zfs: add an option to the bootloader to rewind the ZFS checkpoint

The checkpoints are another way of keeping the state of ZFS.
During the rewind, the pool has to be exported.
This makes checkpoints unusable when using ZFS as root.
Add the option to rewind the ZFS checkpoint at the boot time.
If checkpoint exists, a new option for rewinding a checkpoint will appear in
the bootloader menu.
We fully support boot environments.
If the rewind option is selected, the boot loader will show a list of
boot environments that existed before the checkpoint.

Reviewed by:	tsoome, allanjude, kevans (ok with high-level overview)
Differential Revision:	https://reviews.freebsd.org/D24920

stand/lua/core.lua

@@ -38,6 +38,8 @@ local default_safe_mode = false
 local default_single_user = false
 local default_verbose = false
 
+local bootenv_list = "bootenvs"
+
 local function composeLoaderCmd(cmd_name, argstr)
 	if argstr ~= nil then
 		cmd_name = cmd_name .. " " .. argstr
@@ -270,7 +272,7 @@ function core.bootenvDefault()
 end
 
 function core.bootenvList()
-	local bootenv_count = tonumber(loader.getenv("bootenvs_count"))
+	local bootenv_count = tonumber(loader.getenv(bootenv_list .. "_count"))
 	local bootenvs = {}
 	local curenv
 	local envcount = 0
@@ -281,7 +283,12 @@ function core.bootenvList()
 	end
 
 	-- Currently selected bootenv is always first/default
-	curenv = core.bootenvDefault()
+	-- On the rewinded list the bootenv may not exists
+	if core.isRewinded() then
+		curenv = core.bootenvDefaultRewinded()
+	else
+		curenv = core.bootenvDefault()
+	end
 	if curenv ~= nil then
 		envcount = envcount + 1
 		bootenvs[envcount] = curenv
@@ -289,7 +296,7 @@ function core.bootenvList()
 	end
 
 	for curenv_idx = 0, bootenv_count - 1 do
-		curenv = loader.getenv("bootenvs[" .. curenv_idx .. "]")
+		curenv = loader.getenv(bootenv_list .. "[" .. curenv_idx .. "]")
 		if curenv ~= nil and unique[curenv] == nil then
 			envcount = envcount + 1
 			bootenvs[envcount] = curenv
@@ -299,6 +306,40 @@ function core.bootenvList()
 	return bootenvs
 end
 
+function core.isCheckpointed()
+	return loader.getenv("zpool_checkpoint") ~= nil
+end
+
+function core.bootenvDefaultRewinded()
+	local defname =  "zfs:!" .. string.sub(core.bootenvDefault(), 5)
+	local bootenv_count = tonumber("bootenvs_check_count")
+
+	if bootenv_count == nil or bootenv_count <= 0 then
+		return defname
+	end
+
+	for curenv_idx = 0, bootenv_count - 1 do
+		curenv = loader.getenv("bootenvs_check[" .. curenv_idx .. "]")
+		if curenv == defname then
+			return defname
+		end
+	end
+
+	return loader.getenv("bootenvs_check[0]")
+end
+
+function core.isRewinded()
+	return bootenv_list == "bootenvs_check"
+end
+
+function core.changeRewindCheckpoint()
+	if core.isRewinded() then
+		bootenv_list = "bootenvs"
+	else
+		bootenv_list = "bootenvs_check"
+	end
+end
+
 function core.setDefaults()
 	core.setACPI(core.getACPIPresent(true))
 	core.setSafeMode(default_safe_mode)