Description of problem:

tempnam() regregresion in glibc affecting RedHat 8.4+.

The C function tempnam() can generate a very small number of unique temporary filenames on RedHat 8.5. As a result an application which is using tempnam() may not function properly under heavy load.  

Based on customers investigations it appears to be a regression in glibc starting with RedHat 8.4. Also RedHat 7.9 is not affected.  We think that latest glibc was already fixed but the fixes were not backported in RedHat 8.5

See below a demo program which outputs the number of unique temporary file names that can be generated on RedHat 8.5 vs RedHat 7.9.

---
#include <map>
#include <iostream>
int main(int argc, char **argv)
{
  std::map<std::string, int> m;
  int i;
  for (i=0; i<1000000; i++) {
    auto ret = m.insert(std::pair<std::string,int>(tempnam(NULL, "a"),1));
    if (ret.second == false) {
      break;
    }
  }
  std::cout << "Generated " << i << " tempnam() distinct entries\n";
  return 0;
}
---
Compile like: 
g++ -o tempnam_demo tempnam_demo.c
---


If the above program is run more times on RedHat 8.5 it outputs results like:
~~~
Generated 4362 tempnam() distinct entries
Generated 3867 tempnam() distinct entries
Generated 4384 tempnam() distinct entries
Generated 5811 tempnam() distinct entries
Generated 13786 tempnam() distinct entries
Generated 10687 tempnam() distinct entries
Generated 13476 tempnam() distinct entries
Generated 5854 tempnam() distinct entries
Generated 7741 tempnam() distinct entries
~~~
If the above program is run more times on RedHat 7.9 it outputs results like:

~~~
Generated 208381 tempnam() distinct entries
Generated 182184 tempnam() distinct entries
Generated 145776 tempnam() distinct entries
Generated 679048 tempnam() distinct entries
Generated 578402 tempnam() distinct entries
Generated 95064 tempnam() distinct entries
Generated 420830 tempnam() distinct entries
Generated 430587 tempnam() distinct entries
Generated 112908 tempnam() distinct entries
~~~


Version-Release number of selected component (if applicable):


How reproducible:


Steps to Reproduce:
1.
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Actual results:


Expected results:


Additional info:

Also customer has  provided detail one scenario(pseudocode) that is affected:
---
process_1
   # init zone
   filename = tempnam();
    ...
   # processing zone
  ...
  createAndFillFile(filename); // file is actually created and filled with information
  ...

process_2
   # init zone
   filename = tempnam();
    ...
   # processing zone
  ...
  createAndFillFile(filename); // file is actually created and filled with information
 ...

process_n
   # init zone
   filename = tempnam();
    ...
   # processing zone
  ...
  createAndFillFile(filename); // file is actually created and filled with information
 ...

---

In the above scenario each process is using tempnam() but is not creating the file(based in filename) immediately. When there are many processes and the load on machine is very high, the time interval between the filename is generated and the file is actually created is increasing for each process(there may be few seconds between). So, there are lot of tempnam() calls in parallel without files from previous calls to be actually created.

This is a scenario which is affected by current reported bug, scenario that works very well on RedHat 7.9, but sometimes is not working properly on RedHat 8.4+.