NAME
   Trace::Mask - Standard for masking frames in stack traces.

DESCRIPTION
   This is a specification packages can follow to define behaviors stack
   tracers may choose to honor. If a module implements this specification
   than any compliant stack tracer will render the stack trace as desired.
   Implementing the spec will have no effect on non-complient stack
   tracers. This specification does not effect "caller()" in any way.

PRACTICAL APPLICATIONS
   Masking stack traces is not something you want to do every day, but
   there are situations where it can be helpful, if not essential.

   Emulate existing language structures
           sub foo {
               if ($cond) { trace() }
           }

       In the example above a stack trace is produced, the call to "foo()"
       will show up, but the "if" block will not. This is useful as the
       conditional is part of the sub, and should not be listed.

       Emulating this behavior would be a useful feature for exception
       tools that provide try/catch/finally or similar control structures.

           try   { ... }
           catch { ... };

       In perl the above would be emulated with 2 subs that take codeblocks
       in their prototype. In a stack trace you see a call to try, and a
       call to an anonymous block. In a stack trace this is distracting at
       best. Further it is hard to distinguish which anonymous block you
       are in, though tools like Sub::Name mitigate this some.

   Testing Tools
       Tools like Test::Exception use Sub::Uplevel to achieve a similar
       effect. This is done by globally overriding "caller()", which can
       have some unfortunate side effects. Using Trace::Mask instead would
       avoid the nasty side effects, would be much faster than overriding
       "caller()", and give more control over what makes it into the trace.

   One interface to many tools
       Currently Carp provides several configuration variables such as
       @CARP_NOT to give you control over where a trace starts. Other
       modules that provide stack traces all provide their own variables.
       If you want to control stack traces you need to account for all the
       possible tracing tools that could be used. Many tracing tools do not
       provide enough control, including "Carp" itself.

SPECIFICATION
   No module (including this one) is required when implementing the spec.
   Though it is a good idea to list the version of the spec you have
   implemented in the runtime recommendations for your module. There are no
   undesired side effects as the specification is completely opt-in, both
   for modules that want to effect stack traces, and for the stack tracers
   themselves.

 %Trace::Mask::MASKS
   Packages that wish to mask stack frames may do so by altering the
   %Trace::Mask::MASKS package variable. Packages may change this variable
   at any time, so consumers should not cache the contents, however they
   may cache the reference to the hash itself.

   This is an overview of the MASKS structure:

       %Trace::Mask::MASKS = (
           FILE => {
               LINE => {
                   SUBNAME => {
                       # Behaviors
                       no_start => BOOL,     # Do not start a trace on this frame
                       stop     => BOOL,     # Stop tracing at this frame
                       restart  => BOOL,     # Start tracing again at this frame
                       hide     => COUNT,    # Hide the frames completely
                       shift    => COUNT,    # Pretend this frame started X frames before it did

                       # Replacements
                       0 => PACKAGE,         # Replace the package listed in the frame
                       1 => FILE,            # Replace the filename listed in the frame
                       2 => LINE,            # Replace the linenum listed in the frame
                       3 => NAME,            # Replace the subname listen in the frame
                       ...,                  # Replace any index listed in the frame
                   }
               }
           }
       );

   No package should ever reset/erase the %Trace::Mask::MASKS variable.
   They should only ever remove entries they added, even that is not
   recommended.

   You CAN add entries for files+lines that are not under your control.
   This is an important allowance as it allows a function to hide the call
   to itself.

   A stack frame is defined based on the return from "caller()" which
   returns the "($package, $file, $line, $subname)" data of a call in the
   stack. To manipulate a call you define the
   $MASKS{$file}->{$line}->{$subname} path in the hash that matches the
   call itself.

   'FILE', 'LINE', and 'SUBNAME' can all be replaced with the wildcard '*'
   string to apply to all:

       # Effect all calls to Foo::foo in any file
       ('*' => { '*' => { Foo::foo => { ... }}})

       # Effect all sub calls in Foo.pm
       ('Foo.pm' => { '*' => { '*' => { ... }}});

   You cannot use 3 wildcards to effect all subs. The 3 wildcard entry will
   be ignored by a compliant tracer.

       # This is not allowed, the entry will be ignored
       ('*' => { '*' => { '*' => { ... }}});

 CALL MASK HASHES
   Numeric keys in the behavior structures are replacement values. If you
   want to replace the package listed in the frame then you specify a value
   for field '0'. If you want to replace the filename you would put a value
   for field '1'. Numeric fields always correspond to the same index in the
   list returned from "caller()".

