Server Native Client 10.0" Hresult: 0x80004005 Description: "Violation of PRIMARY KEY constraint 'PK_SalesPerson_SalesPersonID'. Cannot insert duplicate key in object 'dbo.SalesPerson'.". SSIS Error Code DTS_E_INDUCEDTRANSFORMFAILUREONERROR. The "input "OLE DB Destination Input" (56)" failed because error code 0xC020907B occurred, and the error row disposition on "input "OLE DB Destination Input" (56)" specifies failure on error. An error occurred on the specified object of the specified component. There may be error messages posted before this with more information about the failure. SSIS Error Code DTS_E_PROCESSINPUTFAILED. The ProcessInput method on component "OLE DB Destination" (43) failed with error code 0xC0209029 while processing input "OLE DB Destination Input" (56). The identified component returned an error from the ProcessInput method. The error is specific to the component, but the error is fatal and will cause the Data Flow task to stop running. There may be error messages posted before this with more information about the failure. Please guide me. Your help is very much appreciated. Thanks

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  • OperationalError: foreign key mismatch

    - by Niek de Klein
    I have two tables that I'm filling, 'msrun' and 'feature'. 'feature' has a foreign key pointing to the 'msrun_name' column of the 'msrun' table. Inserting in the tables works fine. But when I try to delete from the 'feature' table I get the following error: pysqlite2.dbapi2.OperationalError: foreign key mismatch From the rules of foreign keys in the manual of SQLite: - The parent table does not exist, or - The parent key columns named in the foreign key constraint do not exist, or - The parent key columns named in the foreign key constraint are not the primary key of the parent table and are not subject to a unique constraint using collating sequence specified in the CREATE TABLE, or - The child table references the primary key of the parent without specifying the primary key columns and the number of primary key columns in the parent do not match the number of child key columns. I can see nothing that I'm violating. My create tables look like this: DROP TABLE IF EXISTS `msrun`; -- ----------------------------------------------------- -- Table `msrun` -- ----------------------------------------------------- CREATE TABLE IF NOT EXISTS `msrun` ( `msrun_name` VARCHAR(40) PRIMARY KEY NOT NULL , `description` VARCHAR(500) NOT NULL ); DROP TABLE IF EXISTS `feature`; -- ----------------------------------------------------- -- Table `feature` -- ----------------------------------------------------- CREATE TABLE IF NOT EXISTS `feature` ( `feature_id` VARCHAR(40) PRIMARY KEY NOT NULL , `intensity` DOUBLE NOT NULL , `overallquality` DOUBLE NOT NULL , `charge` INT NOT NULL , `content` VARCHAR(45) NOT NULL , `msrun_msrun_name` VARCHAR(40) NOT NULL , CONSTRAINT `fk_feature_msrun1` FOREIGN KEY (`msrun_msrun_name` ) REFERENCES `msrun` (`msrun_name` ) ON DELETE NO ACTION ON UPDATE NO ACTION); CREATE UNIQUE INDEX `id_UNIQUE` ON `feature` (`feature_id` ASC); CREATE INDEX `fk_feature_msrun1` ON `feature` (`msrun_msrun_name` ASC); As far as I can see the parent table exists, the foreign key is pointing to the right parent key, the parent key is a primary key and the foreign key specifies the primary key column. The script that produces the error: from pysqlite2 import dbapi2 as sqlite import parseFeatureXML connection = sqlite.connect('example.db') cursor = connection.cursor() cursor.execute("PRAGMA foreign_keys=ON") inputValues = ('example', 'description') cursor.execute("INSERT INTO `msrun` VALUES(?, ?)", inputValues) featureXML = parseFeatureXML.Reader('../example_scripts/example_files/input/featureXML_example.featureXML') for feature in featureXML.getSimpleFeatureInfo(): inputValues = (featureXML['id'], featureXML['intensity'], featureXML['overallquality'], featureXML['charge'], featureXML['content'], 'example') # insert the values into msrun using ? for sql injection safety cursor.execute("INSERT INTO `feature` VALUES(?,?,?,?,?,?)", inputValues) connection.commit() for feature in featureXML.getSimpleFeatureInfo(): cursor.execute("DELETE FROM `feature` WHERE feature_id = ?", (str(featureXML['id']),))

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  • How to attach to the jQuery Autocomplete plugin's mouse events?

