sfdisk: fix previous --append patch, improve man page

- fix stupid typo (!has_container_or_unused(sf))
- use fdisk_is_partition_used() as fdisk_get_partition() returns
  nothing for unused partition
- update tests
- add more hints to the man page

Addresses: https://github.com/calamares/calamares/issues/1332
Signed-off-by: Karel Zak <kzak@redhat.com>

disk-utils/sfdisk.8

@@ -74,6 +74,10 @@ the event handling on the device.  For example:
 .sp
 .RE
 Note, this semantic is not currently supported by udevd for MD and DM devices.
+.PP
+The sfdisk prompt is only a hint for users and a displayed partition number does
+not mean that the same partition table entry will be created (if -N not
+specified), especially for tables with gaps.
 
 .SH COMMANDS
 The commands are mutually exclusive.
@@ -189,6 +193,10 @@ tools can automatically relocate the header to follow the standard.
 .TP
 .BR \-a , " \-\-append"
 Don't create a new partition table, but only append the specified partitions.
+.sp
+Note that unused partition maybe be re-used in this case although it is not the
+last partition in the partition table. See also \fB\-N\fR to specify entry in
+the partition table.
 .TP
 .BR \-b , " \-\-backup"
 Back up the current partition table sectors before starting the partitioning.

disk-utils/sfdisk.c

@@ -1530,10 +1530,13 @@ static int has_container_or_unused(struct sfdisk *sf)
 
 	nparts = fdisk_get_npartitions(sf->cxt);
 	for (i = 0; i < nparts; i++) {
+
+		if (!fdisk_is_partition_used(sf->cxt, i)) {
+			sf->unused = 1;
+			continue;
+		}
 		if (fdisk_get_partition(sf->cxt, i, &pa) != 0)
 			continue;
-		if (!fdisk_partition_is_used(pa))
-			sf->unused = 1;
 		if (fdisk_partition_is_container(pa))
 			sf->container = 1;
 	}
@@ -1840,7 +1843,7 @@ static int command_fdisk(struct sfdisk *sf, int argc, char **argv)
 		if (created
 		    && partno < 0
 		    && next_partno == fdisk_get_npartitions(sf->cxt)
-		    && has_container_or_unused(sf)) {
+		    && !has_container_or_unused(sf)) {
 			fdisk_info(sf->cxt, _("All partitions used."));
 			rc = SFDISK_DONE_ASK;
 			break;

tests/expected/sfdisk/dos-dump-gaps

@@ -12,7 +12,7 @@ I/O size (minimum/optimal): 4096 bytes / <removed> bytes
 <removed>2: Ignoring partition.
 <removed>3: Created a new <removed>.
 <removed>4: Ignoring partition.
-All partitions used.
+<removed>5: Done.
 
 New situation:
 Disklabel type: dos

tests/expected/sfdisk/dos-dump-old

@@ -12,7 +12,7 @@ I/O size (minimum/optimal): 4096 bytes / <removed> bytes
 <removed>2: Ignoring partition.
 <removed>3: Ignoring partition.
 <removed>4: Ignoring partition.
-All partitions used.
+<removed>5: Done.
 
 New situation:
 Disklabel type: dos