1Trace::Mask(3) User Contributed Perl Documentation Trace::Mask(3)
2
6 Trace::Mask - Standard for masking frames in stack traces.
7
9 This is a specification packages can follow to define behaviors stack
10 tracers may choose to honor. If a module implements this specification
11 than any compliant stack tracer will render the stack trace as desired.
12 Implementing the spec will have no effect on non-complient stack
13 tracers. This specification does not effect caller() in any way.
14
16 Masking stack traces is not something you want to do every day, but
17 there are situations where it can be helpful, if not essential.
18 Emulate existing language structures
20 sub foo {
21 if ($cond) { trace() }
22 }
23 In the example above a stack trace is produced, the call to foo()
25 will show up, but the "if" block will not. This is useful as the
26 conditional is part of the sub, and should not be listed.
27 Emulating this behavior would be a useful feature for exception
29 tools that provide try/catch/finally or similar control structures.
30 try { ... }
32 catch { ... };
33 In perl the above would be emulated with 2 subs that take
35 codeblocks in their prototype. In a stack trace you see a call to
36 try, and a call to an anonymous block. In a stack trace this is
37 distracting at best. Further it is hard to distinguish which
38 anonymous block you are in, though tools like Sub::Name mitigate
39 this some.
40 Testing Tools
42 Tools like Test::Exception use Sub::Uplevel to achieve a similar
43 effect. This is done by globally overriding caller(), which can
44 have some unfortunate side effects. Using Trace::Mask instead would
45 avoid the nasty side effects, would be much faster than overriding
46 caller(), and give more control over what makes it into the trace.
47 One interface to many tools
49 Currently Carp provides several configuration variables such as
50 @CARP_NOT to give you control over where a trace starts. Other
51 modules that provide stack traces all provide their own variables.
52 If you want to control stack traces you need to account for all the
53 possible tracing tools that could be used. Many tracing tools do
54 not provide enough control, including "Carp" itself.
55
57 No module (including this one) is required when implementing the spec.
58 Though it is a good idea to list the version of the spec you have
59 implemented in the runtime recommendations for your module. There are
60 no undesired side effects as the specification is completely opt-in,
61 both for modules that want to effect stack traces, and for the stack
62 tracers themselves.
63 %Trace::Mask::MASKS
65 Packages that wish to mask stack frames may do so by altering the
66 %Trace::Mask::MASKS package variable. Packages may change this variable
67 at any time, so consumers should not cache the contents, however they
68 may cache the reference to the hash itself.
69 This is an overview of the MASKS structure:
71 %Trace::Mask::MASKS = (
73 FILE => {
74 LINE => {
75 SUBNAME => {
76 # Behaviors
77 no_start => BOOL, # Do not start a trace on this frame
78 stop => BOOL, # Stop tracing at this frame
79 pause => BOOL, # Stop tracing at this frame until you see a restart
80 restart => BOOL, # Start tracing again at this frame
81 hide => COUNT, # Hide the frames completely
82 shift => COUNT, # Pretend this frame started X frames before it did
83 lock => BOOL, # Prevent the frame from being hidden or modified
84 # Replacements
86 0 => PACKAGE, # Replace the package listed in the frame
87 1 => FILE, # Replace the filename listed in the frame
88 2 => LINE, # Replace the linenum listed in the frame
89 3 => NAME, # Replace the subname listen in the frame
90 ..., # Replace any index listed in the frame
91 }
92 }
93 }
94 );
95 No package should ever reset/erase the %Trace::Mask::MASKS variable.
97 They should only ever remove entries they added, even that is not
98 recommended.
99 You CAN add entries for files+lines that are not under your control.
101 This is an important allowance as it allows a function to hide the call
102 to itself.
103 A stack frame is defined based on the return from caller() which
105 returns the "($package, $file, $line, $subname)" data of a call in the
106 stack. To manipulate a call you define the
107 "$MASKS{$file}->{$line}->{$subname}" path in the hash that matches the
108 call itself.
109 'FILE', 'LINE', and 'SUBNAME' can all be replaced with the wildcard '*'
111 string to apply to all:
112 # Effect all calls to Foo::foo in any file
114 ('*' => { '*' => { Foo::foo => { ... }}})
115 # Effect all sub calls in Foo.pm
117 ('Foo.pm' => { '*' => { '*' => { ... }}});
118 You cannot use 3 wildcards to effect all subs. The 3 wildcard entry
120 will be ignored by a compliant tracer.
121 # This is not allowed, the entry will be ignored
123 ('*' => { '*' => { '*' => { ... }}});
124 CALL MASK HASHES
126 Numeric keys in the behavior structures are replacement values. If you
127 want to replace the package listed in the frame then you specify a
128 value for field '0'. If you want to replace the filename you would put
129 a value for field '1'. Numeric fields always correspond to the same
130 index in the list returned from caller().
131 {
133 # Behaviors
134 no_start => BOOL, # Do not start a trace on this frame
135 stop => BOOL, # Stop tracing at this frame
136 pause => BOOL, # Stop tracing at this frame until you see a restart
137 restart => BOOL, # Start tracing again at this frame
138 hide => COUNT, # Hide the frames completely
139 shift => COUNT, # Pretend this frame started X frames before it did
140 lock => BOOL, # Prevent the frame from being hidden or modified
141 # Replacements
143 0 => PACKAGE, # Replace the package listed in the frame
144 1 => FILE, # Replace the filename listed in the frame
145 2 => LINE, # Replace the linenum listed in the frame
146 3 => NAME, # Replace the subname listen in the frame
147 ..., # Replace any index listed in the frame
148 }
149 The following additional behaviors may be specified:
151 no_start => $BOOL
153 This prevents a stack trace from starting at the given call. This
154 is similar to Carp's @CARP_NOT variable. These frames will still
155 appear in the stack trace if they are not the start.
