basic: split out dev_t related calls into new devno-util.[ch]

No actual code changes, just splitting out of some dev_t handling
related calls from stat-util.[ch], they are quite a number already, and
deserve their own module now I think.

Also, try to settle on the name "devnum" as the name for the concept,
instead of "devno" or "dev" or "devid". "devnum" is the name exported in
udev APIs, hence probably best to stick to that. (this just renames a
few symbols to "devum", local variables are left untouched, to make the
patch not too invasive)

No actual code changes.

src/basic/devnum-util.c

@@ -0,0 +1,133 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include <string.h>
+#include <sys/stat.h>
+
+#include "chase-symlinks.h"
+#include "devnum-util.h"
+#include "parse-util.h"
+#include "path-util.h"
+#include "string-util.h"
+
+int parse_devnum(const char *s, dev_t *ret) {
+        const char *major;
+        unsigned x, y;
+        size_t n;
+        int r;
+
+        n = strspn(s, DIGITS);
+        if (n == 0)
+                return -EINVAL;
+        if (s[n] != ':')
+                return -EINVAL;
+
+        major = strndupa_safe(s, n);
+        r = safe_atou(major, &x);
+        if (r < 0)
+                return r;
+
+        r = safe_atou(s + n + 1, &y);
+        if (r < 0)
+                return r;
+
+        if (!DEVICE_MAJOR_VALID(x) || !DEVICE_MINOR_VALID(y))
+                return -ERANGE;
+
+        *ret = makedev(x, y);
+        return 0;
+}
+
+int device_path_make_major_minor(mode_t mode, dev_t devnum, char **ret) {
+        const char *t;
+
+        /* Generates the /dev/{char|block}/MAJOR:MINOR path for a dev_t */
+
+        if (S_ISCHR(mode))
+                t = "char";
+        else if (S_ISBLK(mode))
+                t = "block";
+        else
+                return -ENODEV;
+
+        if (asprintf(ret, "/dev/%s/%u:%u", t, major(devnum), minor(devnum)) < 0)
+                return -ENOMEM;
+
+        return 0;
+}
+
+int device_path_make_canonical(mode_t mode, dev_t devnum, char **ret) {
+        _cleanup_free_ char *p = NULL;
+        int r;
+
+        /* Finds the canonical path for a device, i.e. resolves the /dev/{char|block}/MAJOR:MINOR path to the end. */
+
+        assert(ret);
+
+        if (major(devnum) == 0 && minor(devnum) == 0) {
+                char *s;
+
+                /* A special hack to make sure our 'inaccessible' device nodes work. They won't have symlinks in
+                 * /dev/block/ and /dev/char/, hence we handle them specially here. */
+
+                if (S_ISCHR(mode))
+                        s = strdup("/run/systemd/inaccessible/chr");
+                else if (S_ISBLK(mode))
+                        s = strdup("/run/systemd/inaccessible/blk");
+                else
+                        return -ENODEV;
+
+                if (!s)
+                        return -ENOMEM;
+
+                *ret = s;
+                return 0;
+        }
+
+        r = device_path_make_major_minor(mode, devnum, &p);
+        if (r < 0)
+                return r;
+
+        return chase_symlinks(p, NULL, 0, ret, NULL);
+}
+
+int device_path_parse_major_minor(const char *path, mode_t *ret_mode, dev_t *ret_devnum) {
+        mode_t mode;
+        dev_t devnum;
+        int r;
+
+        /* Tries to extract the major/minor directly from the device path if we can. Handles /dev/block/ and /dev/char/
+         * paths, as well out synthetic inaccessible device nodes. Never goes to disk. Returns -ENODEV if the device
+         * path cannot be parsed like this.  */
+
+        if (path_equal(path, "/run/systemd/inaccessible/chr")) {
+                mode = S_IFCHR;
+                devnum = makedev(0, 0);
+        } else if (path_equal(path, "/run/systemd/inaccessible/blk")) {
+                mode = S_IFBLK;
+                devnum = makedev(0, 0);
+        } else {
+                const char *w;
+
+                w = path_startswith(path, "/dev/block/");
+                if (w)
+                        mode = S_IFBLK;
+                else {
+                        w = path_startswith(path, "/dev/char/");
+                        if (!w)
+                                return -ENODEV;
+
+                        mode = S_IFCHR;
+                }
+
+                r = parse_devnum(w, &devnum);
+                if (r < 0)
+                        return r;
+        }
+
+        if (ret_mode)
+                *ret_mode = mode;
+        if (ret_devnum)
+                *ret_devnum = devnum;
+
+        return 0;
+}