[BUG] Minimum benchmark time with multiple threads

[hahnjo]

Describe the bug
#1836 fixed #1834 and now the reported output makes more sense. However, the minimum benchmark time doesn't seem to be respected.

System
Arch Linux, GCC 15.2.1, benchmark 1.9.5-1

To reproduce
Consider

#include <benchmark/benchmark.h>

static void BM_MultiThreaded(benchmark::State& state) {
  int a = 0;

  for (auto _ : state) {
    for (int i = 0; i < 1000; i++) {
      benchmark::DoNotOptimize(a++);
    }
  }
}
BENCHMARK(BM_MultiThreaded)->ThreadRange(1, 8);

BENCHMARK_MAIN();

The output is somewhat expected:

---------------------------------------------------------------------
Benchmark                           Time             CPU   Iterations
---------------------------------------------------------------------
BM_MultiThreaded/threads:1        275 ns          274 ns      2585850
BM_MultiThreaded/threads:2        273 ns          272 ns      2259184
BM_MultiThreaded/threads:4        279 ns          278 ns      2449204
BM_MultiThreaded/threads:8        308 ns          307 ns      2148808

However, notice that the number of iterations stays constant - it should increase as more threads are used to run the iterations. For example:

 $ /usr/bin/time -v ./benchmark-multithreaded --benchmark_min_time=1s --benchmark_filter=threads:8
[...]
	User time (seconds): 1.72
	Elapsed (wall clock) time (h:mm:ss or m:ss): 0:00.25

With previous versions of benchmark, this could be fixed with UseRealTime() but that doesn't seem to work anymore since (probably) #1836.

Expected behavior
Each benchmark should spend the minimum time (per thread).

[dmah42]

what does the same table look like after your fix?

[hahnjo]

With the change in #2118, the output is

---------------------------------------------------------------------
Benchmark                           Time             CPU   Iterations
---------------------------------------------------------------------
BM_MultiThreaded/threads:1        273 ns          271 ns      2586614
BM_MultiThreaded/threads:2        272 ns          272 ns      5127964
BM_MultiThreaded/threads:4        273 ns          272 ns     10337092
BM_MultiThreaded/threads:8        273 ns          273 ns     19537480

and each benchmark is run for at least one second if I ask for it:

 $ /usr/bin/time -v ./benchmark-multithreaded --benchmark_min_time=1s --benchmark_filter=threads:8
[...]
        User time (seconds): 13.40
        Elapsed (wall clock) time (h:mm:ss or m:ss): 0:01.76