gptprio_test: check GPT is repaired when appropriate

tests/gptprio_test.in

@@ -66,8 +66,9 @@ prio_uuid[3]="1aa5a658-5b02-414d-9b71-f7e6c151f0cd"
 prio_uuid[4]="8aa0240d-98af-42b0-b32a-ccbe0572d62b"
 
 create_disk_image () {
+    size=$1
     rm -f "${img1}"
-    dd if=/dev/zero of="${img1}" bs=512 count=1 seek=100 status=none
+    dd if=/dev/zero of="${img1}" bs=512 count=1 seek=$((size - 1)) status=none
     ${sgdisk} \
         -n 1:0:+1 -c 1:ESP -t 1:ef00 \
         -n 2:0:+1 -c 2:A -t 2:"${prio_type}" -u 2:"${prio_uuid[2]}" \
@@ -76,6 +77,35 @@ create_disk_image () {
         "${img1}" >/dev/null
 }
 
+wipe_disk_area () {
+    sector=$1
+    size=$2
+    dd if=/dev/zero of="${img1}" bs=512 count=${size} seek=${sector} conv=notrunc status=none
+}
+
+is_zero () {
+    sector=$1
+    size=$2
+    cmp -s -i $((sector * 512)) -n $((size * 512)) /dev/zero "${img1}"
+}
+
+check_is_zero () {
+    sector=$1
+    size=$2
+    if ! is_zero "$@"; then
+        echo "$size sector(s) starting at $sector should be all zero"
+        exit 1
+    fi
+}
+
+check_not_zero () {
+    sector=$1
+    size=$2
+    if is_zero "$@"; then
+        echo "$size sector(s) starting at $sector should not be all zero"
+        exit 1
+    fi
+}
 
 fmt_prio () {
     priority=$(( ( $1 & 15 ) << 48 ))
@@ -93,10 +123,10 @@ set_prio () {
 check_prio () {
     part="$1"
     expect=$(fmt_prio $2 $3 $4)
-    result=$(LANG=C ${sgdisk} -i "${part}" "${img1}" \
+    result=$(LANG=C ${sgdisk} -i "${part}" "${img1}" 2>&1 \
             | awk '/^Attribute flags: / {print $3}')
     if [[ "${expect}" != "${result}" ]]; then
-        echo "Partition ${part} has attributes ${result}, not ${expect}" >&2
+        echo "Partition ${part} has attributes ${result:-??}, not ${expect}"
         exit 1
     fi
 }
@@ -133,6 +163,33 @@ create_disk_image 100
 set_prio 2 3 2 1
 check_prio 2 3 2 1
 
+# Check gptprio works without modifying the disk when no update is required.
+# Leaves any existing corruption as is, repairing in the OS is better.
+create_disk_image 100
+set_prio 2 1 0 1
+wipe_disk_area 99 1
+check_next 2 1 0 1
+check_is_zero 99 1
+
+create_disk_image 100
+set_prio 2 1 0 1
+wipe_disk_area 1 1
+check_next 2 1 0 1
+check_is_zero 1 1
+
+# When writes do need to be made go ahead and perform the repair.
+create_disk_image 100
+set_prio 2 1 1 0
+wipe_disk_area 99 1
+check_next 2 1 0 0
+check_not_zero 99 1
+
+create_disk_image 100
+set_prio 2 1 1 0
+wipe_disk_area 1 1
+check_next 2 1 0 0
+check_not_zero 1 1
+
 # Try two partitions before falling before falling back to a third
 create_disk_image 100
 set_prio 2 3 3 0