lib/deploy: Use a FIFREEZE/FITHAW cycle for /boot

See: http://marc.info/?l=linux-fsdevel&m=149520244919284&w=2

XFS doesn't flush the journal on `syncfs()`. GRUB doesn't know how to follow the
XFS journal, so if the filesystem is in a dirty state (possible with xfs
`/boot`, extremely likely with `/`, if the journaled data includes content for
`/boot`, the system may be unbootable if a system crash occurs.

Fix this by doing a `FIFREEZE`+`FITHAW` cycle.  Now, most people
probably would have replaced the `syncfs()` invocation with those two
ioctls.  But this would have become (I believe) the *only* place in
libostree where we weren't safe against interruption.  The failure
mode would be ugly; nothing else would be able to write to the filesystem
until manual intervention.

The real fix here I think is to land an atomic `FIFREEZETHAW` ioctl
in the kernel.  I might try a patch.

In the meantime though, let's jump through some hoops and set up
a "watchdog" child process that acts as a fallback unfreezer.

Closes: https://github.com/ostreedev/ostree/issues/876

Closes: #1049
Approved by: jlebon

tests/admin-test.sh

@@ -249,3 +249,11 @@ ${CMD_PREFIX} ostree --sysroot=${deployment} remote add --set=gpg-verify=false r
 assert_not_file_has_content sysroot/ostree/repo/config remote-test-nonphysical
 assert_file_has_content ${deployment}/etc/ostree/remotes.d/remote-test-nonphysical.conf testos-repo
 echo "ok remote add nonphysical sysroot"
+
+if env OSTREE_SYSROOT_DEBUG="${OSTREE_SYSROOT_DEBUG},test-fifreeze" \
+       ${CMD_PREFIX} ostree admin deploy --os=testos testos:testos/buildmaster/x86_64-runtime 2>err.txt; then
+    fatal "fifreeze-test exited successfully?"
+fi
+assert_file_has_content err.txt "fifreeze watchdog was run"
+assert_file_has_content err.txt "During fsfreeze-thaw: aborting due to test-fifreeze"
+echo "ok fifreeze test"

tests/test-admin-deploy-grub2.sh

@@ -19,7 +19,7 @@
 
 set -euo pipefail
 
-echo "1..18"
+echo "1..19"
 
 . $(dirname $0)/libtest.sh
 

tests/test-admin-deploy-syslinux.sh

@@ -19,7 +19,7 @@
 
 set -euo pipefail
 
-echo "1..18"
+echo "1..19"
 
 . $(dirname $0)/libtest.sh
 

tests/test-admin-deploy-uboot.sh

@@ -20,7 +20,7 @@
 
 set -euo pipefail
 
-echo "1..19"
+echo "1..20"
 
 . $(dirname $0)/libtest.sh