Re: [PATCH] Cygwin: getdtablesize: always return OPEN_MAX_MAX

[Ken Brown <kbrown@cornell.edu>]

On 1/28/2021 5:20 AM, Corinna Vinschen via Cygwin-patches wrote:
> On Jan 27 21:51, Ken Brown via Cygwin-patches wrote:
>> According to the Linux man page for getdtablesize(3), the latter is
>> supposed to return "the maximum number of files a process can have
>> open, one more than the largest possible value for a file descriptor."
>> The constant OPEN_MAX_MAX is the only limit enforced by Cygwin, so we
>> now return that.
>>
>> Previously getdtablesize returned the current size of cygheap->fdtab,
>> Cygwin's internal file descriptor table.  But this is a dynamically
>> growing table, and its current size does not reflect an actual limit
>> on the number of open files.
>>
>> With this change, gnulib now reports that getdtablesize and
>> fcntl(F_DUPFD) work on Cygwin.  Packages like GNU tar that use the
>> corresponding gnulib modules will no longer use gnulib replacements on
>> Cygwin.
>> ---
>>   winsup/cygwin/syscalls.cc | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/winsup/cygwin/syscalls.cc b/winsup/cygwin/syscalls.cc
>> index 5da05b18a..1f16d54b9 100644
>> --- a/winsup/cygwin/syscalls.cc
>> +++ b/winsup/cygwin/syscalls.cc
>> @@ -2887,7 +2887,7 @@ setdtablesize (int size)
>>   extern "C" int
>>   getdtablesize ()
>>   {
>> -  return cygheap->fdtab.size;
>> +  return OPEN_MAX_MAX;
>>   }
> 
> getdtablesize is used internally, too.  After this change, the values
> returned by sysconf and getrlimit should be revisited as well.

They will now return OPEN_MAX_MAX, as I think they should.  The only question in 
my mind is whether to simplify the code by removing the calls to getdtablesize, 
something like this (untested):

diff --git a/winsup/cygwin/resource.cc b/winsup/cygwin/resource.cc
index 9e39d3a04..ac56acf8c 100644
--- a/winsup/cygwin/resource.cc
+++ b/winsup/cygwin/resource.cc
@@ -182,10 +182,7 @@ getrlimit (int resource, struct rlimit *rlp)
           __get_rlimit_stack (rlp);
           break;
         case RLIMIT_NOFILE:
-         rlp->rlim_cur = getdtablesize ();
-         if (rlp->rlim_cur < OPEN_MAX)
-           rlp->rlim_cur = OPEN_MAX;
-         rlp->rlim_max = OPEN_MAX_MAX;
+         rlp->rlim_cur = rlp->rlim_max = OPEN_MAX_MAX;
           break;
         case RLIMIT_CORE:
           rlp->rlim_cur = cygheap->rlim_core;
diff --git a/winsup/cygwin/sysconf.cc b/winsup/cygwin/sysconf.cc
index 001da96ad..d5d82bb4a 100644
--- a/winsup/cygwin/sysconf.cc
+++ b/winsup/cygwin/sysconf.cc
@@ -21,15 +21,6 @@ details. */
  #include "cpuid.h"
  #include "clock.h"

-static long
-get_open_max (int in)
-{
-  long max = getdtablesize ();
-  if (max < OPEN_MAX)
-    max = OPEN_MAX;
-  return max;
-}
-
  static long
  get_page_size (int in)
  {
@@ -520,7 +511,7 @@ static struct
    {cons, {c:CHILD_MAX}},               /*   1, _SC_CHILD_MAX */
    {cons, {c:CLOCKS_PER_SEC}},          /*   2, _SC_CLK_TCK */
    {cons, {c:NGROUPS_MAX}},             /*   3, _SC_NGROUPS_MAX */
-  {func, {f:get_open_max}},            /*   4, _SC_OPEN_MAX */
+  {cons, {c:OPEN_MAX_MAX}},            /*   4, _SC_OPEN_MAX */
    {cons, {c:_POSIX_JOB_CONTROL}},      /*   5, _SC_JOB_CONTROL */
    {cons, {c:_POSIX_SAVED_IDS}},                /*   6, _SC_SAVED_IDS */
    {cons, {c:_POSIX_VERSION}},          /*   7, _SC_VERSION */

WDYT?

Ken