… | |
… | |
53 | |
53 | |
54 | For example the deliantra game server uses a variant of this technique |
54 | For example the deliantra game server uses a variant of this technique |
55 | to interrupt background processes regularly to send map updates to game |
55 | to interrupt background processes regularly to send map updates to game |
56 | clients. |
56 | clients. |
57 | |
57 | |
|
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58 | Or L<EV::Loop::Async> uses an interrupt object to wake up perl when new |
|
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59 | events have arrived. |
|
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60 | |
58 | L<IO::AIO> and L<BDB> could also use this to speed up result reporting. |
61 | L<IO::AIO> and L<BDB> could also use this to speed up result reporting. |
59 | |
62 | |
60 | =item Speedy event loop invocation |
63 | =item Speedy event loop invocation |
61 | |
64 | |
62 | One could use this module e.g. in L<Coro> to interrupt a running coro-thread |
65 | One could use this module e.g. in L<Coro> to interrupt a running coro-thread |
… | |
… | |
88 | I<running> interpreter, there is optional support for signalling a pipe |
91 | I<running> interpreter, there is optional support for signalling a pipe |
89 | - that means you can also wait for the pipe to become readable (e.g. via |
92 | - that means you can also wait for the pipe to become readable (e.g. via |
90 | L<EV> or L<AnyEvent>). This, of course, incurs the overhead of a C<read> |
93 | L<EV> or L<AnyEvent>). This, of course, incurs the overhead of a C<read> |
91 | and C<write> syscall. |
94 | and C<write> syscall. |
92 | |
95 | |
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96 | =head1 USAGE EXAMPLES |
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97 | |
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98 | =head2 Implementing race-free signal handling |
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99 | |
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100 | This example uses a single event pipe for all signals, and one |
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101 | Async::Interrupt per signal. This code is actually what the L<AnyEvent> |
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102 | module uses itself when Async::Interrupt is available. |
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103 | |
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104 | First, create the event pipe and hook it into the event loop |
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105 | |
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106 | $SIGPIPE = new Async::Interrupt::EventPipe; |
|
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107 | $SIGPIPE_W = AnyEvent->io ( |
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108 | fh => $SIGPIPE->fileno, |
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109 | poll => "r", |
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110 | cb => \&_signal_check, # defined later |
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111 | ); |
|
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112 | |
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113 | Then, for each signal to hook, create an Async::Interrupt object. The |
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114 | callback just sets a global variable, as we are only interested in |
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115 | synchronous signals (i.e. when the event loop polls), which is why the |
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116 | pipe draining is not done automatically. |
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117 | |
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118 | my $interrupt = new Async::Interrupt |
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119 | cb => sub { undef $SIGNAL_RECEIVED{$signum} }, |
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120 | signal => $signum, |
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121 | pipe => [$SIGPIPE->filenos], |
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122 | pipe_autodrain => 0, |
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123 | ; |
|
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124 | |
|
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125 | Finally, the I/O callback for the event pipe handles the signals: |
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126 | |
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127 | sub _signal_check { |
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128 | # drain the pipe first |
|
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129 | $SIGPIPE->drain; |
|
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130 | |
|
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131 | # two loops, just to be sure |
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132 | while (%SIGNAL_RECEIVED) { |
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133 | for (keys %SIGNAL_RECEIVED) { |
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134 | delete $SIGNAL_RECEIVED{$_}; |
|
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135 | warn "signal $_ received\n"; |
|
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136 | } |
|
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137 | } |
|
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138 | } |
|
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139 | |
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140 | =head2 Interrupt perl from another thread |
|
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141 | |
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142 | This example interrupts the Perl interpreter from another thread, via the |
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143 | XS API. This is used by e.g. the L<EV::Loop::Async> module. |
|
