dissect: add small "systemd-dissect" tool as wrapper around dissect-image.c

This adds a small tool that may be used to look into OS images, and mount them
to any place. This is mostly a friendlier version of test-dissect-image.c. I am
not sure this should really become a proper command of systemd, hence for now
do not install it into bindir, but simply libexecdir.

This tool is already pretty useful since you can mount image files with it,
honouring the various partitions correctly. I figure this is going to become
more interesting if the dissctor learns luks and verity support.

2 files changed, 14 insertions, 3 deletions

diff --git a/src/shared/loop-util.c b/src/shared/loop-util.c
index 8be4dbf938..047e213634 100644
--- a/src/shared/loop-util.c
+++ b/src/shared/loop-util.c
@@ -129,7 +129,7 @@ LoopDevice* loop_device_unref(LoopDevice *d) {
 
         if (d->fd >= 0) {
 
-                if (d->nr >= 0) {
+                if (d->nr >= 0 && !d->relinquished) {
                         if (ioctl(d->fd, LOOP_CLR_FD) < 0)
                                 log_debug_errno(errno, "Failed to clear loop device: %m");
 
@@ -138,7 +138,7 @@ LoopDevice* loop_device_unref(LoopDevice *d) {
                 safe_close(d->fd);
         }
 
-        if (d->nr >= 0) {
+        if (d->nr >= 0 && !d->relinquished) {
                 _cleanup_close_ int control = -1;
 
                 control = open("/dev/loop-control", O_RDWR|O_CLOEXEC|O_NOCTTY|O_NONBLOCK);
@@ -155,3 +155,12 @@ LoopDevice* loop_device_unref(LoopDevice *d) {
 
         return NULL;
 }
+
+void loop_device_relinquish(LoopDevice *d) {
+        assert(d);
+
+        /* Don't attempt to clean up the loop device anymore from this point on. Leave the clean-ing up to the kernel
+         * itself, using the loop device "auto-clear" logic we already turned on when creating the device. */
+
+        d->relinquished = true;
+}
diff --git a/src/shared/loop-util.h b/src/shared/loop-util.h
index 5c847c4ac3..45fead5f18 100644
--- a/src/shared/loop-util.h
+++ b/src/shared/loop-util.h
@@ -29,11 +29,13 @@ struct LoopDevice {
         int fd;
         int nr;
         char *node;
+        bool relinquished;
 };
 
 int loop_device_make(int fd, int open_flags, LoopDevice **ret);
 int loop_device_make_by_path(const char *path, int open_flags, LoopDevice **ret);
 
 LoopDevice* loop_device_unref(LoopDevice *d);
-
 DEFINE_TRIVIAL_CLEANUP_FUNC(LoopDevice*, loop_device_unref);
+
+void loop_device_relinquish(LoopDevice *d);