lualoader: allow the local module to filter out the BE list

This allows something like the following local.lua to install a filter
to implement its own notion of hidden BEs using a naming convention of
a leading dot to hide them:

-- file: /boot/lua/local.lua
local core = require("core")

local function be_hide(be)
    if core.isSingleUserBoot() then
        -- All BEs are accepted for single-user
        return true
    end

    local name = be:match("/([^/]+)$")
    if not name then
        -- Accept malformed BEs, for whatever reason
        return true
    end

    return name:match("^%.") == nil
end

if core.bootenvFilter then
    -- Just in case we need to be compatible with older versions of
    -- core.lua without the filtering functionality.
    core.bootenvFilter(be_hide)
end
-- EOF

Requested by:	Marek Zarychta
Reviewed by:	imp

(cherry picked from commit bf0881060ecd75ce79683b82ebcd4809eadf7cf5)

1 files changed, 12 insertions, 1 deletions

diff --git a/stand/lua/core.lua.8 b/stand/lua/core.lua.8
index de43d3e2b220..325320b2fce8 100644
--- a/stand/lua/core.lua.8
+++ b/stand/lua/core.lua.8
@@ -24,7 +24,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd March 29, 2025
+.Dd April 8, 2026
 .Dt CORE.LUA 8
 .Os
 .Sh NAME
@@ -164,6 +164,17 @@ is set in
 kernels will be autodetected from the current system.
 .It Fn core.bootenvDefault
 Returns the default boot environment, nil if unset.
+.It Fn core.bootenvFilter func
+Installs a filter
+.Fa func
+into
+.Fn core.bootenvList .
+If the
+.Fa func
+returns true, then the boot environment is retained in the list.
+Otherwise, the boot environment is hidden.
+The old filter, if any, is returned to allow the caller to compose a filter on
+top of another filter.
 .It Fn core.bootenvList
 Returns a table of boot environments, or an empty table.
 These will be picked up using the