systemctl: add a way to explicitly request client-side unit installing

This adds support for a new environment variable
SYSTEMCTL_INSTALL_CLIENT_SIDE, that ensures that systemctl executes
install operations client-side instead of passing them to PID1. This is
useful in debugging situations, but even beyond that. However, we don't
want to make it official API, hence let's just make it an undocumented
environment variable.

Similar, add a second variable, SYSTEMCTL_SKIP_SYSV which allows
skipping the SysV chkconfig fall-back if set. This is useful for similar
reasons, and exposed as undocumented as environment variable for similar
reasons, too.

src/basic/env-util.c

@@ -25,6 +25,7 @@
 #include "alloc-util.h"
 #include "def.h"
 #include "env-util.h"
+#include "parse-util.h"
 #include "string-util.h"
 #include "strv.h"
 #include "utf8.h"
@@ -594,3 +595,13 @@ char **replace_env_argv(char **argv, char **env) {
         ret[k] = NULL;
         return ret;
 }
+
+int getenv_bool(const char *p) {
+        const char *e;
+
+        e = getenv(p);
+        if (!e)
+                return -ENXIO;
+
+        return parse_boolean(e);
+}

src/basic/env-util.h

@@ -47,3 +47,5 @@ char **strv_env_unset_many(char **l, ...) _sentinel_;
 
 char *strv_env_get_n(char **l, const char *name, size_t k) _pure_;
 char *strv_env_get(char **x, const char *n) _pure_;
+
+int getenv_bool(const char *p);

src/systemctl/systemctl.c

@@ -296,6 +296,10 @@ static bool install_client_side(void) {
         if (arg_scope == UNIT_FILE_GLOBAL)
                 return true;
 
+        /* Unsupported environment variable, mostly for debugging purposes */
+        if (getenv_bool("SYSTEMCTL_INSTALL_CLIENT_SIDE") > 0)
+                return true;
+
         return false;
 }
 
@@ -5317,6 +5321,9 @@ static int enable_sysv_units(const char *verb, char **args) {
         if (arg_scope != UNIT_FILE_SYSTEM)
                 return 0;
 
+        if (getenv_bool("SYSTEMCTL_SKIP_SYSV") > 0)
+                return 0;
+
         if (!STR_IN_SET(verb,
                         "enable",
                         "disable",