Horton

Re: Cleaning up historical portability baggage

Thomas Munro <thomas.munro@gmail.com>

View thread View on postgr.es
From: Thomas Munro <thomas.munro@gmail.com>
To: Andres Freund <andres@anarazel.de>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Andrew Dunstan <andrew@dunslane.net>, Robert Haas <robertmhaas@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2022-08-16T04:14:27Z
Lists: pgsql-hackers 
On Tue, Aug 16, 2022 at 1:16 PM Andres Freund <andres@anarazel.de> wrote:
> > But let's suppose we want to play by a timid interpretation of that page's
> > "do not issue low-level or STDIO.H I/O routines".  It also says that SIGINT
> > is special and runs the handler in a new thread (in a big warning box
> > because that has other hazards that would break other kinds of code).  Well,
> > we *know* it's safe to unlink files in another thread... so... how cheesy
> > would it be if we just did raise(SIGINT) in the real handlers?
>
> Not quite sure I understand. You're proposing to raise(SIGINT) for all other
> handlers, so that signal_remove_temp() gets called in another thread, because
> we assume that'd be safe because doing file IO in other threads is safe? That
> assumes the signal handler invocation infrastructure isn't the problem...

That's what I was thinking about, yeah.  But after some more reading,
now I'm wondering if we'd even need to do that, or what I'm missing.
The 6 listed signals in the manual are SIGABRT, SIGFPE, SIGILL,
SIGINT, SIGSEGV and SIGTERM (the 6 required by C).  We want to run
signal_remove_temp() on SIGHUP (doesn't exist, we made it up), SIGINT,
SIGPIPE (doesn't exist, we made it up), and SIGTERM (exists for C spec
compliance but will never be raised by the system according to the
manual, and we don't raise it ourselves IIUC).  So the only case we
actually have to consider is SIGINT, and SIGINT handlers run in a
thread, so if we assume it is therefore exempt from those
very-hard-to-comply-with rules, aren't we good already?  What am I
missing?

> Looks like we could register a "native" ctrl-c handler:
> https://docs.microsoft.com/en-us/windows/console/setconsolectrlhandler
> they're documented to run in a different thread, but without any of the
> file-io warnings.
> https://docs.microsoft.com/en-us/windows/console/handlerroutine

Sounds better in general, considering the extreme constraints of the
signal system, but it'd be nice to see if the current system is truly
unsafe before writing more alien code.

Someone who wants to handle more than one SIGINT would certainly need
to consider that, because there doesn't seem to be a race-free way to
reinstall the signal handler when you receive it[1].  Races aside, for
any signal except SIGINT (assuming the above-mentioned exemption),
you're probably also not even allowed to try because raise() might be
a system call and they're banned.  Fortunately we don't care, we
wanted SIG_DFL next anyway.

[1] https://wiki.sei.cmu.edu/confluence/display/c/SIG01-C.+Understand+implementation-specific+details+regarding+signal+handler+persistence

Commits

  1. Make sure IOV_MAX is defined.

  2. Doc: Abstract AF_UNIX sockets don't work on Windows.

  3. Clean up inconsistent use of fflush().

  4. Remove configure probe for sockaddr_in6 and require AF_INET6.

  5. Remove configure probes for sockaddr_storage members.

  6. Remove configure probe for netinet/tcp.h.

  7. Fix macro problem with gai_strerror on Windows.

  8. Remove configure probe for net/if.h.

  9. mstcpip.h is not missing on MinGW.

  10. Remove dead ifaddr.c fallback code.

  11. Remove configure probe for gethostbyname_r.

  12. Remove replacement code for getaddrinfo.

  13. Remove configure probe for struct sockaddr_storage.

  14. Remove HAVE_UNIX_SOCKETS.

  15. Remove configurability of PPC spinlock assembly code.

  16. Remove configure probe for shl_load library.

  17. Remove configure probe for sys/select.h.

  18. Remove configure probes for sys/un.h and struct sockaddr_un.

  19. Remove configure probes for sys/ipc.h, sys/sem.h, sys/shm.h.

  20. Remove configure probe for sys/resource.h and refactor.

  21. Remove configure probe for sys/uio.h.

  22. Fix obsolete comments in instr_time.h.

  23. aix: fix misreading of condition in 8f12a4e7add

  24. aix: Remove checks for very old OS versions

  25. windows: Remove HAVE_MINIDUMP_TYPE test

  26. solaris: Remove unnecessary gcc / gnu ld vs sun studio differences

  27. Trust a few system headers to stand on their own

  28. Rely on __func__ being supported

  29. Simplify replacement code for strtof.

  30. Simplify gettimeofday for Windows.

  31. Simplify gettimeofday() fallback logic.

  32. Remove fallbacks for strtoll, strtoull.

  33. Remove configure probe for fdatasync.

  34. Remove configure probe for clock_gettime.

  35. Simplify replacement code for preadv and pwritev.

  36. Remove dead pread and pwrite replacement code.

  37. Remove dead setenv, unsetenv replacement code.

  38. Remove configure probes for poll and poll.h.

  39. Remove configure probe for link.

  40. Remove configure probes for symlink/readlink, and dead code.

  41. Remove configure probe for setsid.

  42. Remove configure probe for shm_open.

  43. Remove configure probe and related tests for getrlimit.

  44. Remove configure probe for dlopen, and refactor.

  45. Remove dead getpwuid_r replacement code.

  46. Remove dead handling for pre-POSIX sigwait().

  47. Remove dead getrusage replacement code.

  48. Cygwin and Mingw floating-point fixes.

  49. Support platforms where strtoll/strtoull are spelled __strtoll/__strtoull.

  50. Arrange to supply declarations for strtoll/strtoull if needed.

  51. Lock down regression testing temporary clusters on Windows.

  52. Secure Unix-domain sockets of "make check" temporary clusters.

  53. Add configure checks for strtoll, strtoull (or strto[u]q). Disable

  54. Portability fix for old SunOS releases: fflush(NULL)