libfdisk: (gpt) reduce number of entries to fit small device

The default is 128 partitions (entries in partitions array). The size
of the entry is 128 bytes, it means 32 sectors (512-byte) for the
array. The default is too large for tiny devices and it seems better to
dynamically reduce the number to fix the device size.

The reduction is reported to user.

Example:

 $ dd if=/dev/zero of=8-sectors.img count=8 bs=512
 $ fdisk 8-sectors.img
 ...
 Command (m for help): g
 Created a new GPT disklabel (GUID: 3DB3EECE-BCCA-6C46-95FA-7E23783BB3B2).
 The maximal number of partitions is 8 (default is 128).

 Command (m for help): x

 Expert command (m for help): p
 Disk 8-sectors.img: 4 KiB, 4096 bytes, 8 sectors
 Units: sectors of 1 * 512 = 512 bytes
 Sector size (logical/physical): 512 bytes / 512 bytes
 I/O size (minimum/optimal): 512 bytes / 512 bytes
 Disklabel type: gpt
 Disk identifier: 3DB3EECE-BCCA-6C46-95FA-7E23783BB3B2
 First LBA: 4
 Last LBA: 4
 Alternative LBA: 7
 Partition entries LBA: 2
 Allocated partition entries: 8

In this case, sectors:
 0   - PMBR
 1   - GPT header
 2-3 - GPT array of partitions (8 entries, 128 bytes each)
 4   - partition data
 5-6 - backup GPT array of partitions
 7   - backup GPT header

The smallest image is 6 sectors with 4 entries in array of partitions.

Note that Linux kernel has no problem to accept 4K image with 1
512-bytes GPT partition.

  # losetup -P -f 4K.img
  # lsblk /dev/loop0
  NAME      MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
  loop0       7:0    0    4K  0 loop
  └─loop0p1 259:5    0  512B  0 part

... but we need to fix libblkid, it ignores so small devices to probe for GPT :)

Addresses: https://github.com/karelzak/util-linux/issues/1147
Signed-off-by: Karel Zak <kzak@redhat.com>

libfdisk/src/gpt.c

@@ -742,10 +742,12 @@ static int get_script_u64(struct fdisk_context *cxt, uint64_t *num, const char *
 }
 
 static int count_first_last_lba(struct fdisk_context *cxt,
-				 uint64_t *first, uint64_t *last)
+				 uint64_t *first, uint64_t *last,
+				 uint32_t *maxents)
 {
 	int rc = 0;
 	uint64_t flba = 0, llba = 0;
+	uint32_t nents;
 
 	assert(cxt);
 	assert(first);
@@ -753,12 +755,24 @@ static int count_first_last_lba(struct fdisk_context *cxt,
 
 	*first = *last = 0;
 
-	/* UEFI default */
-	rc = gpt_calculate_last_lba(NULL, GPT_NPARTITIONS, &llba, cxt);
-	if (rc == 0)
-		gpt_calculate_first_lba(NULL, GPT_NPARTITIONS, &flba, cxt);
+	/* The default is GPT_NPARTITIONS, if the device is not large enough
+	 * than reduce this number of partitions and try to recalculate it
+	 * again, until we get something useful or return error.
+	 */
+	for (nents = GPT_NPARTITIONS; nents > 0; nents--) {
+		rc = gpt_calculate_last_lba(NULL, nents, &llba, cxt);
+		if (rc == 0)
+			rc = gpt_calculate_first_lba(NULL, nents, &flba, cxt);
+		if (llba < flba)
+			rc = -ENOSPC;
+		else if (rc == 0)
+			break;
+	}
+
 	if (rc)
 		return rc;
+	if (maxents)
+		*maxents = nents;
 
 	/* script default */
 	if (cxt->script) {
@@ -809,6 +823,7 @@ static int gpt_mknew_header(struct fdisk_context *cxt,
 			    struct gpt_header *header, uint64_t lba)
 {
 	uint64_t first, last;
+	uint32_t nents = 0;
 	int has_id = 0, rc;
 
 	if (!cxt || !header)
@@ -825,17 +840,15 @@ static int gpt_mknew_header(struct fdisk_context *cxt,
 	header->size = cpu_to_le32(sizeof(struct gpt_header)
 				- sizeof(header->reserved2));
 
-	/*
-	 * 128 partitions are the default. It can go beyond that, but
-	 * we're creating a de facto header here, so no funny business.
-	 */
-	header->npartition_entries     = cpu_to_le32(GPT_NPARTITIONS);
-	header->sizeof_partition_entry = cpu_to_le32(sizeof(struct gpt_entry));
-
-	rc = count_first_last_lba(cxt, &first, &last);
+	/* Set {First,Last}LBA and number of the partitions
+	 * (default is GPT_NPARTITIONS) */
+	rc = count_first_last_lba(cxt, &first, &last, &nents);
 	if (rc)
 		return rc;
 
+	header->npartition_entries     = cpu_to_le32(nents);
+	header->sizeof_partition_entry = cpu_to_le32(sizeof(struct gpt_entry));
+
 	header->first_usable_lba = cpu_to_le64(first);
 	header->last_usable_lba  = cpu_to_le64(last);
 
@@ -2637,6 +2650,10 @@ static int gpt_create_disklabel(struct fdisk_context *cxt)
 	guid_to_string(&guid, str);
 	fdisk_label_set_changed(cxt->label, 1);
 	fdisk_info(cxt, _("Created a new GPT disklabel (GUID: %s)."), str);
+
+	if (gpt_get_nentries(gpt) < GPT_NPARTITIONS)
+		fdisk_info(cxt, _("The maximal number of partitions is %d (default is %d)."),
+				gpt_get_nentries(gpt), GPT_NPARTITIONS);
 done:
 	return rc;
 }
@@ -3089,7 +3106,7 @@ static int gpt_reset_alignment(struct fdisk_context *cxt)
 		/* estimate ranges for GPT */
 		uint64_t first, last;
 
-		count_first_last_lba(cxt, &first, &last);
+		count_first_last_lba(cxt, &first, &last, NULL);
 		if (cxt->first_lba < first)
 			cxt->first_lba = first;
 		if (cxt->last_lba > last)