    - by Tommy B
    My problem is this: I need to update some text in a div with the value of what the mouse pointer is hovering over in the autocomplete drop down list. Can it be done easily, or do I have to make fundamental changes to the Autocomplete plugin? I am using the jQuery Autocomplete plugin version 1.1 by Jörn Zaefferer. Any and all help will be greatly appreciated!

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  • gwt back button browser

    - by msaif
    for example current page is www.google.com. but i typed a different website address in address bar and clicked.This site has fully GWT code. but i like to back to the previous page of www.google.com. so i clicked back button of browser.but how can i get event of back button from current GWT code.Can i set any backbutton event handler in GWT of current page?? which notifies an alert to me that back button was pressed is there any solution from GWT??

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  • Android Developing App crashes on startup

    - by alexnavratil
    I currently develop an application which contains a custom ListView. I developed a custom array adapter. I think my app crashes here: ListView DirectoryView = (ListView) findViewById(R.id.fileListView); So i think the error is in the activity_main.xml: <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" android:layout_width="fill_parent" android:layout_height="fill_parent" > <ListView android:id="@+id/fileListView" android:layout_width="fill_parent" android:layout_height="fill_parent" android:layout_weight="1" > </ListView> Here is my LogCat: 09-09 11:19:21.254: E/Trace(1152): error opening trace file: No such file or directory (2) 09-09 11:19:21.484: D/AndroidRuntime(1152): Shutting down VM 09-09 11:19:21.484: W/dalvikvm(1152): threadid=1: thread exiting with uncaught exception (group=0x40a13300) 09-09 11:19:21.504: E/AndroidRuntime(1152): FATAL EXCEPTION: main 09-09 11:19:21.504: E/AndroidRuntime(1152): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.teamdroid.explorer/com.teamdroid.explorer.MainActivity}: java.lang.NullPointerException 09-09 11:19:21.504: E/AndroidRuntime(1152): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2059) 09-09 11:19:21.504: E/AndroidRuntime(1152): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084) 09-09 11:19:21.504: E/AndroidRuntime(1152): at android.app.ActivityThread.access$600(ActivityThread.java:130) 09-09 11:19:21.504: E/AndroidRuntime(1152): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195) 09-09 11:19:21.504: E/AndroidRuntime(1152): at android.os.Handler.dispatchMessage(Handler.java:99) 09-09 11:19:21.504: E/AndroidRuntime(1152): at android.os.Looper.loop(Looper.java:137) 09-09 11:19:21.504: E/AndroidRuntime(1152): at android.app.ActivityThread.main(ActivityThread.java:4745) 09-09 11:19:21.504: E/AndroidRuntime(1152): at java.lang.reflect.Method.invokeNative(Native Method) 09-09 11:19:21.504: E/AndroidRuntime(1152): at java.lang.reflect.Method.invoke(Method.java:511) 09-09 11:19:21.504: E/AndroidRuntime(1152): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786) 09-09 11:19:21.504: E/AndroidRuntime(1152): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553) 09-09 11:19:21.504: E/AndroidRuntime(1152): at dalvik.system.NativeStart.main(Native Method) 09-09 11:19:21.504: E/AndroidRuntime(1152): Caused by: java.lang.NullPointerException 09-09 11:19:21.504: E/AndroidRuntime(1152): at android.app.Activity.findViewById(Activity.java:1825) 09-09 11:19:21.504: E/AndroidRuntime(1152): at com.teamdroid.explorer.listDirectory.getDirectory(listDirectory.java:20) 09-09 11:19:21.504: E/AndroidRuntime(1152): at com.teamdroid.explorer.MainActivity.onCreate(MainActivity.java:33) 09-09 11:19:21.504: E/AndroidRuntime(1152): at android.app.Activity.performCreate(Activity.java:5008) 09-09 11:19:21.504: E/AndroidRuntime(1152): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079) 09-09 11:19:21.504: E/AndroidRuntime(1152): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2023) 09-09 11:19:21.504: E/AndroidRuntime(1152): ... 11 more Please can you help me. I am searching this error for 2 day. thanks!

