Add $SYSTEMD_IN_INITRD=yes|no override for debugging

docs/ENVIRONMENT.md

@@ -37,6 +37,10 @@ All tools:
   useful for debugging, in order to test generators and other code against
   specific kernel command lines.
 
+* `$SYSTEMD_IN_INITRD` — takes a boolean. If set, overrides initrd detection.
+  This is useful for debugging and testing initrd-only programs in the main
+  system.
+
 * `$SYSTEMD_BUS_TIMEOUT=SECS` — specifies the maximum time to wait for method call
   completion. If no time unit is specified, assumes seconds. The usual other units
   are understood, too (us, ms, s, min, h, d, w, month, y). If it is not set or set

src/basic/util.c

@@ -23,6 +23,7 @@
 #include "def.h"
 #include "device-nodes.h"
 #include "dirent-util.h"
+#include "env-util.h"
 #include "fd-util.h"
 #include "fileio.h"
 #include "format-util.h"
@@ -106,6 +107,7 @@ int prot_from_flags(int flags) {
 
 bool in_initrd(void) {
         struct statfs s;
+        int r;
 
         if (saved_in_initrd >= 0)
                 return saved_in_initrd;
@@ -120,9 +122,16 @@ bool in_initrd(void) {
          * emptying when transititioning to the main systemd.
          */
 
-        saved_in_initrd = access("/etc/initrd-release", F_OK) >= 0 &&
-                          statfs("/", &s) >= 0 &&
-                          is_temporary_fs(&s);
+        r = getenv_bool_secure("SYSTEMD_IN_INITRD");
+        if (r < 0 && r != -ENXIO)
+                log_debug_errno(r, "Failed to parse $SYSTEMD_IN_INITRD, ignoring: %m");
+
+        if (r >= 0)
+                saved_in_initrd = r > 0;
+        else
+                saved_in_initrd = access("/etc/initrd-release", F_OK) >= 0 &&
+                                  statfs("/", &s) >= 0 &&
+                                  is_temporary_fs(&s);
 
         return saved_in_initrd;
 }