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144 | |
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145 | On the Perl level, a new loop object (which contains the thread) |
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146 | is created, by first calling some XS constructor, querying the |
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147 | C-level callback function and feeding that as the C<c_cb> into the |
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148 | Async::Interrupt constructor: |
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149 | |
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150 | my $self = XS_thread_constructor; |
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151 | my ($c_func, $c_arg) = _c_func $self; # return the c callback |
|
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152 | my $asy = new Async::Interrupt c_cb => [$c_func, $c_arg]; |
|
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153 | |
|
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154 | Then the newly created Interrupt object is queried for the signaling |
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155 | function that the newly created thread should call, and this is in turn |
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156 | told to the thread object: |
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157 | |
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158 | _attach $self, $asy->signal_func; |
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159 | |
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160 | So to repeat: first the XS object is created, then it is queried for the |
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161 | callback that should be called when the Interrupt object gets signalled. |
|
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162 | |
|
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163 | Then the interrupt object is queried for the callback fucntion that the |
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164 | thread should call to signal the Interrupt object, and this callback is |
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165 | then attached to the thread. |
|
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166 | |
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167 | You have to be careful that your new thread is not signalling before the |
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168 | signal function was configured, for example by starting the background |
|
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169 | thread only within C<_attach>. |
|
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170 | |
|
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171 | That concludes the Perl part. |
|
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172 | |
|
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173 | The XS part consists of the actual constructor which creates a thread, |
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174 | which is not relevant for this example, and two functions, C<_c_func>, |
|
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175 | which returns the Perl-side callback, and C<_attach>, which configures |
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176 | the signalling functioon that is safe toc all from another thread. For |
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177 | simplicity, we will use global variables to store the functions, normally |
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178 | you would somehow attach them to C<$self>. |
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179 | |
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180 | The C<c_func> simply returns the address of a static function and arranges |
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181 | for the object pointed to by C<$self> to be passed to it, as an integer: |
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182 | |
|
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183 | void |
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184 | _c_func (SV *loop) |
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185 | PPCODE: |
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186 | EXTEND (SP, 2); |
|
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187 | PUSHs (sv_2mortal (newSViv (PTR2IV (c_func)))); |
|
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188 | PUSHs (sv_2mortal (newSViv (SvRV (loop)))); |
|
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189 | |
|
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190 | This would be the callback (since it runs in a normal Perl context, it is |
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191 | permissible to manipulate Perl values): |
|
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192 | |
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193 | static void |
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194 | c_func (pTHX_ void *loop_, int value) |
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195 | { |
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196 | SV *loop_object = (SV *)loop_; |
|
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197 | ... |
|
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198 | } |
|
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199 | |
|
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200 | And this attaches the signalling callback: |
|
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201 | |
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202 | static void (*my_sig_func) (void *signal_arg, int value); |
|
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203 | static void *my_sig_arg; |
|
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204 | |
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205 | void |
