nspawn: support ephemeral boots from images

Previously --ephemeral was only supported with container trees in btrfs
subvolumes (i.e. in combination with --directory=). This adds support for
--ephemeral in conjunction with disk images (i.e. --image=) too.

As side effect this fixes that --ephemeral was accepted but ignored when using
-M on a container that turned out to be an image.

Fixes: #4664

3 files changed, 63 insertions, 33 deletions

diff --git a/man/systemd-nspawn.xml b/man/systemd-nspawn.xml
index f153034296..c29542236d 100644
--- a/man/systemd-nspawn.xml
+++ b/man/systemd-nspawn.xml
@@ -211,13 +211,8 @@
         <term><option>-x</option></term>
         <term><option>--ephemeral</option></term>
 
-        <listitem><para>If specified, the container is run with a
-        temporary <literal>btrfs</literal> snapshot of its root
-        directory (as configured with <option>--directory=</option>),
-        that is removed immediately when the container terminates.
-        This option is only supported if the root file system is
-        <literal>btrfs</literal>. May not be specified together with
-        <option>--image=</option> or
+        <listitem><para>If specified, the container is run with a temporary snapshot of its file system that is removed
+        immediately when the container terminates. May not be specified together with
         <option>--template=</option>.</para>
         <para>Note that this switch leaves host name, machine ID and
         all other settings that could identify the instance
@@ -252,11 +247,12 @@
           Partitions Specification</ulink>.</para></listitem>
         </itemizedlist>
 
-        <para>Any other partitions, such as foreign partitions, swap
-        partitions or EFI system partitions are not mounted. May not
-        be specified together with <option>--directory=</option>,
-        <option>--template=</option> or
-        <option>--ephemeral</option>.</para></listitem>
+        <para>On GPT images, if an EFI System Partition (ESP) is discovered, it is automatically mounted to
+        <filename>/efi</filename> (or <filename>/boot</filename> as fallback) in case a directory by this name exists
+        and is empty.</para>
+
+        <para>Any other partitions, such as foreign partitions or swap partitions are not mounted. May not be specified
+        together with <option>--directory=</option>, <option>--template=</option>.</para></listitem>
       </varlistentry>
 
       <varlistentry>
diff --git a/src/basic/missing.h b/src/basic/missing.h
index a5ae5d9e79..8833617dc6 100644
--- a/src/basic/missing.h
+++ b/src/basic/missing.h
@@ -143,6 +143,10 @@
 #define GRND_RANDOM 0x0002
 #endif
 
+#ifndef FS_NOCOW_FL
+#define FS_NOCOW_FL 0x00800000
+#endif
+
 #ifndef BTRFS_IOCTL_MAGIC
 #define BTRFS_IOCTL_MAGIC 0x94
 #endif
diff --git a/src/nspawn/nspawn.c b/src/nspawn/nspawn.c
index a6adbbe879..0ca0b2f4c8 100644
--- a/src/nspawn/nspawn.c
+++ b/src/nspawn/nspawn.c
@@ -1143,11 +1143,6 @@ static int parse_argv(int argc, char *argv[]) {
                 return -EINVAL;
         }
 
-        if (arg_ephemeral && arg_image) {
-                log_error("--ephemeral and --image= may not be combined.");
-                return -EINVAL;
-        }
-
         if (arg_ephemeral && !IN_SET(arg_link_journal, LINK_NO, LINK_AUTO)) {
                 log_error("--ephemeral and --link-journal= may not be combined.");
                 return -EINVAL;
@@ -2605,7 +2600,7 @@ static int determine_names(void) {
                         r = image_find(arg_machine, &i);
                         if (r < 0)
                                 return log_error_errno(r, "Failed to find image for machine '%s': %m", arg_machine);
-                        else if (r == 0) {
+                        if (r == 0) {
                                 log_error("No image for machine '%s': %m", arg_machine);
                                 return -ENOENT;
                         }
@@ -2615,14 +2610,14 @@ static int determine_names(void) {
                         else
                                 r = free_and_strdup(&arg_directory, i->path);
                         if (r < 0)
-                                return log_error_errno(r, "Invalid image directory: %m");
+                                return log_oom();
 
                         if (!arg_ephemeral)
                                 arg_read_only = arg_read_only || i->read_only;
                 } else
                         arg_directory = get_current_dir_name();
 
