procfs-util: fix confusion wrt. quantity limit and maximum value

From packit/rawhide-arm64 logs:
Assertion 'limit >= INT_MAX || get_process_ppid(limit+1, NULL) == -ESRCH' failed at src/test/test-process-util.c:855, function test_get_process_ppid(). Aborting.
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The kernel has a few different limits. In particular kernel.threads-max can be
set to some lower value, and kernel.pid_max can be set to a higher value. This
is nice because it reduces PID reuse, even if the number of threads that is
allowed is limited. But the tests assumed that we cannot have a thread with
PID above MIN(kernel.threads-max, kernel.pid_max-1), which is not valid.

So let's rework the whole thing: let's expose the helpers to read
kernel.threads-max and kernel.pid_max, and print what they return in tests.
procfs_tasks_get_limit() was something that is only used in tests, and wasn't
very well defined, so let's drop it.

Fixes #21193.

src/basic/procfs-util.c

@@ -13,54 +13,34 @@
 #include "stdio-util.h"
 #include "string-util.h"
 
-int procfs_tasks_get_limit(uint64_t *ret) {
+int procfs_get_pid_max(uint64_t *ret) {
         _cleanup_free_ char *value = NULL;
-        uint64_t pid_max, threads_max;
         int r;
 
         assert(ret);
 
-        /* So there are two sysctl files that control the system limit of processes:
-         *
-         * 1. kernel.threads-max: this is probably the sysctl that makes more sense, as it directly puts a limit on
-         *    concurrent tasks.
-         *
-         * 2. kernel.pid_max: this limits the numeric range PIDs can take, and thus indirectly also limits the number
-         *    of concurrent threads. AFAICS it's primarily a compatibility concept: some crappy old code used a signed
-         *    16bit type for PIDs, hence the kernel provides a way to ensure the PIDs never go beyond INT16_MAX by
-         *    default.
-         *
-         * By default #2 is set to much lower values than #1, hence the limit people come into contact with first, as
-         * it's the lowest boundary they need to bump when they want higher number of processes.
-         *
-         * Also note the weird definition of #2: PIDs assigned will be kept below this value, which means the number of
-         * tasks that can be created is one lower, as PID 0 is not a valid process ID. */
-
         r = read_one_line_file("/proc/sys/kernel/pid_max", &value);
         if (r < 0)
                 return r;
 
-        r = safe_atou64(value, &pid_max);
-        if (r < 0)
-                return r;
+        return safe_atou64(value, ret);
+}
+
+int procfs_get_threads_max(uint64_t *ret) {
+        _cleanup_free_ char *value = NULL;
+        int r;
+
+        assert(ret);
 
-        value = mfree(value);
         r = read_one_line_file("/proc/sys/kernel/threads-max", &value);
         if (r < 0)
                 return r;
 
-        r = safe_atou64(value, &threads_max);
-        if (r < 0)
-                return r;
-
-        /* Subtract one from pid_max, since PID 0 is not a valid PID */
-        *ret = MIN(pid_max-1, threads_max);
-        return 0;
+        return safe_atou64(value, ret);
 }
 
 int procfs_tasks_set_limit(uint64_t limit) {
         char buffer[DECIMAL_STR_MAX(uint64_t)+1];
-        _cleanup_free_ char *value = NULL;
         uint64_t pid_max;
         int r;
 
@@ -75,10 +55,7 @@ int procfs_tasks_set_limit(uint64_t limit) {
          * set it to the maximum. */
         limit = CLAMP(limit, 20U, TASKS_MAX);
 
-        r = read_one_line_file("/proc/sys/kernel/pid_max", &value);
-        if (r < 0)
-                return r;
-        r = safe_atou64(value, &pid_max);
+        r = procfs_get_pid_max(&pid_max);
         if (r < 0)
                 return r;
 
@@ -99,14 +76,10 @@ int procfs_tasks_set_limit(uint64_t limit) {
                 /* Hmm, we couldn't write this? If so, maybe it was already set properly? In that case let's not
                  * generate an error */
 
-                value = mfree(value);
-                if (read_one_line_file("/proc/sys/kernel/threads-max", &value) < 0)
+                if (procfs_get_threads_max(&threads_max) < 0)
                         return r; /* return original error */
 
-                if (safe_atou64(value, &threads_max) < 0)
-                        return r; /* return original error */
-
-                if (MIN(pid_max-1, threads_max) != limit)
+                if (MIN(pid_max - 1, threads_max) != limit)
                         return r; /* return original error */
 
                 /* Yay! Value set already matches what we were trying to set, hence consider this a success. */