fstab-generator: add support for volatile boots

This adds support for a new kernel command line option "systemd.volatile=" that
provides the same functionality that systemd-nspawn's --volatile= switch
provides, but for host systems (i.e. systems booting with a kernel).

It takes the same parameter and has the same effect.

In order to implement systemd.volatile=yes a new service
systemd-volatile-root.service is introduced that only runs in the initrd and
rearranges the root directory as needed to become a tmpfs instance. Note that
systemd.volatile=state is implemented different: it simply generates a
var.mount unit file that is part of the normal boot and has no effect on the
initrd execution.

The way this is implemented ensures that other explicit configuration for /var
can always override the effect of these options.  Specifically, the var.mount
unit is generated in the "late" generator directory, so that it only is in
effect if nothing else overrides it.

2 files changed, 29 insertions, 0 deletions

diff --git a/src/shared/volatile-util.c b/src/shared/volatile-util.c
index 1329b51f4e..e7e9721411 100644
--- a/src/shared/volatile-util.c
+++ b/src/shared/volatile-util.c
@@ -17,8 +17,10 @@
   along with systemd; If not, see <http://www.gnu.org/licenses/>.
 ***/
 
+#include "alloc-util.h"
 #include "macro.h"
 #include "parse-util.h"
+#include "proc-cmdline.h"
 #include "string-util.h"
 #include "volatile-util.h"
 
@@ -39,3 +41,28 @@ VolatileMode volatile_mode_from_string(const char *s) {
 
         return _VOLATILE_MODE_INVALID;
 }
+
+int query_volatile_mode(VolatileMode *ret) {
+        _cleanup_free_ char *mode = NULL;
+        VolatileMode m = VOLATILE_NO;
+        int r;
+
+        r = proc_cmdline_get_key("systemd.volatile", PROC_CMDLINE_VALUE_OPTIONAL, &mode);
+        if (r < 0)
+                return r;
+        if (r == 0)
+                goto finish;
+
+        if (mode) {
+                m = volatile_mode_from_string(mode);
+                if (m < 0)
+                        return -EINVAL;
+        } else
+                m = VOLATILE_YES;
+
+        r = 1;
+
+finish:
+        *ret = m;
+        return r;
+}
diff --git a/src/shared/volatile-util.h b/src/shared/volatile-util.h
index d012940c76..17930ba6ae 100644
--- a/src/shared/volatile-util.h
+++ b/src/shared/volatile-util.h
@@ -28,3 +28,5 @@ typedef enum VolatileMode {
 } VolatileMode;
 
 VolatileMode volatile_mode_from_string(const char *s);
+
+int query_volatile_mode(VolatileMode *ret);