156 stop => $BOOL
158 This tells the stack tracer to stop tracing at this frame. The
159 frame itself will be listed in the trace, unless this is combined
160 with the 'hide' option.
161 Usually you want pause.
163 pause => $BOOL
165 Same as stop, except that things can restart it.
166 restart => $BOOL
168 This tells the stack tracer to start again after a pause,
169 effectively skipping all the frames between the pause and this
170 restart. This may be combined with 'pause' in order to show a
171 single frame.
172 hide => $COUNT
174 This completely hides the frame from a stack trace. This does not
175 modify the values of any surrounding frames, the frame is simply
176 dropped from the trace. If $COUNT is greater than 1, then
177 additional frames below the hidden one will also be dropped.
178 This has the same effect on a stack trace as Sub::Uplevel.
180 shift => $COUNT
182 This is like hide with one important difference, all components of
183 the shifted call, except for package, file, and line, will replace
184 the values of the next frame to be kept in the trace. If $COUNT is
185 large than 1, the shift will hide frames between the shifted frame
186 and the new frame. If $COUNT is larger than the remaining stack,
187 the lowest unhidden/unshifted stack frame will be the recipient of
188 the shift operation, even if the shift frame itself is the lowest.
189 This has the same effect on a stack trace as "goto &sub".
191 lock => $BOOL
193 Locking a frame means that it must be displayed, and cannot be
194 modified. If it is lower than a stop, or in the middle of a
195 hide/shift span it must be shown anyway. No replacements will have
196 any effect, and it cannot be modified by a shift.
197 MASK RESOLUTION
199 Multiple masks in the %Trace::Mask::MASKS structure may apply to any
200 given stack frame, a compliant tracer will account for all of them. A
201 simple hash merge is sufficient so long as they are merged in the
202 correct order. Here is an example:
203 my $masks_ref = \%Trace::Mask::MASKS;
205 my @all = grep { defined $_ } (
207 $masks_ref->{$file}->{'*'}->{'*'},
208 $masks_ref->{$file}->{$line}->{'*'},
209 $masks_ref->{'*'}->{'*'}->{$name},
210 $masks_ref->{$file}->{'*'}->{$name},
211 $masks_ref->{$file}->{$line}->{$name},
212 );
213 my %final = map { %{$_} } @all;
215 The most specific path should win out (override others). Rightmost path
217 component is considered the most important. More wildcards means less
218 specific. Paths may never have wildcards for all 3 components.
219 $ENV{'NO_TRACE_MASK'}
221 If this environment variable is set to true then all masking rules
222 should be ignored, tracers should produce full and complete stack
223 traces.
224 TRACES STARTING AT $LEVEL
226 If a tracing tool starts at the call to the tool (such as
227 Carp::confess()) then it should account for all the masks starting with
228 the call to confess itself going all the way until the bottom of the
229 stack, or until a mask with 'stop' is found. If a tracing tool allows
230 you to start tracing from a specific level, the tracer should still
231 account for the masks of the frames at the top of the stack on which it
232 is not reporting.
233 MASK NUMERIC KEYS
235 Numeric keys in a mask represent items in the list returned from
236 caller(). If you provide numeric keys their values will replace the
237 corresponding value in the caller list before it is used in the trace.
238 You can use this to replace the package, file, etc. This will work for
239 any VALID index into the list. This cannot be used to extend the list.
240 Numeric keys outside the bounds of the list are simply ignored, this is
241 for compatability as different perl versions may have a different size
242 list.
243 SPECIAL/MAGIC subs
245 Traces must NEVER hide or alter the following special/magic subs, they
246 should be considered the same as any "lock" frame.
247 BEGIN
249 UNITCHECK
250 CHECK
251 INIT
252 END
253 DESTROY
254 import
255 unimport
256 These subs are all special in one way or another, hiding them would be
258 hiding critical information.
259
261 The masks() method is defined in Trace::Mask, it returns a reference to
262 the %Trace::Mask::MASKS hash for easy access. It is fine to cache this
263 reference, but not the data it contains.
264
266 Trace::Mask::Reference is included in this distribution. The Reference
267 module contains example tracers, and example tools that benefit from
268 masking stack traces. The examples in this module should NOT be used in
269 production code.
270
272 Trace::Mask::Util is included in this distribution. The util module
273 provides utilities for adding stack trace masking behavior. The
274 utilities provided by this module are considered usable in production
275 code.
276
278 Trace::Mask::Test is included in this distribution. This module
279 provides test cases and tools useful for verifying your tracing tools
280 are compliant with the spec.
281
283 Carp
284 Trace::Mask::Carp is included in this distribution. This module can
285 make Carp compliant with Trace::Mask.
286 Try::Tiny
288 Trace::Mask::TryTiny is included in this ditribution. Simply loading
289 theis module will cause Try::Tiny framework to be hidden in compliant
290 stack traces.
291
293 Sub::Uplevel - Tool for hiding stack frames from all callers, not just
294 stack traces.
295
297 The source code repository for Trace-Mask can be found at
298 http://github.com/exodist/Trace-Mask.
299
301 Chad Granum <exodist@cpan.org>
302
304 Chad Granum <exodist@cpan.org>
305
307 Copyright 2015 Chad Granum <exodist7@gmail.com>.
308 This program is free software; you can redistribute it and/or modify it
310 under the same terms as Perl itself.
311 See http://www.perl.com/perl/misc/Artistic.html
313perl v5.38.0 2023-07-21 Trace::Mask(3)