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
47 trace.
48 One interface to many tools
50 Currently Carp provides several configuration variables such as
51 @CARP_NOT to give you control over where a trace starts. Other
52 modules that provide stack traces all provide their own variables.
53 If you want to control stack traces you need to account for all the
54 possible tracing tools that could be used. Many tracing tools do
55 not provide enough control, including "Carp" itself.
56
58 No module (including this one) is required when implementing the spec.
59 Though it is a good idea to list the version of the spec you have
60 implemented in the runtime recommendations for your module. There are
61 no undesired side effects as the specification is completely opt-in,
62 both for modules that want to effect stack traces, and for the stack
63 tracers themselves.
64 %Trace::Mask::MASKS
66 Packages that wish to mask stack frames may do so by altering the
67 %Trace::Mask::MASKS package variable. Packages may change this variable
68 at any time, so consumers should not cache the contents, however they
69 may cache the reference to the hash itself.
70 This is an overview of the MASKS structure:
72 %Trace::Mask::MASKS = (
74 FILE => {
75 LINE => {
76 SUBNAME => {
77 # Behaviors
78 no_start => BOOL, # Do not start a trace on this frame
79 stop => BOOL, # Stop tracing at this frame
80 pause => BOOL, # Stop tracing at this frame until you see a restart
81 restart => BOOL, # Start tracing again at this frame
82 hide => COUNT, # Hide the frames completely
83 shift => COUNT, # Pretend this frame started X frames before it did
84 lock => BOOL, # Prevent the frame from being hidden or modified
85 # Replacements
87 0 => PACKAGE, # Replace the package listed in the frame
88 1 => FILE, # Replace the filename listed in the frame
89 2 => LINE, # Replace the linenum listed in the frame
90 3 => NAME, # Replace the subname listen in the frame
91 ..., # Replace any index listed in the frame
92 }
93 }
94 }
95 );
96 No package should ever reset/erase the %Trace::Mask::MASKS variable.
98 They should only ever remove entries they added, even that is not
99 recommended.
100 You CAN add entries for files+lines that are not under your control.
102 This is an important allowance as it allows a function to hide the call
103 to itself.
104 A stack frame is defined based on the return from "caller()" which
106 returns the "($package, $file, $line, $subname)" data of a call in the
107 stack. To manipulate a call you define the
108 $MASKS{$file}->{$line}->{$subname} path in the hash that matches the
109 call itself.
110 'FILE', 'LINE', and 'SUBNAME' can all be replaced with the wildcard '*'
112 string to apply to all:
113 # Effect all calls to Foo::foo in any file
115 ('*' => { '*' => { Foo::foo => { ... }}})
116 # Effect all sub calls in Foo.pm
118 ('Foo.pm' => { '*' => { '*' => { ... }}});
119 You cannot use 3 wildcards to effect all subs. The 3 wildcard entry
121 will be ignored by a compliant tracer.
122 # This is not allowed, the entry will be ignored
124 ('*' => { '*' => { '*' => { ... }}});
125 CALL MASK HASHES
127 Numeric keys in the behavior structures are replacement values. If you
128 want to replace the package listed in the frame then you specify a
129 value for field '0'. If you want to replace the filename you would put
130 a value for field '1'. Numeric fields always correspond to the same
131 index in the list returned from "caller()".
132 {
134 # Behaviors
135 no_start => BOOL, # Do not start a trace on this frame
136 stop => BOOL, # Stop tracing at this frame
137 pause => BOOL, # Stop tracing at this frame until you see a restart
138 restart => BOOL, # Start tracing again at this frame
139 hide => COUNT, # Hide the frames completely
140 shift => COUNT, # Pretend this frame started X frames before it did
141 lock => BOOL, # Prevent the frame from being hidden or modified
142 # Replacements
144 0 => PACKAGE, # Replace the package listed in the frame
145 1 => FILE, # Replace the filename listed in the frame
146 2 => LINE, # Replace the linenum listed in the frame
147 3 => NAME, # Replace the subname listen in the frame
148 ..., # Replace any index listed in the frame
149 }
150 The following additional behaviors may be specified:
152 no_start => $BOOL
154 This prevents a stack trace from starting at the given call. This
155 is similar to Carp's @CARP_NOT variable. These frames will still
156 appear in the stack trace if they are not the start.
157 stop => $BOOL
159 This tells the stack tracer to stop tracing at this frame. The
160 frame itself will be listed in the trace, unless this is combined
161 with the 'hide' option.
162 Usually you want pause.
164 pause => $BOOL
166 Same as stop, except that things can restart it.
167 restart => $BOOL
169 This tells the stack tracer to start again after a pause,
170 effectively skipping all the frames between the pause and this
171 restart. This may be combined with 'pause' in order to show a
172 single frame.
173 hide => $COUNT
175 This completely hides the frame from a stack trace. This does not
176 modify the values of any surrounding frames, the frame is simply
177 dropped from the trace. If $COUNT is greater than 1, then
178 additional frames below the hidden one will also be dropped.