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206 | _attach (SV *loop_, IV sig_func, void *sig_arg) |
|
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207 | CODE: |
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208 | { |
|
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209 | my_sig_func = sig_func; |
|
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210 | my_sig_arg = sig_arg; |
|
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211 | |
|
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212 | /* now run the thread */ |
|
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213 | thread_create (&u->tid, l_run, 0); |
|
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214 | } |
|
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215 | |
|
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216 | And C<l_run> (the background thread) would eventually call the signaling |
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217 | function: |
|
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218 | |
|
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219 | my_sig_func (my_sig_arg, 0); |
|
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220 | |
|
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221 | You can have a look at L<EV::Loop::Async> for an actual example using |
|
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222 | intra-thread communication, locking and so on. |
|
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223 | |
|
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224 | |
|
|
225 | =head1 THE Async::Interrupt CLASS |
|
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226 | |
93 | =over 4 |
227 | =over 4 |
94 | |
228 | |
95 | =cut |
229 | =cut |
96 | |
230 | |
97 | package Async::Interrupt; |
231 | package Async::Interrupt; |
98 | |
232 | |
99 | use common::sense; |
233 | use common::sense; |
100 | |
234 | |
101 | BEGIN { |
235 | BEGIN { |
102 | # the next line forces initialisation of internal |
236 | # the next line forces initialisation of internal |
103 | # signal handling # variables |
237 | # signal handling variables, otherwise, PL_sig_pending |
|
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238 | # etc. might be null pointers. |
104 | $SIG{KILL} = sub { }; |
239 | $SIG{KILL} = sub { }; |
105 | |
240 | |
106 | our $VERSION = '0.501'; |
241 | our $VERSION = '1.2'; |
107 | |
242 | |
108 | require XSLoader; |
243 | require XSLoader; |
109 | XSLoader::load ("Async::Interrupt", $VERSION); |
244 | XSLoader::load ("Async::Interrupt", $VERSION); |
110 | } |
245 | } |
111 | |
246 | |
… | |
… | |
134 | The exceptions are C<$!> and C<$@>, which are saved and restored by |
269 | The exceptions are C<$!> and C<$@>, which are saved and restored by |
135 | Async::Interrupt. |
270 | Async::Interrupt. |
136 | |
271 | |
137 | If the callback should throw an exception, then it will be caught, |
272 | If the callback should throw an exception, then it will be caught, |
138 | and C<$Async::Interrupt::DIED> will be called with C<$@> containing |
273 | and C<$Async::Interrupt::DIED> will be called with C<$@> containing |
139 | the exception. The default will simply C<warn> about the message and |
274 | the exception. The default will simply C<warn> about the message and |
140 | continue. |
275 | continue. |
141 | |
276 | |
142 | =item c_cb => [$c_func, $c_arg] |
277 | =item c_cb => [$c_func, $c_arg] |
143 | |
278 | |
144 | Registers a C callback the be invoked whenever the async interrupt is |
279 | Registers a C callback the be invoked whenever the async interrupt is |
… | |
… | |
160 | which case the requirements set out for C<cb> apply as well). |
295 | which case the requirements set out for C<cb> apply as well). |
161 | |
296 | |
162 | =item var => $scalar_ref |
297 | =item var => $scalar_ref |
163 | |
298 | |
164 | When specified, then the given argument must be a reference to a |
299 | When specified, then the given argument must be a reference to a |
165 | scalar. The scalar will be set to C<0> intiially. Signalling the interrupt |
300 | scalar. The scalar will be set to C<0> initially. Signalling the interrupt |
166 | object will set it to the passed value, handling the interrupt will reset |
301 | object will set it to the passed value, handling the interrupt will reset |
167 | it to C<0> again. |
302 | it to C<0> again. |
168 | |
303 | |
169 | Note that the only thing you are legally allowed to do is to is to check |
304 | Note that the only thing you are legally allowed to do is to is to check |
170 | the variable in a boolean or integer context (e.g. comparing it with a |
305 | the variable in a boolean or integer context (e.g. comparing it with a |
… | |
… | |
177 | given signal, that is, it will effectively call C<< ->signal (0) >> each time |
312 | given signal, that is, it will effectively call C<< ->signal (0) >> each time |
178 | the given signal is caught by the process. |
313 | the given signal is caught by the process. |
179 | |
314 | |
180 | Only one async can hook a given signal, and the signal will be restored to |
315 | Only one async can hook a given signal, and the signal will be restored to |
181 | defaults when the Async::Interrupt object gets destroyed. |
316 | defaults when the Async::Interrupt object gets destroyed. |
|
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317 | |
|
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318 | =item signal_hysteresis => $boolean |
|
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319 | |
|
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320 | Sets the initial signal hysteresis state, see the C<signal_hysteresis> |
|
|
321 | method, below. |
182 | |
322 | |
183 | =item pipe => [$fileno_or_fh_for_reading, $fileno_or_fh_for_writing] |
323 | =item pipe => [$fileno_or_fh_for_reading, $fileno_or_fh_for_writing] |
184 | |
324 | |
185 | Specifies two file descriptors (or file handles) that should be signalled |