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  • linux firewall in c

    - by asha
    i m implementing a firewall for linux,i want to block packets using loadable kernal module(lkm). please help me how can i do it ? i just know that it can be done by lkm by using some function in it,like init(),clean_up. i want to know how to use DROP,ACCEPT in blocking function within lkm. i want to do it without iptable command.

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  • How to upgrade a VB6 app with .NET components

    - by Craig Johnston
    I want to make a change to a VB6 app which consists of a .EXE, no VB6 DLLs but a handful of .NET DLLs. The interop is achieved by a one of the .NET dlls being referenced by the VB6 app which seems to require REGASM-ing of an associated .tlb file. If I want to change only the VB6 app .exe, could I just compile it and drop it into the app folder on existing installations or are there going to be binding issues?

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  • Why not use PHP's built-in session handling?

    - by Dougal
    Is there currently - or has there ever been - any serious or significant problem with PHP's built-in session handling? I mean, it's always worked for me and my projects. But I see some codebases and frameworks out there seem to use a custom handler. Is this reinventing the wheel? Or improving on some flaws? What flaws?

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  • C# Event handling In mutiple level /classes

    - by Amitd
    Hi, I have 3 classes A,B,C . Class A creates B .. class B creates class C. Class C raises events after some action/operation with some data, which is handled by event handler in Class B. Now I want to be handle or pass the same raised event data to Class A. I know i can raise another event from class B and handle it in A but is there a better way of handling such events?? Thx

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  • WPF Best point to udpate a progress bar from backgorundworker

    - by toni
    Hi! I have a task that takes a long time executing. In order to inform about the progress to the user I have a progress bar that I update inside DoWork but I would like anybody tells me if it is the best way to update the progress bar. I have heard that there is a reportprogress event handler but I am confused because I don't know what is the utility of the ReportProgress. Thanks.

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  • How to use msdeploy and target a specific site?

    - by Chris Marisic
    When using msdeploy and the msdeploy.axd handler any attempts to deploy to my remote server is met with a 401 not authorized error and the server logs IISWMSVC_AUTHORIZATION_SERVER_NOT_ALLOWED Only Windows Administrators are allowed to connect using a server connection. Other users should use the 'Connect To Site or Application' task to be able to connect. Process:WMSvc What is the correct way to target the axd for the site level?

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  • How can I access the JavaScript global object ("window") inside an embedded browser object (the IWebBrowser2 interface)?

    - by John Factorial
    I have an HTML page which embeds an IWebBrowser2 ActiveX (i.e. the control is essentially an Internet Explorer browser). I need to write JavaScript in this HTML page which will remove any window.onresize handler from the page loaded in the IWebBrowser2 control. IWebBrowser2 exposes the DOM through IWebBrowser2::Document, but this is equivalent to window.document. Is there any way I can get access to window or window.onresize?

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  • Gridview - Is it necessary to grab data from database every time a filter, sort, or paging event occ

    - by hamlin11
    Regarding gridviews that are not bound to a Data Source Control: In most GridView tutorials that I have seen, when just about any GridView event occurs, the end of the event handler will include BindDataGrid(). In some form, these BindDataGrid() functions 1) Grab data from the database 2) Assign any Filter or Sort expressions to the data, and 3) Bind the gridview to that data source (usually a DataView or DataTable. Is there a better way to provide filterable & sortable data to a GridView without having to hit the database so often? Thanks

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  • Totally Sweet Horizontal Rules in LaTeX

    - by Arcane
    I have been trying to find a way to easily drop a nice horizontal rule into a LaTeX document. \hline just makes a line across the page. It would seem that some package must provide something that is maybe half a page wide, with little bedknobs on the ends or something to act as a nice section marker for paragraphs. Any ideas? Or am I boned, and need to come up with my own macro to create such a beast?

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