util-lib: add new procfs-util.[ch] API for dealing with tasks limits

As it turns out the limit on concurrent tasks on Linux nasty to
determine, hence let's appropriate helpers for this.

src/basic/meson.build

@@ -150,6 +150,8 @@ basic_sources = files('''
         proc-cmdline.h
         process-util.c
         process-util.h
+        procfs-util.c
+        procfs-util.h
         random-util.c
         random-util.h
         ratelimit.c

src/basic/procfs-util.c

@@ -0,0 +1,138 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+
+#include <errno.h>
+
+#include "alloc-util.h"
+#include "fileio.h"
+#include "parse-util.h"
+#include "process-util.h"
+#include "procfs-util.h"
+#include "stdio-util.h"
+#include "string-util.h"
+
+int procfs_tasks_get_limit(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;
+
+        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;
+}
+
+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;
+
+        if (limit == 0) /* This makes no sense, we are userspace and hence count as tasks too, and we want to live,
+                         * hence the limit conceptually has to be above 0. Also, most likely if anyone asks for a zero
+                         * limit he/she probably means "no limit", hence let's better refuse this to avoid
+                         * confusion. */
+                return -EINVAL;
+
+        /* The Linux kernel doesn't allow this value to go below 20, hence don't allow this either, higher values than
+         * TASKS_MAX are not accepted by the pid_max sysctl. We'll treat anything this high as "unbounded" and hence
+         * 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);
+        if (r < 0)
+                return r;
+
+        /* As pid_max is about the numeric pid_t range we'll bump it if necessary, but only ever increase it, never
+         * decrease it, as threads-max is the much more relevant sysctl. */
+        if (limit > pid_max-1) {
+                sprintf(buffer, "%" PRIu64, limit+1); /* Add one, since PID 0 is not a valid PID */
+                r = write_string_file("/proc/sys/kernel/pid_max", buffer, WRITE_STRING_FILE_DISABLE_BUFFER);
+                if (r < 0)
+                        return r;
+        }
+
+        sprintf(buffer, "%" PRIu64, limit);
+        r = write_string_file("/proc/sys/kernel/threads-max", buffer, WRITE_STRING_FILE_DISABLE_BUFFER);
+        if (r < 0) {
+                uint64_t threads_max;
+
+                /* 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)
+                        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)
+                        return r; /* return original error */
+
+                /* Yay! Value set already matches what we were trying to set, hence consider this a success. */
+        }
+
+        return 0;
+}
+
+int procfs_tasks_get_current(uint64_t *ret) {
+        _cleanup_free_ char *value = NULL;
+        const char *p, *nr;
+        size_t n;
+        int r;
+
+        assert(ret);
+
+        r = read_one_line_file("/proc/loadavg", &value);
+        if (r < 0)
+                return r;
+
+        /* Look for the second part of the fourth field, which is separated by a slash from the first part. None of the
+         * earlier fields use a slash, hence let's use this to find the right spot. */
+        p = strchr(value, '/');
+        if (!p)
+                return -EINVAL;
+
+        p++;
+        n = strspn(p, DIGITS);
+        nr = strndupa(p, n);
+
+        return safe_atou64(nr, ret);
+}

src/basic/procfs-util.h

@@ -0,0 +1,8 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+#pragma once
+
+#include <inttypes.h>
+
+int procfs_tasks_get_limit(uint64_t *ret);
+int procfs_tasks_set_limit(uint64_t limit);
+int procfs_tasks_get_current(uint64_t *ret);

src/test/meson.build

@@ -400,6 +400,10 @@ tests += [
          [],
          []],
 
+        [['src/test/test-procfs-util.c'],
+         [],
+         []],
+
         [['src/test/test-unaligned.c'],
          [],
          []],

src/test/test-procfs-util.c

@@ -0,0 +1,38 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+
+#include <errno.h>
+
+#include "log.h"
+#include "procfs-util.h"
+
+int main(int argc, char *argv[]) {
+        uint64_t v;
+        int r;
+
+        log_parse_environment();
+        log_open();
+
+        assert_se(procfs_tasks_get_current(&v) >= 0);
+        log_info("Current number of tasks: %" PRIu64, v);
+
+        assert_se(procfs_tasks_get_limit(&v) >= 0);
+        log_info("Limit of tasks: %" PRIu64, v);
+        assert_se(v > 0);
+        assert_se(procfs_tasks_set_limit(v) >= 0);
+
+        if (v > 100) {
+                uint64_t w;
+                r = procfs_tasks_set_limit(v-1);
+                assert_se(IN_SET(r, 0, -EPERM, -EACCES, -EROFS));
+
+                assert_se(procfs_tasks_get_limit(&w) >= 0);
+                assert_se((r == 0 && w == v - 1) || (r < 0 && w == v));
+
+                assert_se(procfs_tasks_set_limit(v) >= 0);
+
+                assert_se(procfs_tasks_get_limit(&w) >= 0);
+                assert_se(v == w);
+        }
+
+        return 0;
+}