Rivet Internals

## Rivet Internals

This section easily falls out of date, as new code is added, old code is removed, and changes are made. The best place to look is the source code itself. If you are interested in the changes themselves, the Subversion revision control system (svn) can provide you with information about what has been happening with the code.

### Initialization

When Apache is started, (or when child Apache processes are started if a threaded Tcl is used), `Rivet_InitTclStuff` is called, which creates a new interpreter, or one interpreter per virtual host, depending on the configuration. It also initializes various things, like the RivetChan channel system, creates the Rivet-specific Tcl commands, and executes Rivet's `init.tcl`. The caching system is also set up, and if there is a GlobalInitScript, it is run.

### RivetChan

The RivetChan system was created in order to have an actual Tcl channel that we could redirect standard output to. This lets us use, for instance, the regular puts command in .rvt pages. It works by creating a channel that buffers output, and, at predetermined times, passes it on to Apache's IO system. Tcl's regular standard output is replaced with an instance of this channel type, so that, by default, output will go to the web page.

### The global Command

Rivet aims to run standard Tcl code with as few surprises as possible. At times this involves some compromises - in this case regarding the global command. The problem is that the command will create truly global variables. If the user is just cut'n'pasting some Tcl code into Rivet, they most likely just want to be able to share the variable in question with other procs, and don't really care if the variable is actually persistant between pages. The solution we have created is to create a proc ::request::global that takes the place of the global command in Rivet templates. If you really need a true global variable, use either ::global or add the :: namespace qualifier to variables you wish to make global.

### Page Parsing, Execution and Caching

When a Rivet page is requested, it is transformed into an ordinary Tcl script by parsing the file for the <? ?> processing instruction tags. Everything outside these tags becomes a large puts statement, and everything inside them remains Tcl code.

Each .rvt file is evaluated in its own `::request` namespace, so that it is not necessary to create and tear down interpreters after each page. By running in its own namespace, though, each page will not run afoul of local variables created by other scripts, because they will be deleted automatically when the namespace goes away after Apache finishes handling the request.

| Note
One current problem with this system is that while variables are garbage collected, file handles are not, so that it is very important that Rivet script authors make sure to close all the files they open.

After a script has been loaded and parsed into it's "pure Tcl" form, it is also cached, so that it may be used in the future without having to reload it (and re-parse it) from the disk. The number of scripts stored in memory is configurable. This feature can significantly improve performance.

### Extending Rivet by developing C procedures implementing new commands

Rivet endows the Tcl interpreter with new commands serving as interface between the application layer and the Apache web server. Many of these commands are meaningful only when a HTTP request is under way and therefore a request_rec object allocated by the framework is existing and was passed to mod_rivet as argument of a callback. In case commands have to gain access to a valid request_rec object the C procedure must check if such a pointer exists and it's initialized with valid data. For this purpose the procedure handling requests (Rivet_SendContent) makes a copy of such pointer and keeps it in an internal structure. The copy is set to NULL just before returning to the framework, right after mod_rivet's has carried out its request processing. When the pointer copy is NULL the module is outside any request processing and this condition invalidates the execution of many of the Rivet commands. In case they are called (for example in a ChildInitScript, GlobalInitScript, ServerInitScript or ChildExitScript) they fail with a Tcl error you can handle with a catch command.

For this purpose in src/rivet.h the macro CHECK_REQUEST_REC was defined accepting two arguments: the copy to the request_rec pointer (stored in the rivet_interp_globals structure) and the command name. If the pointer is NULL the macro calls Tcl_NoRequestRec and returns TCL_ERROR causing the command to fail. These are the step to follow to implement a new C language command for mod_rivet

- Define the command and associated C language procedure in src/rivetcmds/rivetCore.c using the macro RIVET_OBJ_CMD

      RIVET_OBJ_CMD("mycmd",Rivet_MyCmd)

  This macro ensures the command is defined as ::rivet::mycmd
- Add the code of Rivet_MyCmd to src/rivetcmd/rivetCore.c (in case the code resides in a different file also src/Makefile.am should be changed to tell the build system how to compile the code and link it into mod_rivet.so)
- If the code must gain access to globals->r put add the macro testing for the pointer

      TCL_CMD_HEADER( Rivet_MyCmd )
      {
          rivet_interp_globals *globals = Tcl_GetAssocData( interp, "rivet", NULL );
          ....
          CHECK_REQUEST_REC(globals->r,"::rivet::mycmd");
          ...
      }

- Add a test for this command in tests/checkfails.tcl. For instance

      ...
      check_fail no_body
      check_fail virtual_filename unkn
      check_fail my_cmd <arg1> <arg2>
      ....

  Where <arg1> <arg2> are optional arguments in case the command needs to check for globals->r in special cases. Then, if ::rivet::mycmd must fail also tests/failtest.tcl should modified as

      virtual_filename->1
      mycmd->1

  The value associated to the test must be 0 in case the command doesn't need to test the globals->r pointer.

### Debugging Rivet and Apache

If you are interested in hacking on Rivet, you're welcome to contribute! Invariably, when working with code, things go wrong, and it's necessary to do some debugging. In a server environment like Apache, it can be a bit more difficult to find the right way to do this. Here are some techniques to try.

The first thing you should know is that Apache can be launched as a single process with the -X argument:

    httpd -X

.

On Linux, one of the first things to try is the system call tracer, strace. You don't even have to recompile Rivet or Apache for this to work.

    strace -o /tmp/outputfile -S 1000 httpd -X

This command will run httpd in the system call tracer, which leaves its output (there is potentially a lot of it) in `/tmp/outputfile`. The -S option tells strace to only record the first 1000 bytes of a syscall. Some calls such as `write` can potentially be much longer than this, so you may want to increase this number. The results are a list of all the system calls made by the program. You want to look at the end, where the failure presumably occured, to see if you can find anything that looks like an error. If you're not sure what to make of the results, you can always ask on the Rivet development mailing list.

If strace (or its equivalent on your operating system) doesn't answer your question, it may be time to debug Apache and Rivet. To do this, you will need to rebuild mod_rivet. First of all you have to configure the build by running the ./configure script with the -enable-symbols option and after you have set the CFLAGS and LDFLAGS environment variables

    export CFLAGS="-g -O0"
    export LDFLAGS="-g"
    ./configure --enable-symbols ......
    make
    make install

Arguments to ./configure must fit your Apache HTTP web server installation. See the output produced by

    ./configure --help

And check the the section called “Apache Rivet Installation” page to have further information. Since it's easier to debug a single process, we'll still run Apache in single process mode with -X:

    @ashland [~] $ gdb /usr/sbin/apache.dbg
    GNU gdb 5.3-debian
    Copyright 2002 Free Software Foundation, Inc.
    GDB is free software, covered by the GNU General Public License, and you are
    welcome to change it and/or distribute copies of it under certain conditions.
    Type "show copying" to see the conditions.
    There is absolutely no warranty for GDB.  Type "show warranty" for details.
    This GDB was configured as "powerpc-linux"...
    (gdb) run -X
    Starting program: /usr/sbin/apache.dbg -X
    [New Thread 16384 (LWP 13598)]
    .
    .
    .

When your apache session is up and running, you can request a web page with the browser, and see where things go wrong (if you are dealing with a crash, for instance).

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