Bug 23737 - Houston... error during web request on Apache 2.4
Summary: Houston... error during web request on Apache 2.4
Status: NEW
Alias: None
Product: Class Libraries
Classification: Mono
Component: System.Web ()
Version: unspecified
Hardware: PC Linux
: --- normal
Target Milestone: Untriaged
Assignee: Bugzilla
Depends on:
Reported: 2014-10-10 06:43 UTC by APS
Modified: 2017-10-20 19:11 UTC (History)
4 users (show)

Is this bug a regression?: ---
Last known good build:

Test case (1.92 KB, application/zip)
2014-10-22 12:13 UTC, APS

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Description APS 2014-10-10 06:43:22 UTC

I installed mono on a Centos 7 box with Apache 2.4 in order to serve aspx pages.
I had to use a patched version of mod_mono because the standard version 2.10 doesn't compile on Apache 2.4. It works pretty well but in some request, I've not been able yet to discover the case because it's not the same page every time, I obtain the following error:

System.Web.HttpUnhandledException: Exception of type 'System.Web.HttpUnhandledException' was thrown. ---> System.Exception: Houston...
  at Mono.WebServer.ModMonoRequest.GetClientBlock (System.Byte[] bytes, Int32 position, Int32 size) [0x00000] in <filename unknown>:0
  at Mono.WebServer.ModMonoWorker.Read (System.Byte[] buffer, Int32 position, Int32 size) [0x00000] in <filename unknown>:0
  at Mono.WebServer.ModMonoWorkerRequest.ReadEntityBody (System.Byte[] buffer, Int32 size) [0x00000] in <filename unknown>:0
  at System.Web.HttpRequest.MakeInputStream () [0x00000] in <filename unknown>:0
  at System.Web.HttpRequest.get_InputStream () [0x00000] in <filename unknown>:0
  at System.Web.HttpRequest.LoadWwwForm () [0x00000] in <filename unknown>:0
  at System.Web.HttpRequest.get_FormUnvalidated () [0x00000] in <filename unknown>:0
  at System.Web.HttpRequest.get_Form () [0x00000] in <filename unknown>:0
  at System.Web.UI.Page.DeterminePostBackMode () [0x00000] in <filename unknown>:0
  at System.Web.UI.Page.InternalProcessRequest () [0x00000] in <filename unknown>:0
  at System.Web.UI.Page.ProcessRequest (System.Web.HttpContext context) [0x00000] in <filename unknown>:0
  --- End of inner exception stack trace ---
  at System.Web.UI.Page.ProcessException (System.Exception e) [0x00000] in <filename unknown>:0
  at System.Web.UI.Page.ProcessRequest (System.Web.HttpContext context) [0x00000] in <filename unknown>:0
  at System.Web.HttpApplication+<Pipeline>c__Iterator3.MoveNext () [0x00000] in <filename unknown>:0
  at System.Web.HttpApplication.Tick () [0x00000] in <filename unknown>:0

I found that the issue is located in xsp-2.10.2/src/Mono.WebServer.Apache/ModMonoRequest.cs

public int GetClientBlock ([Out] byte [] bytes, int position, int size) 
    if (SetupClientBlock () != 0) return 0;

     * turns out that that GET_CLIENT_BLOCK (ap_get_client_block) can
     * return -1 if a socket is closed
    writer.Write ((int) ModMonoCmd.GET_CLIENT_BLOCK);
    writer.Write (size);
    Send ();

    FillBuffer (4);
    int i = reader.ReadInt32 ();
    if (i == -1)
        return -1;

    if (i > size)
        throw new Exception ("Houston...");

    FillBuffer ((uint)i);
    return reader.Read (bytes, position, i);

I inserted some more log into the exception and this is the result:

System.Exception: Houston... 1363232375 vs 28360
System.Exception: Houston... 1835812708 vs 28525 

The exception is thrown because the value returned to mod_mono from apache (1363232375, 1835812708) is greater than the size requested by mono (28360, 28525, ...). 

I checked the history of mod_mono 3.8 that enables Apache 2.4 support but I saw nothing related to this kind of problem.
Comment 1 Zoltan Varga 2014-10-15 12:58:01 UTC
-> xsp.
Comment 2 APS 2014-10-22 12:13:16 UTC
Created attachment 8472 [details]
Test case

I created a small test-case. It consists in a page that performs consecutive postbacks increasing every time the size by adding text to a textbox.
After some variable loop count, less than 50 on my machine, the request fails with the reported error.
Hope it helps.
Comment 3 APS 2014-10-23 06:36:51 UTC
I see that the bug has been moved to xsp but I can confirm that the problem is on mod_mono because I tested the same test-case with xsp and it never throws errors.
Comment 4 APS 2014-10-23 12:46:54 UTC
Same problem is reported also in this old bug report https://bugzilla.xamarin.com/show_bug.cgi?id=16796
Comment 5 APS 2014-11-05 11:06:47 UTC
After further investigations I think that the problem is in the management of the socket that connects mod_mono with xsp.
For some (yet) unknown reason sometimes xsp reads the result of GET_CLIENT_BLOCK before mod_modo ends to write data. It happens to me only when the size of the page is more than 32Kb.
I've found a small non-elegant workaround adding a sleep before reading data. So I changed the above function in this way:

Send ();
FillBuffer (4);

It seems that a 1ms sleep it's enough to workaround the problem. Hope it helps to correct the problem definitively.
Comment 6 Carl 2015-09-02 17:30:33 UTC
I am no longer getting this error, thanks!  Adding the 1ms sleep fixes it.
Comment 7 Carl 2017-10-20 19:11:10 UTC
Unfortunately I am getting this bug again when the upload is very quick.  It just doesn't happen as often now.