[Bug-xorriso] PMBR sizing with appended_part_as=gpt

[Paul Robins]

Thomas et al,
 I am attempting to duplicate the work done by grub-mkrescue, but 
using xorriso directly and without its emulation mode. The purpose of 
this is to attach a self-contained grub install to an appended 
partition. I have encountered issues when adding appended_part_as=gpt 
and believe I have isolated the cause of the problem. I also have a 
couple of questions relating to an additional feature. I've managed to 
get very close to my goal very quickly though thanks to your hard work.

 The command I am using at the moment (tested on 1.5.0, 1.5.1 / HEAD):

xorriso -report_about ALL \
        -compliance no_emul_toc \
        -padding included \
        -hfsplus on \
        -outdev stdio:new.iso \
        -map $PREFIX/ / \
        -boot_image grub grub2_mbr=/usr/lib/grub/i386-pc/boot_hybrid.img \
        -boot_image grub bin_path=/boot/grub/i386-pc/eltorito.img \
        -boot_image grub grub2_boot_info=on \
        -boot_image any next \
        -boot_image grub apm_block_size=2048 \
        -boot_image grub efi_path=/efi.img \
        -boot_image grub efi_boot_part=--efi-boot-image \
        -boot_image any appended_part_as=gpt \
        -boot_image any appended_part_as=apm \
        -append_partition 2 Linux tmpfs/partition

(I'm aware there are missing bless commands for apm, but for now I'm 
just focusing on i386-pc, x86_64-efi and El Torito booting)

 This seems to work correctly excepting for the addition of 
appended_part_as=gpt. Whereas before the PMBR contained the appended 
partition and thereby covered the entirety of the space the GPT did, 
now the PMBR doesn't contain the appended partition, and the protective 
partition entry is not extended to the end of the ISO.

 Expected result:
        MBR partition table:   N Status  Type        Start       Blocks
        MBR partition      :   1   0x00  0xee            1     10515971

 Actual result:
        MBR partition table:   N Status  Type        Start       Blocks
        MBR partition      :   1   0x00  0xee            1      2127363


 There is code to do this in libisofs in a couple of places:
	1. For isohybrid: 
https://dev.lovelyhq.com/libburnia/libisofs/blob/master/libisofs/system_area.c#L2004-2009
	2. When no requested MBR partitions: 
https://dev.lovelyhq.com/libburnia/libisofs/blob/master/libisofs/system_area.c#L2054-2064

 In this instance, the first will not fire as the bootloader is only 
grub. The second instance appears as if it should fire, but due to a 
code path called earlier, mbr_req is not actually empty, but already 
includes a prototype PMBR that is set to the length of the ISO data, 
not including the appended partition.

 This mbr_req is created through the 
partprepend_writer_compute_data_blocks call with the following stack 
trace (running a bit of a hacked up version so apologies for the line 
numbers):
	#0  iso_quick_mbr_entry (req_array=0x5555555cb6c8, 
mbr_req_count=0x5555555cb6e8, start_block=1, block_count=0, 
type_byte=238 '\356', status_byte=0 '\000', desired_slot=0) at 
libisofs/system_area.c:1094
	#1  0x00007ffff7e3f2bd in partprepend_writer_compute_data_blocks 
(writer=0x555555584d50) at libisofs/system_area.c:3065
	#2  0x00007ffff7e1e63d in ecma119_image_new (src=0x55555556c2c0, 
in_opts=0x5555555c9b60, img=0x7fffffffddc8) at libisofs/ecma119.c:3039
	#3  0x00007ffff7e1f39d in iso_image_create_burn_source 
(image=0x55555556c2c0, opts=0x5555555c9b60, burn_src=0x7fffffffde40) at 
libisofs/ecma119.c:3449
	#4  0x00007ffff7e97a88 in isoburn_prepare_disc_aux 
(in_d=0x7ffff7d45660, out_d=0x7ffff7d45660, disc=0x7fffffffdf48, 
opts=0x5555555e4c10, flag=0) at libisoburn/isoburn.c:652
	#5  0x00007ffff7e97db2 in isoburn_prepare_disc (d=0x7ffff7d45660, 
disc=0x7fffffffdf48, opts=0x5555555e4c10) at libisoburn/isoburn.c:716
	#6  0x00007ffff7f3f671 in Xorriso_write_session 
(xorriso=0x7ffff7f9a010, flag=0) at xorriso/write_run.c:1359
	#7  0x00007ffff7ee977c in Xorriso_option_commit 
(xorriso=0x7ffff7f9a010, flag=1) at xorriso/opts_a_c.c:2151
	#8  0x00007ffff7eed531 in Xorriso_option_end (xorriso=0x7ffff7f9a010, 
flag=2) at xorriso/opts_d_h.c:427
	#9  0x000055555555562a in main (argc=41, argv=0x55555555a630) at 
xorriso/xorriso_main.c:280

I have hoisted the conditional I highlighted in #2 above out of the 
enclosing if statement and confirmed that it does produce an ISO with 
the PMBR extended to the end of the appended partition which appears to 
be UEFI compliant. I don't have a thorough enough knowledge of libisofs 
to know whether that is an appropriate solution. I'd appreciate advice 
on whether the mbr_req behaviour is as intended, or whether the data 
block calculation side-effect is causing this failure.

In addition, I'd like to ask about a couple of features I'm interested 
in adding. Specifically, I need the ability to align the appended 
partition to a particular start position in order to ensure there is 
some free space for the contents of the ISO, EFI etc... partitions to 
grow. The intention behind this is to write the ISO to a USB stick and 
at a later date (after a grub update for example) overwrite only the 
portions of the USB at issue. Ultimately I need to have a unified image 
which can be partially overwritten with updated versions.

As an add-on nicety to the above, it would be useful to be able to use 
more than 4 GPT partitions, and set the type ID so I can implement the 
Discoverable Partitions Specification ( 
https://www.freedesktop.org/wiki/Specifications/DiscoverablePartitionsSpec/ 
)

Hopefully you'd be amenable to these changes, and if you have 
preferences on how they should be implemented, please let me know. I 
hope the bug report above is correctly diagnosed, and if I can provide 
any more info please let me know.

Thanks again!