      {
          # Behaviors
          no_start => BOOL,     # Do not start a trace on this frame
          stop     => BOOL,     # Stop tracing at this frame
          restart  => BOOL,     # Start tracing again at this frame
          hide     => COUNT,    # Hide the frames completely
          shift    => COUNT,    # Pretend this frame started X frames before it did

          # Replacements
          0 => PACKAGE,         # Replace the package listed in the frame
          1 => FILE,            # Replace the filename listed in the frame
          2 => LINE,            # Replace the linenum listed in the frame
          3 => NAME,            # Replace the subname listen in the frame
          ...,                  # Replace any index listed in the frame
      }

   The following additional behaviors may be specified:

   no_start => $BOOL
       This prevents a stack trace from starting at the given call. This is
       similar to Carp's @CARP_NOT variable. These frames will still appear
       in the stack trace if they are not the start.

   stop => $BOOL
       This tells the stack tracer to stop tracing at this frame. The frame
       itself will be listed in the trace, unless this is combined with the
       'hide' option.

   restart => $BOOL
       This tells the stack tracer to start again after a stop, effectively
       skipping all the frames between the stop and this start. This may be
       combined with 'stop' in order to show a single frame.

   hide => $COUNT
       This completely hides the frame from a stack trace. This does not
       modify the values of any surrounding frames, the frame is simply
       dropped from the trace. If $COUNT is greater than 1, then additional
       frames below the hidden one will also be dropped.

       This has the same effect on a stack trace as Sub::Uplevel.

   shift => $COUNT
       This is like hide with one important difference, all components of
       the shifted call, except for package, file, and line, will replace
       the values of the next frame to be kept in the trace. If $COUNT is
       large than 1, the shift will hide frames between the shifted frame
       and the new frame. If $COUNT is larger than the remaining stack, the
       lowest unhidden/unshifted stack frame will be the recipient of the
       shift operation, even if the shift frame itself is the lowest.

       This has the same effect on a stack trace as "goto &sub".

 MASK RESOLUTION
   Multiple masks in the %Trace::Mask::MASKS structure may apply to any
   given stack frame, a compliant tracer will account for all of them. A
   simple hash merge is sufficient so long as they are merged in the
   correct order. Here is an example:

       my $masks_ref = \%Trace::Mask::MASKS;

       my @all = grep { defined $_ } (
           $masks_ref->{$file}->{'*'}->{'*'},
           $masks_ref->{$file}->{$line}->{'*'},
           $masks_ref->{'*'}->{'*'}->{$name},
           $masks_ref->{$file}->{'*'}->{$name},
           $masks_ref->{$file}->{$line}->{$name},
       );

       my %final = map { %{$_} } @all;

   The most specific path should win out (override others). Rightmost path
   component is considered the most important. More wildcards means less
   specific. Paths may never have wildcards for all 3 components.

 $ENV{'NO_TRACE_MASK'}
   If this environment variable is set to true then all masking rules
   should be ignored, tracers should produce full and complete stack
   traces.

 TRACES STARTING AT $LEVEL
   If a tracing tool starts at the call to the tool (such as
   "Carp::confess()") then it should account for all the masks starting
   with the call to confess itself going all the way until the bottom of
   the stack, or until a mask with 'stop' is found. If a tracing tool
   allows you to start tracing from a specific level, the tracer should
   still account for the masks of the frames at the top of the stack on
   which it is not reporting.

 MASK NUMERIC KEYS
   Numeric keys in a mask represent items in the list returned from
   "caller()". If you provide numeric keys their values will replace the
   corresponding value in the caller list before it is used in the trace.
   You can use this to replace the package, file, etc. This will work for
   any VALID index into the list. This cannot be used to extend the list.
   Numeric keys outside the bounds of the list are simply ignored, this is
   for compatability as different perl versions may have a different size
   list.

 SPECIAL/MAGIC subs
   Traces must NEVER hide or alter the following special/magic subs:

   BEGIN
   UNITCHECK
   CHECK
   INIT
   END
   DESTROY
   import
   unimport

   These subs are all special in one way or another, hiding them would be
   hiding critical information.

CLASS METHODS
   The "masks()" method is defined in Trace::Mask, it returns a reference
   to the %Trace::Mask::MASKS hash for easy access. It is fine to cache
   this reference, but not the data it contains.

REFERENCE
   Trace::Mask::Reference is included in this distribution. The Reference
   module contains example tracers, and example tools that benefit from
   masking stack traces. The examples in this module should NOT be used in
   production code.

UTILS
   Trace::Mask::Util is included in this distribution. The util module
   provides utilities for adding stack trace masking behavior. The
   utilities provided by this module are considered usable in production
   code.

TEST
   Trace::Mask::Test is included in this distribution. This module provides
   test cases and tools useful for verifying your tracing tools are
   compliant with the spec.

PLUGINS
 Carp
   Trace::Mask::Carp is included in this distribution. This module can make
   Carp compliant with Trace::Mask.

 Try::Tiny
   Trace::Mask::TryTiny is included in this ditribution. Simply loading
   theis module will cause Try::Tiny framework to be hidden in compliant
   stack traces.

SEE ALSO
   Sub::Uplevel - Tool for hiding stack frames from all callers, not just
   stack traces.

SOURCE
   The source code repository for Trace-Mask can be found at
   http://github.com/exodist/Trace-Mask.

MAINTAINERS
   Chad Granum <[email protected]>

AUTHORS
   Chad Granum <[email protected]>

COPYRIGHT
   Copyright 2015 Chad Granum <[email protected]>.

   This program is free software; you can redistribute it and/or modify it
   under the same terms as Perl itself.

   See http://www.perl.com/perl/misc/Artistic.html

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