base-files: board_detect: Make robust against power-cuts

If board_detect is interrupted by cutting power on first boot,
board.json might only be half-way written and the file will not be
written again correctly on subsequent boards.

Write to a temporary file first, then rename. Since a rename on the same
file system is an atomic operation, it ensures that either no
/etc/board.json or a complete /etc/board.json exists.

Signed-off-by: Andreas Gnau <andreas.gnau@iopsys.eu>

Author: Rondom

package/base-files/files/bin/board_detect

@@ -1,14 +1,22 @@
 #!/bin/sh
 
-CFG=$1
+REAL_CFG=$1
 
-[ -n "$CFG" ] || CFG=/etc/board.json
+[ -n "$REAL_CFG" ] || REAL_CFG=/etc/board.json
 
-if [ -d "/etc/board.d/" ] && [ ! -s "$CFG" ]; then
+if [ -d "/etc/board.d/" -a ! -s "$REAL_CFG" ]; then
+	# Use temp file to prevent incomplete file on power-cut
+	CFG="$(mktemp -p "$(dirname "$REAL_CFG")" "~$(basename "$REAL_CFG").XXXXXX")"
+	[ -n "$CFG" ] || return
 	for a in $(ls /etc/board.d/*); do
 		[ -s "$a" ] || continue;
 		(. "$a")
 	done
 fi
 
-[ -s "$CFG" ] || return 1
+if [ -s "$CFG" ]; then
+	mv "$CFG" "$REAL_CFG" || return
+else
+	rm -f "$CFG"
+	return 1
+fi