325 | Specifies two file descriptors (or file handles) that should be signalled |
186 | whenever the async interrupt is signalled. This means a single octet will |
326 | whenever the async interrupt is signalled. This means a single octet will |
… | |
… | |
194 | This can be used to ensure that async notifications will interrupt event |
334 | This can be used to ensure that async notifications will interrupt event |
195 | frameworks as well. |
335 | frameworks as well. |
196 | |
336 | |
197 | Note that C<Async::Interrupt> will create a suitable signal fd |
337 | Note that C<Async::Interrupt> will create a suitable signal fd |
198 | automatically when your program requests one, so you don't have to specify |
338 | automatically when your program requests one, so you don't have to specify |
199 | this agrument when all you want is an extra file descriptor to watch. |
339 | this argument when all you want is an extra file descriptor to watch. |
|
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340 | |
|
|
341 | If you want to share a single event pipe between multiple Async::Interrupt |
|
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342 | objects, you can use the C<Async::Interrupt::EventPipe> class to manage |
|
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343 | those. |
|
|
344 | |
|
|
345 | =item pipe_autodrain => $boolean |
|
|
346 | |
|
|
347 | Sets the initial autodrain state, see the C<pipe_autodrain> method, below. |
200 | |
348 | |
201 | =back |
349 | =back |
202 | |
350 | |
203 | =cut |
351 | =cut |
204 | |
352 | |
205 | sub new { |
353 | sub new { |
206 | my ($class, %arg) = @_; |
354 | my ($class, %arg) = @_; |
207 | |
355 | |
208 | bless \(_alloc $arg{cb}, @{$arg{c_cb}}[0,1], @{$arg{pipe}}[0,1], $arg{signal}, $arg{var}), $class |
356 | my $self = bless \(_alloc $arg{cb}, @{$arg{c_cb}}[0,1], @{$arg{pipe}}[0,1], $arg{signal}, $arg{var}), $class; |
|
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357 | |
|
|
358 | # urgs, reminds me of Event |
|
|
359 | for my $attr (qw(pipe_autodrain signal_hysteresis)) { |
|
|
360 | $self->$attr ($arg{$attr}) if exists $arg{$attr}; |
|
|
361 | } |
|
|
362 | |
|
|
363 | $self |
209 | } |
364 | } |
210 | |
365 | |
211 | =item ($signal_func, $signal_arg) = $async->signal_func |
366 | =item ($signal_func, $signal_arg) = $async->signal_func |
212 | |
367 | |
213 | Returns the address of a function to call asynchronously. The function has |
368 | Returns the address of a function to call asynchronously. The function |
214 | the following prototype and needs to be passed the specified C<$c_arg>, |
369 | has the following prototype and needs to be passed the specified |
215 | which is a C<void *> cast to C<IV>: |
370 | C<$signal_arg>, which is a C<void *> cast to C<IV>: |
216 | |
371 | |
217 | void (*signal_func) (void *signal_arg, int value) |
372 | void (*signal_func) (void *signal_arg, int value) |
218 | |
373 | |
219 | An example call would look like: |
374 | An example call would look like: |
220 | |
375 | |
… | |
… | |
253 | CODE: |
408 | CODE: |
254 | valuep = (IV *)addr; |
409 | valuep = (IV *)addr; |
255 | |
410 | |
256 | // code in a loop, waiting |
411 | // code in a loop, waiting |
257 | while (!*valuep) |
412 | while (!*valuep) |
258 | ; // do soemthing |
413 | ; // do something |
259 | |
414 | |
260 | =item $async->signal ($value=1) |
415 | =item $async->signal ($value=1) |
261 | |
416 | |
262 | This signals the given async object from Perl code. Semi-obviously, this |
417 | This signals the given async object from Perl code. Semi-obviously, this |
263 | will instantly trigger the callback invocation. |
418 | will instantly trigger the callback invocation (it does not, as the name |
|
|
419 | might imply, do anything with POSIX signals). |
264 | |
420 | |
265 | C<$value> must be in the valid range for a C<sig_atomic_t>, except C<0> |
421 | C<$value> must be in the valid range for a C<sig_atomic_t>, except C<0> |
266 | (1..127 is portable). |
422 | (1..127 is portable). |
|
|
423 | |
|
|
424 | =item $async->handle |
|
|
425 | |
|
|
426 | Calls the callback if the object is pending. |
|
|
427 | |
|
|
428 | This method does not need to be called normally, as it will be invoked |
|
|
429 | automatically. However, it can be used to force handling of outstanding |
|
|
430 | interrupts while the object is blocked. |
|
|
431 | |
|
|
432 | One reason why one might want to do that is when you want to switch |
|
|
433 | from asynchronous interruptions to synchronous one, using e.g. an event |
|
|
434 | loop. To do that, one would first C<< $async->block >> the interrupt |
|
|
435 | object, then register a read watcher on the C<pipe_fileno> that calls C<< |
|
|
436 | $async->handle >>. |
|
|
437 | |
|
|
438 | This disables asynchronous interruptions, but ensures that interrupts are |
|
|
439 | handled by the event loop. |
|
|
440 | |
|
|
441 | =item $async->signal_hysteresis ($enable) |
|
|
442 | |
|
|
443 | Enables or disables signal hysteresis (default: disabled). If a POSIX |
|
|
444 | signal is used as a signal source for the interrupt object, then enabling |
|
|
445 | signal hysteresis causes Async::Interrupt to reset the signal action to |
|
|
446 | C<SIG_IGN> in the signal handler and restore it just before handling the |
|
|
447 | interruption. |
|
|
448 | |
|
|
449 | When you expect a lot of signals (e.g. when using SIGIO), then enabling |
|
|
450 | signal hysteresis can reduce the number of handler invocations |
|
|
451 | considerably, at the cost of two extra syscalls. |
|
|
452 | |
|
|
453 | Note that setting the signal to C<SIG_IGN> can have unintended side |
|
|
454 | effects when you fork and exec other programs, as often they do not expect |
|
|
455 | signals to be ignored by default. |
267 | |
456 | |
268 | =item $async->block |
457 | =item $async->block |
269 | |
458 | |
270 | =item $async->unblock |
459 | =item $async->unblock |
271 | |
460 | |
… | |
… | |
286 | This call C<< $async->block >> and installs a handler that is called when |
