Fix partition initialization bug.

After creating a partition, the toolkit must wait for the partition
device to be created (under /dev). However, the naming scheme of
partitions is inconsistent, with both `/dev/<name>XX` and
`/dev/<name>pXX` being used, depending on the device driver. So, the
toolkit checks for both.

However, a problem occurs when the device name itself ends in a digit.
If the disk device path is say `/dev/loop1`, then `/dev/loop11` is also
a valid disk device path. For such disks, the `/dev/<name>pXX` form
must be used for partitions.

While the toolkit prioritizes `/dev/loop1p1` over `/dev/loop11`, if the
`/dev/loop1p1` device doesn't exist yet and `/dev/loop11` is in use,
then the toolkit may pick the wrong device path for the partition.

This change fixes this by ignoring the `/dev/<name>XX` variant if the
disk device path ends in a digit.

toolkit/tools/imagegen/diskutils/diskutils.go

@@ -682,13 +682,19 @@ func InitializeSinglePartition(diskDevPath string, partitionNumber int,
 	partitionNumberStr := strconv.Itoa(partitionNumber)
 
 	// There are two primary partition naming conventions:
-	// /dev/sdN<y> style or /dev/loopNp<x> style
+	// - /dev/sdN<y>
+	// - /dev/loopNp<x>
 	// Detect the exact one we are using.
-	// Make sure we check for /dev/loopNp<x> FIRST, since /dev/loop1 would generate /dev/loop11 as a partition
-	// device which may be a valid device. We want to select /dev/loop1p1 first.
 	testPartDevPaths := []string{
 		fmt.Sprintf("%sp%s", diskDevPath, partitionNumberStr),
-		fmt.Sprintf("%s%s", diskDevPath, partitionNumberStr),
+	}
+
+	// If disk path ends in a digit, then the 'p<x>' style must be used.
+	// So, don't check the other style to avoid ambiguities. For example, /dev/loop1 vs. /dev/loop11.
+	// The is particularly relevant on Ubuntu, due to snap's use of loopback devices.
+	if !isDigit(diskDevPath[len(diskDevPath)-1]) {
+		devPath := fmt.Sprintf("%s%s", diskDevPath, partitionNumberStr)
+		testPartDevPaths = append(testPartDevPaths, devPath)
 	}
 
 	err = retry.Run(func() error {
@@ -759,6 +765,10 @@ func InitializeSinglePartition(diskDevPath string, partitionNumber int,
 	return
 }
 
+func isDigit(c byte) bool {
+	return c >= '0' && c <= '9'
+}
+
 func setGptPartitionType(partition configuration.Partition, timeoutInSeconds, diskDevPath, partitionNumberStr string) (err error) {
 	if supports, _ := PartedSupportsTypeCommand(); !supports {
 		logger.Log.Warn("parted version <3.6 does not support the 'type' session command - skipping this operation")