mcuboot multi-image, but only one image to update

[mastupristi]

On a Cortex M7 MCU I use mcuboot 2.2.0, configured to support 2 images (no dependency between them).

When only mcuboot is on flash, and the remaining part is blank, it write:

hello sbl.
Bootloader Version 2.2.0
Non-optimal sector distribution, slot0 has 254 usable sectors (256 assigned) but slot1 has 256 assigned
Primary image: magic=unset, swap_type=0x1, copy_done=0x3, image_ok=0x3
Secondary image: magic=unset, swap_type=0x1, copy_done=0x3, image_ok=0x3
Boot source: none
Image index: 0, Swap type: none
Scrambling trailer; fa_id=1
Image index: 0, Swap type: none
Non-optimal sector distribution, slot0 has 254 usable sectors (256 assigned) but slot1 has 256 assigned
Primary image: magic=unset, swap_type=0x1, copy_done=0x3, image_ok=0x3
Secondary image: magic=unset, swap_type=0x1, copy_done=0x3, image_ok=0x3
Boot source: none
Image index: 1, Swap type: none
Scrambling trailer; fa_id=3
Image index: 1, Swap type: none
Unable to find bootable image

Then I prepare an image using:

python3 /path/to/imgtool.py sign \
	--key /path/to/sign-rsa2048-priv.pem \
	--align 4 \
	--header-size 0x400 \
	--pad-header \
	--slot-size 0x100000 \
	--max-sectors 800 \
	--version "1.0" \
        fw_image.bin \
	fw_image_signed.bin

...and I write the signed image into slot_1-image_0. I would have expected an attempt to update image 0 (copy from slot 1 to slot 0), but instead this happens:

hello sbl.
Bootloader Version 2.2.0
Non-optimal sector distribution, slot0 has 254 usable sectors (256 assigned) but slot1 has 256 assigned
Primary image: magic=unset, swap_type=0x1, copy_done=0x3, image_ok=0x3
Secondary image: magic=good, swap_type=0x1, copy_done=0x3, image_ok=0x3
Boot source: none
Image index: 0, Swap type: test
Non-optimal sector distribution, slot0 has 254 usable sectors (256 assigned) but slot1 has 256 assigned
Primary image: magic=unset, swap_type=0x1, copy_done=0x3, image_ok=0x3
Secondary image: magic=unset, swap_type=0x1, copy_done=0x3, image_ok=0x3
Boot source: none
Image index: 1, Swap type: none
Scrambling trailer; fa_id=3
Image index: 1, Swap type: none
Unable to find bootable image

slot_1-image_0 has a valid magic and the detected swap type is test, so why shouldn't the copy start?

Then I tried to populate slot_1-image_1 as well (using another signed image). With both images to be updated, the updates (copies from slot_1 to slot_0) take place

hello sbl.
Bootloader Version 2.2.0
Non-optimal sector distribution, slot0 has 254 usable sectors (256 assigned) but slot1 has 256 assigned
Primary image: magic=unset, swap_type=0x1, copy_done=0x3, image_ok=0x3
Secondary image: magic=good, swap_type=0x1, copy_done=0x3, image_ok=0x3
Boot source: none
Image index: 0, Swap type: test
Non-optimal sector distribution, slot0 has 254 usable sectors (256 assigned) but slot1 has 256 assigned
Primary image: magic=unset, swap_type=0x1, copy_done=0x3, image_ok=0x3
Secondary image: magic=good, swap_type=0x1, copy_done=0x3, image_ok=0x3
Boot source: none
Image index: 1, Swap type: test
Image 0 upgrade secondary slot -> primary slot
Erasing the primary slot
Image 0 copying the secondary slot to the primary slot: 0x100000 bytes
erasing secondary header
erasing secondary trailer
writing copy_done; fa_id=0 off=0xfffe0 (0x13ffe0)
Image 1 upgrade secondary slot -> primary slot
Erasing the primary slot
Image 1 copying the secondary slot to the primary slot: 0x100000 bytes
erasing secondary header
erasing secondary trailer
writing copy_done; fa_id=2 off=0xfffe0 (0x33ffe0)
Bootloader chainload address offset: 0x40000
Reset_Handler address offset: 0x40400
Jumping to the image
[...]

So does mcuboot require that all the images it manages must be populated? Or am I doing something wrong? How can I make mcuboot accept empty image_1?

best regards

Max