efi: cache test results of boolean EFI state functions

EFI variable access is nowadays subject to rate limiting by the kernel.
Thus, let's cache the results of checking them, in order to minimize how
often we access them.

Fixes: systemd#14828

src/basic/efivars.c

@@ -244,10 +244,16 @@ int efi_set_variable_string(sd_id128_t vendor, const char *name, const char *v)
 }
 
 bool is_efi_boot(void) {
-        if (detect_container() > 0)
-                return false;
+        static int cache = -1;
 
-        return access("/sys/firmware/efi/", F_OK) >= 0;
+        if (cache < 0) {
+                if (detect_container() > 0)
+                        cache = false;
+                else
+                        cache = access("/sys/firmware/efi/", F_OK) >= 0;
+        }
+
+        return cache;
 }
 
 static int read_flag(const char *varname) {
@@ -271,11 +277,21 @@ static int read_flag(const char *varname) {
 }
 
 bool is_efi_secure_boot(void) {
-        return read_flag("SecureBoot") > 0;
+        static int cache = -1;
+
+        if (cache < 0)
+                cache = read_flag("SecureBoot");
+
+        return cache > 0;
 }
 
 bool is_efi_secure_boot_setup_mode(void) {
-        return read_flag("SetupMode") > 0;
+        static int cache = -1;
+
+        if (cache < 0)
+                cache = read_flag("SetupMode");
+
+        return cache > 0;
 }
 
 int systemd_efi_options_variable(char **line) {