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Re: [PATCH 01/10] strbuf: Add strbuf_read_noblock

From
Jeff King <peff@peff.net>
Date
Sep 17, 2015, 16:57 UTC
Message-ID
<20150917165724.GA27102@sigill.intra.peff.net>
In-Reply-To
<CAPc5daVY=t27wkX2x7DSfObwxZuv37VYxi1XD-RKiS1mv4=6KA@mail.gmail.com>
On Thu, Sep 17, 2015 at 09:44:19AM -0700, Junio C Hamano wrote:
Show 5 quoted lines
> > Arguably we should actually return the number of bytes we _did_ read,
> > but then caller cannot easily tell the difference between EOF and
> > EAGAIN.
> 
> Why can't it check errno==EAGAIN/EWOULDBLOCK?

Is it trustworthy to check errno if read() has not actually ever returned -1? E.g., consider this sequence:

  1. somebody (maybe even us calling strbuf_read) calls read() and it
     returns -1, setting errno to EAGAIN
  2. we call strbuf_read()
    2a. it calls read(), which returns N bytes
    2b. it calls read() again, which returns 0 for EOF
    2c. it returns N, because that's how many bytes it read
  3. we wonder if we hit EOF, or if we simply need to read again. We
     check errno == EAGAIN

I don't think the read calls in step 2 are guaranteed to clear errno, and we might read the cruft from step 1.

-Peff
Previous: Stefan BellerNext: Junio C Hamano
Message 7 of 37 in “fetch submodules in parallel and a preview on parallel "submodule update"”
  1. 00/10 fetch submodules in parallel and a preview on parallel "submodule update"Stefan Beller, Sep 17, 2015
  2. 01/10 strbuf: Add strbuf_read_noblockStefan Beller, Sep 17, 2015
  3. Junio C HamanoSep 17, 2015
  4. Jeff KingSep 17, 2015
  5. Junio C HamanoSep 17, 2015
  6. Stefan BellerSep 17, 2015
  7. Jeff KingSep 17, 2015
  8. Junio C HamanoSep 17, 2015
  9. Jeff KingSep 17, 2015
  10. Stefan BellerSep 17, 2015
  11. Jeff KingSep 17, 2015
  12. Stefan BellerSep 17, 2015
  13. Jeff KingSep 17, 2015
  14. Stefan BellerSep 17, 2015
  15. Junio C HamanoSep 17, 2015
  16. Junio C HamanoSep 17, 2015
  17. Jeff KingSep 17, 2015
  18. Stefan BellerSep 17, 2015
  19. 02/10 run-command: factor out return value computationStefan Beller, Sep 17, 2015
  20. Jeff KingSep 17, 2015
  21. 03/10 run-command: add an asynchronous parallel child processorStefan Beller, Sep 17, 2015
  22. Junio C HamanoSep 17, 2015
  23. Stefan BellerSep 17, 2015
  24. Junio C HamanoSep 18, 2015
  25. Stefan BellerSep 18, 2015
  26. 04/10 fetch_populated_submodules: use new parallel job processingStefan Beller, Sep 17, 2015
  27. 05/10 submodules: Allow parallel fetching, add tests and documentationStefan Beller, Sep 17, 2015
  28. 06/10 git submodule update: Redirect any output to stderrStefan Beller, Sep 17, 2015
  29. Eric SunshineSep 17, 2015
  30. Stefan BellerSep 17, 2015
  31. 07/10 git submodule update: pass --prefix only with a non empty prefixStefan Beller, Sep 17, 2015
  32. Eric SunshineSep 17, 2015
  33. 08/10 git submodule update: cmd_update_recursiveStefan Beller, Sep 17, 2015
  34. 09/10 git submodule update: cmd_update_recursiveStefan Beller, Sep 17, 2015
  35. Eric SunshineSep 17, 2015
  36. 10/10 git submodule update: cmd_update_fetchStefan Beller, Sep 17, 2015
  37. Jacob KellerSep 17, 2015

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