psi-util: fix error handling

We checked ERRNO_IS_NOT_SUPPORTED on a possible positive non-error code,
which isn't right.

Fix that. Also add caching, since we are about to call this more often.

(cherry picked from commit 90ec8eb)
(cherry picked from commit 5ee19fd)
(cherry picked from commit 8e62340)
(cherry picked from commit cb4f512)

Author: poettering

src/shared/psi-util.c

@@ -106,20 +106,25 @@ int read_resource_pressure(const char *path, PressureType type, ResourcePressure
 }
 
 int is_pressure_supported(void) {
+        static thread_local int cached = -1;
         const char *p;
+        int r;
 
-        /* The pressure files, both under /proc and in cgroups, will exist
-         * even if the kernel has PSI support disabled; we have to read
-         * the file to make sure it doesn't return -EOPNOTSUPP */
-        FOREACH_STRING(p, "/proc/pressure/cpu", "/proc/pressure/io", "/proc/pressure/memory") {
-                int r;
+        /* The pressure files, both under /proc/ and in cgroups, will exist even if the kernel has PSI
+         * support disabled; we have to read the file to make sure it doesn't return -EOPNOTSUPP */
 
+        if (cached >= 0)
+                return cached;
+
+        FOREACH_STRING(p, "/proc/pressure/cpu", "/proc/pressure/io", "/proc/pressure/memory") {
                 r = read_virtual_file(p, 0, NULL, NULL);
-                if (r == -ENOENT || ERRNO_IS_NOT_SUPPORTED(r))
-                        return 0;
-                if (r < 0)
+                if (r < 0) {
+                        if (r == -ENOENT || ERRNO_IS_NOT_SUPPORTED(r))
+                                return (cached = false);
+
                         return r;
+                }
         }
 
-        return 1;
+        return (cached = true);
 }