Boot on read-only snapshot (#115)

* Create a initramfs folder & configuration files
  * create a initramfs folder
  * create Arch Linux folder
  * Add `HOOK/INSTALL` files to generate a custom initrafms for boot on read-only snapshot for Arch Linux.
  * Add readme file to initramfs folder

* Makefile changes
  * Add readme file
  * Add readme file of initramfs-overlayfs
  * Add HOOK/INSTALL files for Arch Linux

* Readme changes
  * Updates the documentation to be able to boot on a read-only snapshot.
  * Redesign readme again.

Makefile

@@ -12,6 +12,10 @@ install:
 	@install -Dm644 -t "$(LIB_DIR)/systemd/system/" grub-btrfs.service
 	@install -Dm644 -t "$(LIB_DIR)/systemd/system/" grub-btrfs.path
 	@install -Dm644 -t "$(SHARE_DIR)/licenses/$(PKGNAME)/" LICENSE
+	@install -Dm644 "initramfs/Arch Linux/overlay_snap_ro-install" "$(LIB_DIR)/initcpio/install/grub-btrfs-overlayfs"	# Arch Linux only
+	@install -Dm644 "initramfs/Arch Linux/overlay_snap_ro-install" "$(LIB_DIR)/initcpio/install/grub-btrfs-overlayfs"	# Arch Linux only
+	@install -Dm644 -t "$(SHARE_DIR)/doc/$(PKGNAME)/" README.md
+	@install -Dm644 "initramfs/readme.md" "$(SHARE_DIR)/doc/$(PKGNAME)/initramfs-overlayfs.md"
 
 uninstall:
 	rm -f "$(DESTDIR)/etc/grub.d/41_snapshots-btrfs"
@@ -20,4 +24,8 @@ uninstall:
 	rm -f "$(LIB_DIR)/systemd/system/grub-btrfs.path"
 	rm -f "$(SHARE_DIR)/licenses/$(PKGNAME)/LICENSE"
 	rm -f "$(DESTDIR)/boot/grub/grub-btrfs.cfg"
+	rm -f "$(LIB_DIR)/initcpio/install/grub-btrfs-overlayfs" # Arch Linux only
+	rm -f "$(LIB_DIR)/initcpio/hooks/grub-btrfs-overlayfs" # Arch Linux only
+	rm -rf "$(SHARE_DIR)/doc/$(PKGNAME)/" README.md
+	rm -rf "$(SHARE_DIR)/doc/$(PKGNAME)/initramfs-overlayfs.md"
 	rmdir --ignore-fail-on-non-empty "$(DESTDIR)/etc/default/grub-btrfs"

README.md

@@ -10,17 +10,17 @@ This is a version 4.xx of grub-btrfs
 Improves Grub by adding "btrfs snapshots" to the Grub menu.
 
 You can start your system on a "snapshot" from the Grub menu.  
-
 Supports manual snapshots, snapper, timeshift ...
 
 ##### Warning: booting on read-only snapshots can be tricky
 
-If you choose to do it, `/var/log` must be on a separate subvolume.
-
+If you choose to do it, `/var/log` or even `/var` must be on a separate subvolume.  
 Otherwise, make sure your snapshots are writeable.  
-
 See [this ticket](https://github.com/Antynea/grub-btrfs/issues/92) for more info.
 
