Allow the fallback hostname to be overriden using an environment variable

See https://bugzilla.redhat.com/show_bug.cgi?id=1893417 for the back story:
the fallback hostname matters a lot in certain environments. Right now the only
way to configure the fallback hostname is by recompiling systemd, which is
obviously problematic in case when the fallback hostname shall differ between
different editions of the same distro that share a single compiled rpm.

By making this overridable through an envvar, we're providing an escape hatch
without making this a top-level api. Later on a way to set this through
os-release is added, but I think the approach with the variable is still
useful. It it very convenient for testing, or to override settings only in a
particular service, etc.

src/basic/hostname-util.c

@@ -3,14 +3,27 @@
 #include <errno.h>
 #include <limits.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include <sys/utsname.h>
 #include <unistd.h>
 
 #include "alloc-util.h"
 #include "hostname-util.h"
+#include "os-util.h"
 #include "string-util.h"
 #include "strv.h"
 
+char* get_default_hostname(void) {
+        const char *e = secure_getenv("SYSTEMD_DEFAULT_HOSTNAME");
+        if (e) {
+                if (hostname_is_valid(e, 0))
+                        return strdup(e);
+                log_debug("Invalid hostname in $SYSTEMD_DEFAULT_HOSTNAME, ignoring: %s", e);
+        }
+
+        return strdup(FALLBACK_HOSTNAME);
+}
+
 char* gethostname_malloc(void) {
         struct utsname u;
         const char *s;
@@ -23,7 +36,7 @@ char* gethostname_malloc(void) {
 
         s = u.nodename;
         if (isempty(s) || streq(s, "(none)"))
-                s = FALLBACK_HOSTNAME;
+                return get_default_hostname();
 
         return strdup(s);
 }
@@ -31,6 +44,7 @@ char* gethostname_malloc(void) {
 char* gethostname_short_malloc(void) {
         struct utsname u;
         const char *s;
+        _cleanup_free_ char *f = NULL;
 
         /* Like above, but kills the FQDN part if present. */
 
@@ -38,7 +52,10 @@ char* gethostname_short_malloc(void) {
 
         s = u.nodename;
         if (isempty(s) || streq(s, "(none)") || s[0] == '.') {
-                s = FALLBACK_HOSTNAME;
+                s = f = get_default_hostname();
+                if (!s)
+                        return NULL;
+
                 assert(s[0] != '.');
         }
 

src/basic/hostname-util.h

@@ -7,6 +7,7 @@
 #include "macro.h"
 #include "strv.h"
 
+char* get_default_hostname(void);
 char* gethostname_malloc(void);
 char* gethostname_short_malloc(void);
 int gethostname_strict(char **ret);