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