Better CPU affinity, if ag is invoked with a nontrivial CPU mask

On a Ryzen 5700U CPU, Linux 5.10.0, Debian bullseye, and `nosmt` on the
kernel commandline, every task is effectively pinned to the CPU mask
0x5555:
```
On-line CPU(s) list:             0,2,4,6,8,10,12,14
Off-line CPU(s) list:            1,3,5,7,9,11,13,15
```

In order to avoid failures of `pthread_setaffinity_np`, check the CPUs
that the main thread has available (and use it to set num_cores!) and
then each time a thread is created, find the next set bit.

This also fixes the behavior when ag is deliberately pinned to some
smaller number of CPUs than those on-line:
```
$ taskset 0x5555 ./ag -D booboo | ag "Using \d+ workers"
DEBUG: Using 7 workers
$ taskset 0x55 ag -D booboo | ag "Using \d+ workers"
DEBUG: Using 3 workers
```

configure.ac

@@ -64,7 +64,7 @@ AC_CHECK_HEADERS([sys/cpuset.h err.h])
 AC_CHECK_MEMBER([struct dirent.d_type], [AC_DEFINE([HAVE_DIRENT_DTYPE], [], [Have dirent struct member d_type])], [], [[#include <dirent.h>]])
 AC_CHECK_MEMBER([struct dirent.d_namlen], [AC_DEFINE([HAVE_DIRENT_DNAMLEN], [], [Have dirent struct member d_namlen])], [], [[#include <dirent.h>]])
 
-AC_CHECK_FUNCS(fgetln fopencookie getline realpath strlcpy strndup vasprintf madvise posix_fadvise pthread_setaffinity_np pledge)
+AC_CHECK_FUNCS(fgetln fopencookie getline realpath strlcpy strndup vasprintf madvise posix_fadvise pthread_getaffinity_np pthread_setaffinity_np pledge)
 
 AC_CONFIG_FILES([Makefile the_silver_searcher.spec])
 AC_CONFIG_HEADERS([src/config.h])

src/main.c

@@ -36,7 +36,7 @@ typedef struct {
 int main(int argc, char **argv) {
     char **base_paths = NULL;
     char **paths = NULL;
-    int i;
+    int i, j;
     int pcre_opts = PCRE_MULTILINE;
     int study_opts = 0;
     worker_t *workers = NULL;
@@ -77,6 +77,18 @@ int main(int argc, char **argv) {
         GetSystemInfo(&si);
         num_cores = si.dwNumberOfProcessors;
     }
+#elif defined(HAVE_PTHREAD_SETAFFINITY_NP) && defined(HAVE_PTHREAD_GETAFFINITY_NP) && (defined(USE_CPU_SET) || defined(HAVE_SYS_CPUSET_H))
+#if defined(__linux__) || defined(__midipix__)
+    cpu_set_t avail_cpu_set;
+#elif __FreeBSD__
+    cpuset_t avail_cpu_set;
+#endif
+    int rv = pthread_getaffinity_np(pthread_self(), sizeof(avail_cpu_set), &avail_cpu_set);
+    for (i = num_cores = 0; i < CPU_SETSIZE; i++) {
+        if (CPU_ISSET(i, &avail_cpu_set)) {
+            num_cores++;
+        }
+    }
 #else
     num_cores = (int)sysconf(_SC_NPROCESSORS_ONLN);
 #endif
@@ -146,13 +158,13 @@ int main(int argc, char **argv) {
     if (opts.search_stream) {
         search_stream(stdin, "");
     } else {
-        for (i = 0; i < workers_len; i++) {
+        for (i = j = 0; i < workers_len; i++) {
             workers[i].id = i;
-            int rv = pthread_create(&(workers[i].thread), NULL, &search_file_worker, &(workers[i].id));
+            rv = pthread_create(&(workers[i].thread), NULL, &search_file_worker, &(workers[i].id));
             if (rv != 0) {
                 die("Error in pthread_create(): %s", strerror(rv));
             }
-#if defined(HAVE_PTHREAD_SETAFFINITY_NP) && (defined(USE_CPU_SET) || defined(HAVE_SYS_CPUSET_H))
+#if defined(HAVE_PTHREAD_SETAFFINITY_NP) && defined(HAVE_PTHREAD_GETAFFINITY_NP) && (defined(USE_CPU_SET) || defined(HAVE_SYS_CPUSET_H))
             if (opts.use_thread_affinity) {
 #if defined(__linux__) || defined(__midipix__)
                 cpu_set_t cpu_set;
@@ -160,13 +172,17 @@ int main(int argc, char **argv) {
                 cpuset_t cpu_set;
 #endif
                 CPU_ZERO(&cpu_set);
-                CPU_SET(i % num_cores, &cpu_set);
+                do {
+                    j = (j + 1) % CPU_SETSIZE;
+                } while (!CPU_ISSET(j, &avail_cpu_set));
+                CPU_SET(j++, &cpu_set);
+
                 rv = pthread_setaffinity_np(workers[i].thread, sizeof(cpu_set), &cpu_set);
                 if (rv) {
                     log_err("Error in pthread_setaffinity_np(): %s", strerror(rv));
                     log_err("Performance may be affected. Use --noaffinity to suppress this message.");
                 } else {
-                    log_debug("Thread %i set to CPU %i", i, i);
+                    log_debug("Thread %i set to CPU %i", i, j);
                 }
             } else {
                 log_debug("Thread affinity disabled.");