util: invert ac_power() source type check

So far we assumed every power source was a battery except for the ones
which definitely are not. I think this logic makes little sense, as
"battery" is kinda the exceptional case here, not the other way round.
Hence let's invert the type check, and denylist "Battery" devices rather
than allowlist "Mains" devices.

This should increase compatibility with alternative types of power
sources, in particular USB ones.

This takes into account that additional power types have been added
since we wrote the original code, and in particular should cover the
siutation discussed here OK:

https://sources.debian.org/src/powermgmt-base/1.36/power_supply.txt/#L31
https://sources.debian.org/src/powermgmt-base/1.36/on_ac_power/#L25

Also, modernizes the code in various was ways.

Inspired by and fixes: #20964

1 files changed, 29 insertions, 34 deletions

diff --git a/src/basic/util.c b/src/basic/util.c
index 955b18bd2a..43e74b773f 100644
--- a/src/basic/util.c
+++ b/src/basic/util.c
@@ -119,62 +119,57 @@ int on_ac_power(void) {
         bool found_offline = false, found_online = false;
         _cleanup_closedir_ DIR *d = NULL;
         struct dirent *de;
+        int r;
 
         d = opendir("/sys/class/power_supply");
         if (!d)
                 return errno == ENOENT ? true : -errno;
 
         FOREACH_DIRENT(de, d, return -errno) {
-                _cleanup_close_ int fd = -1, device = -1;
-                char contents[6];
-                ssize_t n;
+                _cleanup_close_ int device_fd = -1;
+                _cleanup_free_ char *contents = NULL;
+                unsigned v;
 
-                device = openat(dirfd(d), de->d_name, O_DIRECTORY|O_RDONLY|O_CLOEXEC|O_NOCTTY);
-                if (device < 0) {
+                device_fd = openat(dirfd(d), de->d_name, O_DIRECTORY|O_RDONLY|O_CLOEXEC);
+                if (device_fd < 0) {
                         if (IN_SET(errno, ENOENT, ENOTDIR))
                                 continue;
 
                         return -errno;
                 }
 
-                fd = openat(device, "type", O_RDONLY|O_CLOEXEC|O_NOCTTY);
-                if (fd < 0) {
-                        if (errno == ENOENT)
-                                continue;
-
-                        return -errno;
-                }
+                r = read_virtual_file_at(device_fd, "type", SIZE_MAX, &contents, NULL);
+                if (r == -ENOENT)
+                        continue;
+                if (r < 0)
+                        return r;
 
-                n = read(fd, contents, sizeof(contents));
-                if (n < 0)
-                        return -errno;
+                delete_trailing_chars(contents, NEWLINE);
 
-                if (n != 6 || memcmp(contents, "Mains\n", 6))
+                /* We assume every power source is AC, except for batteries. See
+                 * https://github.com/torvalds/linux/blob/4eef766b7d4d88f0b984781bc1bcb574a6eafdc7/include/linux/power_supply.h#L176
+                 * for defined power source types. Also see:
+                 * https://www.kernel.org/doc/Documentation/ABI/testing/sysfs-class-power */
+                if (streq(contents, "Battery"))
                         continue;
 
-                safe_close(fd);
-                fd = openat(device, "online", O_RDONLY|O_CLOEXEC|O_NOCTTY);
-                if (fd < 0) {
-                        if (errno == ENOENT)
-                                continue;
-
-                        return -errno;
-                }
+                contents = mfree(contents);
 
-                n = read(fd, contents, sizeof(contents));
-                if (n < 0)
-                        return -errno;
+                r = read_virtual_file_at(device_fd, "online", SIZE_MAX, &contents, NULL);
+                if (r == -ENOENT)
+                        continue;
+                if (r < 0)
+                        return r;
 
-                if (n != 2 || contents[1] != '\n')
-                        return -EIO;
+                delete_trailing_chars(contents, NEWLINE);
 
-                if (contents[0] == '1') {
+                r = safe_atou(contents, &v);
+                if (r < 0)
+                        return r;
+                if (v > 0) /* At least 1 and 2 are defined as different types of 'online' */
                         found_online = true;
-                        break;
-                } else if (contents[0] == '0')
-                        found_offline = true;
                 else
-                        return -EIO;
+                        found_offline = true;
         }
 
         return found_online || !found_offline;