475 | This call C<< $async->block >> and installs a handler that is called when |
287 | the current scope is exited (via an exception, by canceling the Coro |
476 | the current scope is exited (via an exception, by canceling the Coro |
288 | thread, by calling last/goto etc.). |
477 | thread, by calling last/goto etc.). |
289 | |
478 | |
290 | This is the recommended (and fastest) way to implement critical sections. |
479 | This is the recommended (and fastest) way to implement critical sections. |
|
|
480 | |
|
|
481 | =item ($block_func, $block_arg) = $async->scope_block_func |
|
|
482 | |
|
|
483 | Returns the address of a function that implements the C<scope_block> |
|
|
484 | functionality. |
|
|
485 | |
|
|
486 | It has the following prototype and needs to be passed the specified |
|
|
487 | C<$block_arg>, which is a C<void *> cast to C<IV>: |
|
|
488 | |
|
|
489 | void (*block_func) (void *block_arg) |
|
|
490 | |
|
|
491 | An example call would look like: |
|
|
492 | |
|
|
493 | block_func (block_arg); |
|
|
494 | |
|
|
495 | The function is safe to call only from within the toplevel of a perl XS |
|
|
496 | function and will call C<LEAVE> and C<ENTER> (in this order!). |
291 | |
497 | |
292 | =item $async->pipe_enable |
498 | =item $async->pipe_enable |
293 | |
499 | |
294 | =item $async->pipe_disable |
500 | =item $async->pipe_disable |
295 | |
501 | |
… | |
… | |
306 | =item $fileno = $async->pipe_fileno |
512 | =item $fileno = $async->pipe_fileno |
307 | |
513 | |
308 | Returns the reading side of the signalling pipe. If no signalling pipe is |
514 | Returns the reading side of the signalling pipe. If no signalling pipe is |
309 | currently attached to the object, it will dynamically create one. |
515 | currently attached to the object, it will dynamically create one. |
310 | |
516 | |
311 | Note that the only valid oepration on this file descriptor is to wait |
517 | Note that the only valid operation on this file descriptor is to wait |
312 | until it is readable. The fd might belong currently to a pipe, a tcp |
518 | until it is readable. The fd might belong currently to a pipe, a tcp |
313 | socket, or an eventfd, depending on the platform, and is guaranteed to be |
519 | socket, or an eventfd, depending on the platform, and is guaranteed to be |
314 | C<select>able. |
520 | C<select>able. |
|
|
521 | |
|
|
522 | =item $async->pipe_autodrain ($enable) |
|
|
523 | |
|
|
524 | Enables (C<1>) or disables (C<0>) automatic draining of the pipe (default: |
|
|
525 | enabled). When automatic draining is enabled, then Async::Interrupt will |
|
|
526 | automatically clear the pipe. Otherwise the user is responsible for this |
|
|
527 | draining. |
|
|
528 | |
|
|
529 | This is useful when you want to share one pipe among many Async::Interrupt |
|
|
530 | objects. |
|
|
531 | |
|
|
532 | =item $async->pipe_drain |
|
|
533 | |
|
|
534 | Drains the pipe manually, for example, when autodrain is disabled. Does |
|
|
535 | nothing when no pipe is enabled. |
315 | |
536 | |
316 | =item $async->post_fork |
537 | =item $async->post_fork |
317 | |
538 | |
318 | The object will not normally be usable after a fork (as the pipe fd is |
539 | The object will not normally be usable after a fork (as the pipe fd is |
319 | shared between processes). Calling this method after a fork in the child |
540 | shared between processes). Calling this method after a fork in the child |
… | |
… | |
323 | This only works when the pipe was created by Async::Interrupt. |
544 | This only works when the pipe was created by Async::Interrupt. |
324 | |
545 | |
325 | Async::Interrupt ensures that the reading file descriptor does not change |
546 | Async::Interrupt ensures that the reading file descriptor does not change |
326 | it's value. |
547 | it's value. |
327 | |
548 | |
|
|
549 | =item $signum = Async::Interrupt::sig2num $signame_or_number |
|
|
550 | |
|
|
551 | =item $signame = Async::Interrupt::sig2name $signame_or_number |
|
|
552 | |
|
|
553 | These two convenience functions simply convert a signal name or number to |
|
|
554 | the corresponding name or number. They are not used by this module and |
|
|
555 | exist just because perl doesn't have a nice way to do this on its own. |
|
|
556 | |
|
|
557 | They will return C<undef> on illegal names or numbers. |
|
|
558 | |
|
|
559 | =back |
|
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560 | |
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561 | =head1 THE Async::Interrupt::EventPipe CLASS |
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562 | |
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563 | Pipes are the predominant utility to make asynchronous signals |
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564 | synchronous. However, pipes are hard to come by: they don't exist on the |
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565 | broken windows platform, and on GNU/Linux systems, you might want to use |
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566 | an C<eventfd> instead. |
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567 | |
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568 | This class creates selectable event pipes in a portable fashion: on |
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569 | windows, it will try to create a tcp socket pair, on GNU/Linux, it will |
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570 | try to create an eventfd and everywhere else it will try to use a normal |
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571 | pipe. |
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572 | |
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573 | =over 4 |
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574 | |
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575 | =item $epipe = new Async::Interrupt::EventPipe |
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576 | |
