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- add missing \label
- add errormessage generation to lonhelp.pm in case it does not find a label for
   the reference

- This should fix #3016, and make it easier to find these errors in the fututre

1: \label{Guts_Apache} 2: 3: The standard mode in which the Apache web server is used is that a 4: URL corresponds to some static file on the file system, which then 5: more or less gets sent out as-is. Slight deviations from that simple 6: principle are however already the directory listing function, virtual 7: servers, and the cgi-bin directory. In the latter case, Apache executes 8: the file in a new process and provides both the input to the process 9: in the environment, and routes the output to the client. Other deviations 10: are the error messages. 11: 12: In a more general view, URLs to Apache are URIs (Uniform Resource 13: Identifiers), which may are may not correspond to a physical file, 14: which in turn may or may not be sent out as-is. 15: 16: As a request for a URI gets sent to the Apache server, it goes through 17: several phases. At each phase ({}``stage'') along the process, handler 18: modules can be defined to deal with the request. Details about these 19: phases are given in {}``Writing Apache Modules with Perl and C'' 20: from O'Reilly (the {}``Eagle book'') in Chapter 3 (page 60). 21: 22: These handler modules are not like cgi-scripts executed in a separate 23: process, but are dynamically linked into the Apache child processes 24: themselves - they run inside of Apache. The mod\_perl mechanism in 25: addition links the Perl interpreter into the Apache child process. 26: Modules are pre-interpreted ({}``compiled'') by the Perl interpreter 27: when they are first loaded, and the pre-interpreted code stays permanently 28: in the memory allocated to that Apache child process. The result is 29: a significant speed-up, and through the flexible mechanism of module 30: registration and different stages of the process, a high degree of 31: customizability. 32: 33: LON-CAPA does \emph{not} use Apache::Registry, and so in addition 34: avoids the unnecessary overhead of emulating a cgi-environment within 35: the handlers. Instead, it makes direct use of the Apache Perl API 36: (Chapter 9, Eagle book). 37: 38: If you intend to add new capabilities to LON-CAPA that require new 39: handlers, as opposed to tweaking current ones, we recommend buying 40: a copy of {}``Writing Apache Modules with Perl and C''; it is not 41: strictly necessary but it is the clearest description of what is going 42: on in the Apache server, and without it you will be challenged to 43: understand everything that is occurring in the server.