+This project includes its own solution.  
+Refer to the [documentation](https://github.com/Antynea/grub-btrfs/blob/boot-read-only-snapshot/initramfs/readme.md).
+
 ##
 ### What does grub-btrfs v4.xx do :
 * Automatically List snapshots existing on root partition (btrfs).
@@ -38,19 +38,17 @@ See [this ticket](https://github.com/Antynea/grub-btrfs/issues/92) for more info
 pacman -S grub-btrfs
 ```
 
-### Manual
+#### Manual
 
 * Run `make install` or look into Makefile for instructions on where to put each file.
 
 NOTE: Generate your Grub menu after installation for the changes to take effect.  
-
 On Arch Linux use `grub-mkconfig -o /boot/grub/grub.cfg`.
 
 ##
 ### Customization:
 
 You have the possibility to modify many parameters in `/etc/default/grub-btrfs/config`.  
-
 See [config file](https://github.com/Antynea/grub-btrfs/blob/master/config) for more information.
 
 ##
@@ -59,7 +57,6 @@ If you would like Grub to automatically update when a snapshot is made or delete
 * Use `systemctl start/enable grub-btrfs.path`.
 * `grub-btrfs.path` automatically (re)generates `grub.cfg` when a modification appears in `/.snapshots` folder (by default).
 * If your snapshots aren't mounted in `/.snapshots`, you must modify the watch folder using `systemctl edit grub-btrfs.path`.  
-
 	* For example: Timeshift mount its snapshots in `/run/timeshift/backup/timeshift-btrfs/snapshots` folder.
 
 		Use `systemctl edit grub-btrfs.path`.
@@ -74,11 +71,5 @@ If you would like Grub to automatically update when a snapshot is made or delete
 ##
 ### Special thanks for assistance and contributions
 * [maximbaz](https://github.com/maximbaz)
-* [crossroads1112](https://github.com/crossroads1112)
-* [penetal](https://github.com/penetal)
-* [wesbarnett](https://github.com/wesbarnett)
-* [Psykar](https://github.com/Psykar)
-* [anyc](https://github.com/anyc)
-* [daftaupe](https://github.com/daftaupe)
-* [N-Parsons](https://github.com/N-Parsons)
+* [All contributors](https://github.com/Antynea/grub-btrfs/graphs/contributors)
 ##

initramfs/Arch Linux/overlay_snap_ro-hook

@@ -0,0 +1,15 @@
+#!/usr/bin/ash
+
+run_latehook() {
+	local root_mnt="/new_root"
+	local current_dev=$(resolve_device "$root"); # resolve devices for blkid
+	if [[ $(blkid "${current_dev}" -s TYPE -o value) = "btrfs" ]] && [[ $(btrfs property get ${root_mnt} ro) != "ro=false" ]]; then # run only on a read only snapshot
+		local lower_dir=$(mktemp -d -p /)
+		local ram_dir=$(mktemp -d -p /)
+		mount --move ${root_mnt} ${lower_dir} # move new_root to lower_dir
+		mount -t tmpfs cowspace ${ram_dir}  #meuh!!! space, you can't test !
+		mkdir -p ${ram_dir}/upper
+		mkdir -p ${ram_dir}/work
+		mount -t overlay -o lowerdir=${lower_dir},upperdir=${ram_dir}/upper,workdir=${ram_dir}/work rootfs ${root_mnt}
+	fi
+}

initramfs/Arch Linux/overlay_snap_ro-install

@@ -0,0 +1,18 @@
+#!/bin/bash
+
+build() {
+    add_module btrfs
+    add_module overlay
+    add_binary btrfs
+    add_binary btrfsck
+    add_binary blkid
+    add_runscript
+}
+
+help() {
+    cat <<HELPEOF
+This hook uses overlayfs to boot on a read only snapshot.
+HELPEOF
+}
+
+# vim: set ft=sh ts=4 sw=4 et:

initramfs/readme.md

@@ -0,0 +1,45 @@
+### Description :
+
+Booting on a snapshot in read-only mode can be tricky.  
+An elegant way is to boot this snapshot using overlayfs (included in the kernel ≥ 3.18).
+
+Using overlayfs, the booted snapshot will behave like a live-cd in non-persistent mode.  
+The snapshot will not be modified, the system will be able to boot correctly, because a writeable folder will be included in the ram.  
+(no more problems due to `/var` not open for writing)
+
+Any changes in this system thus started will be lost when the system is rebooted/shutdown.
+
+To do this, it is necessary to modify the initramfs.  
+This means that any snapshot that does not include this modified initramfs will not be able to benefit from it.  
+(except for separate boot partitions)
+#
+### Installation :
+#### Arch Linux
+1.
+`Pacman -S grub-btrfs`  
+Or if you use git  
+copy the `overlay_snap_ro-install` file to `/etc/initcpio/install/grub-btrfs-overlayfs`  
+copy the `overlay_snap_ro-hook` file to `/etc/initcpio/hooks/grub-btrfs-overlayfs`  
+You must rename the files. (I did it above)
+
+For example :  
+`overlay_snap_ro-install` to `grub-btrfs-overlayfs`  
+`overlay_snap_ro-hook` to `grub-btrfs-overlayfs`  
+Keep in mind that the files must have exactly the same name to ensure a match.
+
+2.
+Edit the file `/etc/mkinitcpio.conf`  
+Added hook `grub-btrfs-overlayfs` at the end of the line `HOOKS`.
+
+For example :  
+`HOOKS=(base udev autodetect modconf block filesystems keyboard fsck grub-btrfs-overlayfs)`  
+You notice that the name of the `hook` must match the name of the 2 installed files. (don't forget it)
+
+3.
+Re-generate your initramfs  
+`mkinitcpio -P` (option -P means, all preset present in `/etc/mkinitcpio.d`)
+
+#### Other distribution
+Refer to your distribution's documentation  
+or contribute to this project to add a paragraph.
+#