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
                        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
           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.

    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.

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 <exodist@cpan.org>

AUTHORS
    Chad Granum <exodist@cpan.org>

COPYRIGHT
    Copyright 2015 Chad Granum <exodist7@gmail.com>.

    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