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577 | This creates and returns an eventpipe object. This object is simply a |
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578 | blessed array reference: |
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579 | |
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580 | =item ($r_fd, $w_fd) = $epipe->filenos |
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581 | |
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582 | Returns the read-side file descriptor and the write-side file descriptor. |
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583 | |
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584 | Example: pass an eventpipe object as pipe to the Async::Interrupt |
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585 | constructor, and create an AnyEvent watcher for the read side. |
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586 | |
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587 | my $epipe = new Async::Interrupt::EventPipe; |
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588 | my $asy = new Async::Interrupt pipe => [$epipe->filenos]; |
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589 | my $iow = AnyEvent->io (fh => $epipe->fileno, poll => 'r', cb => sub { }); |
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590 | |
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591 | =item $r_fd = $epipe->fileno |
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592 | |
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593 | Return only the reading/listening side. |
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594 | |
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595 | =item $epipe->signal |
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596 | |
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597 | Write something to the pipe, in a portable fashion. |
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598 | |
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599 | =item $epipe->drain |
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600 | |
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601 | Drain (empty) the pipe. |
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602 | |
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603 | =item ($c_func, $c_arg) = $epipe->signal_func |
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604 | |
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605 | =item ($c_func, $c_arg) = $epipe->drain_func |
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606 | |
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607 | These two methods returns a function pointer and C<void *> argument |
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608 | that can be called to have the effect of C<< $epipe->signal >> or C<< |
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609 | $epipe->drain >>, respectively, on the XS level. |
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610 | |
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611 | They both have the following prototype and need to be passed their |
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612 | C<$c_arg>, which is a C<void *> cast to an C<IV>: |
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613 | |
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614 | void (*c_func) (void *c_arg) |
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615 | |
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616 | An example call would look like: |
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617 | |
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618 | c_func (c_arg); |
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619 | |
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620 | =item $epipe->renew |
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621 | |
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622 | Recreates the pipe (useful after a fork). The reading side will not change |
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623 | it's file descriptor number, but the writing side might. |
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624 | |
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625 | =item $epipe->wait |
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626 | |
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627 | This method blocks the process until there are events on the pipe. This is |
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628 | not a very event-based or ncie way of usign an event pipe, but it can be |
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629 | occasionally useful. |
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630 | |
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631 | =back |
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632 | |
328 | =cut |
633 | =cut |
329 | |
634 | |
330 | 1; |
635 | 1; |
331 | |
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332 | =back |
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333 | |
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334 | =head1 EXAMPLE |
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335 | |
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336 | There really should be a complete C/XS example. Bug me about it. Better |
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337 | yet, create one. |
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338 | |
636 | |
339 | =head1 IMPLEMENTATION DETAILS AND LIMITATIONS |
637 | =head1 IMPLEMENTATION DETAILS AND LIMITATIONS |
340 | |
638 | |
341 | This module works by "hijacking" SIGKILL, which is guaranteed to always |
639 | This module works by "hijacking" SIGKILL, which is guaranteed to always |
342 | exist, but also cannot be caught, so is always available. |
640 | exist, but also cannot be caught, so is always available. |