-                if (!arg_directory && !arg_machine) {
+                if (!arg_directory && !arg_image) {
                         log_error("Failed to determine path, please use -D or -i.");
                         return -EINVAL;
                 }
@@ -2633,7 +2628,6 @@ static int determine_names(void) {
                         arg_machine = gethostname_malloc();
                 else
                         arg_machine = strdup(basename(arg_image ?: arg_directory));
-
                 if (!arg_machine)
                         return log_oom();
 
@@ -4077,7 +4071,7 @@ int main(int argc, char *argv[]) {
         _cleanup_fdset_free_ FDSet *fds = NULL;
         int r, n_fd_passed, loop_nr = -1, ret = EXIT_SUCCESS;
         char veth_name[IFNAMSIZ] = "";
-        bool secondary = false, remove_subvol = false;
+        bool secondary = false, remove_subvol = false, remove_image = false;
         pid_t pid = 0;
         union in_addr_union exposed = {};
         _cleanup_release_lock_file_ LockFile tree_global_lock = LOCK_FILE_INIT, tree_local_lock = LOCK_FILE_INIT;
@@ -4148,7 +4142,7 @@ int main(int argc, char *argv[]) {
                         else
                                 r = tempfn_random(arg_directory, "machine.", &np);
                         if (r < 0) {
-                                log_error_errno(r, "Failed to generate name for snapshot: %m");
+                                log_error_errno(r, "Failed to generate name for directory snapshot: %m");
                                 goto finish;
                         }
 
@@ -4219,19 +4213,46 @@ int main(int argc, char *argv[]) {
                 assert(arg_image);
                 assert(!arg_template);
 
-                r = image_path_lock(arg_image, (arg_read_only ? LOCK_SH : LOCK_EX) | LOCK_NB, &tree_global_lock, &tree_local_lock);
-                if (r == -EBUSY) {
-                        r = log_error_errno(r, "Disk image %s is currently busy.", arg_image);
-                        goto finish;
-                }
-                if (r < 0) {
-                        r = log_error_errno(r, "Failed to create image lock: %m");
-                        goto finish;
+                if (arg_ephemeral)  {
+                        _cleanup_free_ char *np = NULL;
+
+                        r = tempfn_random(arg_image, "machine.", &np);
+                        if (r < 0) {
+                                log_error_errno(r, "Failed to generate name for image snapshot: %m");
+                                goto finish;
+                        }
+
+                        r = image_path_lock(np, (arg_read_only ? LOCK_SH : LOCK_EX) | LOCK_NB, &tree_global_lock, &tree_local_lock);
+                        if (r < 0) {
+                                r = log_error_errno(r, "Failed to create image lock: %m");
+                                goto finish;
+                        }
+
+                        r = copy_file(arg_image, np, O_EXCL, arg_read_only ? 0400 : 0600, FS_NOCOW_FL);
+                        if (r < 0) {
+                                r = log_error_errno(r, "Failed to copy image file: %m");
+                                goto finish;
+                        }
+
+                        free(arg_image);
+                        arg_image = np;
+                        np = NULL;
+
+                        remove_image = true;
+                } else {
+                        r = image_path_lock(arg_image, (arg_read_only ? LOCK_SH : LOCK_EX) | LOCK_NB, &tree_global_lock, &tree_local_lock);
+                        if (r == -EBUSY) {
+                                r = log_error_errno(r, "Disk image %s is currently busy.", arg_image);
+                                goto finish;
+                        }
+                        if (r < 0) {
+                                r = log_error_errno(r, "Failed to create image lock: %m");
+                                goto finish;
+                        }
                 }
 
                 if (!mkdtemp(template)) {
-                        log_error_errno(errno, "Failed to create temporary directory: %m");
-                        r = -errno;
+                        r = log_error_errno(errno, "Failed to create temporary directory: %m");
                         goto finish;
                 }
 
@@ -4255,6 +4276,10 @@ int main(int argc, char *argv[]) {
                                   &secondary);
                 if (r < 0)
                         goto finish;
+
+                /* Now that we mounted the image, let's try to remove it again, if it is ephemeral */
+                if (remove_image && unlink(arg_image) >= 0)
+                        remove_image = false;
         }
 
         r = custom_mounts_prepare();
@@ -4337,6 +4362,11 @@ finish:
                         log_warning_errno(k, "Cannot remove subvolume '%s', ignoring: %m", arg_directory);
         }
 
+        if (remove_image && arg_image) {
+                if (unlink(arg_image) < 0)
+                        log_warning_errno(errno, "Can't remove image file '%s', ignoring: %m", arg_image);
+        }
+
         if (arg_machine) {
                 const char *p;