179 This has the same effect on a stack trace as Sub::Uplevel.
181 shift => $COUNT
183 This is like hide with one important difference, all components of
184 the shifted call, except for package, file, and line, will replace
185 the values of the next frame to be kept in the trace. If $COUNT is
186 large than 1, the shift will hide frames between the shifted frame
187 and the new frame. If $COUNT is larger than the remaining stack,
188 the lowest unhidden/unshifted stack frame will be the recipient of
189 the shift operation, even if the shift frame itself is the lowest.
190 This has the same effect on a stack trace as "goto &sub".
192 lock => $BOOL
194 Locking a frame means that it must be displayed, and cannot be
195 modified. If it is lower than a stop, or in the middle of a
196 hide/shift span it must be shown anyway. No replacements will have
197 any effect, and it cannot be modified by a shift.
198 MASK RESOLUTION
200 Multiple masks in the %Trace::Mask::MASKS structure may apply to any
201 given stack frame, a compliant tracer will account for all of them. A
202 simple hash merge is sufficient so long as they are merged in the
203 correct order. Here is an example:
204 my $masks_ref = \%Trace::Mask::MASKS;
206 my @all = grep { defined $_ } (
208 $masks_ref->{$file}->{'*'}->{'*'},
209 $masks_ref->{$file}->{$line}->{'*'},
210 $masks_ref->{'*'}->{'*'}->{$name},
211 $masks_ref->{$file}->{'*'}->{$name},
212 $masks_ref->{$file}->{$line}->{$name},
213 );
214 my %final = map { %{$_} } @all;
216 The most specific path should win out (override others). Rightmost path
218 component is considered the most important. More wildcards means less
219 specific. Paths may never have wildcards for all 3 components.
220 $ENV{'NO_TRACE_MASK'}
222 If this environment variable is set to true then all masking rules
223 should be ignored, tracers should produce full and complete stack
224 traces.
225 TRACES STARTING AT $LEVEL
227 If a tracing tool starts at the call to the tool (such as
228 "Carp::confess()") then it should account for all the masks starting
229 with the call to confess itself going all the way until the bottom of
230 the stack, or until a mask with 'stop' is found. If a tracing tool
231 allows you to start tracing from a specific level, the tracer should
232 still account for the masks of the frames at the top of the stack on
233 which it is not reporting.
234 MASK NUMERIC KEYS
236 Numeric keys in a mask represent items in the list returned from
237 "caller()". If you provide numeric keys their values will replace the
238 corresponding value in the caller list before it is used in the trace.
239 You can use this to replace the package, file, etc. This will work for
240 any VALID index into the list. This cannot be used to extend the list.
241 Numeric keys outside the bounds of the list are simply ignored, this is
242 for compatability as different perl versions may have a different size
243 list.
244 SPECIAL/MAGIC subs
246 Traces must NEVER hide or alter the following special/magic subs, they
247 should be considered the same as any "lock" frame.
248 BEGIN
250 UNITCHECK
251 CHECK
252 INIT
253 END
254 DESTROY
255 import
256 unimport
257 These subs are all special in one way or another, hiding them would be
259 hiding critical information.
260
262 The "masks()" method is defined in Trace::Mask, it returns a reference
263 to the %Trace::Mask::MASKS hash for easy access. It is fine to cache
264 this reference, but not the data it contains.
265
267 Trace::Mask::Reference is included in this distribution. The Reference
268 module contains example tracers, and example tools that benefit from
269 masking stack traces. The examples in this module should NOT be used in
270 production code.
271
273 Trace::Mask::Util is included in this distribution. The util module
274 provides utilities for adding stack trace masking behavior. The
275 utilities provided by this module are considered usable in production
276 code.
277
279 Trace::Mask::Test is included in this distribution. This module
280 provides test cases and tools useful for verifying your tracing tools
281 are compliant with the spec.
282
284 Carp
285 Trace::Mask::Carp is included in this distribution. This module can
286 make Carp compliant with Trace::Mask.
287 Try::Tiny
289 Trace::Mask::TryTiny is included in this ditribution. Simply loading
290 theis module will cause Try::Tiny framework to be hidden in compliant
291 stack traces.
292
294 Sub::Uplevel - Tool for hiding stack frames from all callers, not just
295 stack traces.
296
298 The source code repository for Trace-Mask can be found at
299 http://github.com/exodist/Trace-Mask.
300
302 Chad Granum <exodist@cpan.org>
303
305 Chad Granum <exodist@cpan.org>
306
308 Copyright 2015 Chad Granum <exodist7@gmail.com>.
309 This program is free software; you can redistribute it and/or modify it
311 under the same terms as Perl itself.
312 See http://www.perl.com/perl/misc/Artistic.html
314perl v5.30.0 2019-07-26 Trace::Mask(3)