| Summary: | select should return when pipe still has readable data | ||||||||
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| Product: | [Fedora] Fedora | Reporter: | Tim Taiwanese Liim <tim.liim> | ||||||
| Component: | perl | Assignee: | Marcela Mašláňová <mmaslano> | ||||||
| Status: | CLOSED NOTABUG | QA Contact: | Fedora Extras Quality Assurance <extras-qa> | ||||||
| Severity: | unspecified | Docs Contact: | |||||||
| Priority: | unspecified | ||||||||
| Version: | 14 | CC: | cweyl, iarnell, kasal, lkundrak, mmaslano, ppisar, psabata, rc040203, tcallawa | ||||||
| Target Milestone: | --- | ||||||||
| Target Release: | --- | ||||||||
| Hardware: | Unspecified | ||||||||
| OS: | Unspecified | ||||||||
| Whiteboard: | |||||||||
| Fixed In Version: | Doc Type: | Bug Fix | |||||||
| Doc Text: | Story Points: | --- | |||||||
| Clone Of: | Environment: | ||||||||
| Last Closed: | 2011-03-28 04:49:26 UTC | Type: | --- | ||||||
| Regression: | --- | Mount Type: | --- | ||||||
| Documentation: | --- | CRM: | |||||||
| Verified Versions: | Category: | --- | |||||||
| oVirt Team: | --- | RHEL 7.3 requirements from Atomic Host: | |||||||
| Cloudforms Team: | --- | Target Upstream Version: | |||||||
| Attachments: |
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Created attachment 488070 [details]
the producer.pl code
Created attachment 488071 [details]
the consumer.pl code
I see what I missed. select() and "$_ = <F>" does not go well together; <F> reads all available data into its own buffer, thus select() sees no more readable fd. Will close this bug now. |
Description of problem: select() should return when a fd is readable. In the attached producer-consumer pair, select() did not return when the pipe still has readable data in it. I am guessing this issue is not really perl specific, but related to kernel or libc. But since I found it in perl first, I am reporting it here. Please assign it to other module as you judge appropriate. If the example perl script is wrong, please correct me and close this bug. What does the example producer-consumer pair do? - producer.pl ("P") prints two lines every 5 sec, with auto flush on. - consumer.pl ("C") spawns producer.pl with open(F, "producer.pl|"), and read from the pipe. - without arguments (so C just does "while (<F>) ..."), C prints the two lines from P just fine, every 5 sec. For example, C got the line with "1 b" at the same second when P produces it. Sun Mar 27 23:37:24 2011: Sun Mar 27 23:37:24 2011 1 a Sun Mar 27 23:37:24 2011: Sun Mar 27 23:37:24 2011 1 b ^^^^^^ timestamp from C ^^^^^^ timestamp from P - with "s" as arg, C uses select() to determine if data is available. If so, reads and prints it. BUT C reads the line "1 b" 5 seconds later than P produces it: Sun Mar 27 23:36:48 2011: Sun Mar 27 23:36:48 2011 1 a Sun Mar 27 23:36:53 2011: Sun Mar 27 23:36:48 2011 1 b ^^ ^^ P produces "1 b" at 23:36:48, but C reads it 5 sec later at 23:36:53. Version-Release number of selected component (if applicable): perl-5.12.3-141.fc14.x86_64 How reproducible: always Steps to Reproduce: 1. download attached consumer.pl, producer.pl 2. run "consumer.pl" for 30 sec, then ctrl-C to interrupt. 3. run "consumer.pl s" for 30 sec, then ctrl-C to interrupt. Actual results: At step 2, C (with "while (<F>) ...") reads from P as soon as P flushes stdout (good): Sun Mar 27 23:58:37 2011: Sun Mar 27 23:58:37 2011 1 a Sun Mar 27 23:58:37 2011: Sun Mar 27 23:58:37 2011 1 b Sun Mar 27 23:58:42 2011: Sun Mar 27 23:58:42 2011 2 a Sun Mar 27 23:58:42 2011: Sun Mar 27 23:58:42 2011 2 b At step 3, C (with select() does not read data from P until next set of data from P arrives (bad): Sun Mar 27 23:58:57 2011: Sun Mar 27 23:58:57 2011 1 a Sun Mar 27 23:59:02 2011: Sun Mar 27 23:58:57 2011 1 b Sun Mar 27 23:59:02 2011: Sun Mar 27 23:59:02 2011 2 a Sun Mar 27 23:59:07 2011: Sun Mar 27 23:59:02 2011 2 b because select() does not return even when pipe from P has readable data in it. Expected results: select() should return as long as there is readable data in fd, so step 3 should behaves just like step 2. Additional info: n/a