[Tcl Wiki] Update of "ExceptionHandling" by MassimoManghi

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The "ExceptionHandling" page has been changed by MassimoManghi.
http://wiki.apache.org/tcl/ExceptionHandling

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New page:
 == Exception handling ==

The definition exception handling could be misleading. This is a mechanism based on the command '''abort_page''', rather than a language specific exception handling as it's done in tcl8.6 or through the 'catch' core command. 

abort_page exits a script and return control to the module core with a special return code. The module checks the error code and skips subsequent scripts (AfterScript) printing an error message and dropping already sent page contents from the output channel.

this is a proposal for a new handling of this mechanism that hopefully should preserve existing code maintaining full compatibility: 

 * AbortScript: script that gets run if abort_page is called by the content
generator or by AfterScript. 
 
 * ForceAfterScript (or FinalAfterScript or whatever): it's run anyway
regardless the completion code of the script that ran before in the request
process

 * Command abort_page will have an internal status flag 'aborting' to be set
in the module globals. This flag is set to 0 at the very beginning of the
request processing (Rivet_SendContent). 'abort_page' tests this flag, if 0 it
gets set as 1 and the command returns TCL_ERROR setting the usual error info
that are trapped in Rivet_ExecuteAndCheck. 

Any subsequent call to the command will return TCL_OK disabling its effects
within ForceAfterScript (just making sure that it cannot be interrupted in
this way).

 * 'abort_page' will accept an argument to be later retrived with a new
command 'abort_condition'. AbortScript may check this condition variable to
branch into different blocks of code.

Basically, the code in Rivet_ExecuteAndCheck where the abort condition is checked

if (strcmp (Tcl_GetString (errorCodeElementObj), "RIVET") == 0) {
 ...

}

should test the definition of a new conf variable rivet_abort_script and
execute it if defined. The whole function has to also check for the new conf
variable rivet_force_script and run it anyway before printing the headers and
flushing the channel.



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