Fix ICO save producing an empty file for images smaller than 16x16

[binggao1230]

Saving an image smaller than the smallest default ICO size (16×16) as ICO, without an explicit sizes argument, silently produces an empty, unreadable file:

import io
from PIL import Image

buf = io.BytesIO()
Image.new("RGBA", (8, 8), (10, 20, 30, 255)).save(buf, "ICO")
print(len(buf.getvalue()))          # 6  -> just the ICONDIR header, idCount=0
buf.seek(0)
Image.open(buf).load()              # UnidentifiedImageError: cannot identify image file

The 8×8 image is representable as an ICO (save(buf, "ICO", sizes=[(8, 8)]) works fine), but the default-sizes path drops it entirely with no error or warning.

Cause

The default sizes start at (16, 16), and _save skips every candidate size larger than the source image. For an image smaller than 16×16 every default candidate is skipped, so frames stays empty and o16(len(frames)) writes idCount=0 — a 6-byte header-only file.

Fix

Filter the requested sizes up front, and if none fit, fall back to the image's own size (capped at the 256×256 ICO maximum) so a valid, readable icon is always written. This also fixes the related case where every explicitly requested size is larger than the image (previously also an empty file) — consistent with the documented behaviour that oversized requested sizes are ignored.

Tests

test_save_smaller_than_default_sizes parametrizes (1, 1), (8, 8), (15, 15) and the non-square (8, 12), asserting each round-trips to a valid ICO of the right size with info["sizes"] and a corner pixel preserved. test_save_all_sizes_larger_than_image pins the explicit-sizes fallback. All fail on main with UnidentifiedImageError and pass with the fix; the full Tests/test_file_ico.py suite stays green (27 passed); ruff format, ruff check and mypy are clean.

[codspeed]

Merging this PR will not alter performance

✅ 331 untouched benchmarks

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_{Comparing gaoflow:fix-ico-save-small-image-empty (5bb1b35) with main (6590b1b)}

[radarhere]

This also fixes the related case where every explicitly requested size is larger than the image

I'm not convinced on this point. If the user requests a specific sets of sizes, and we can't provide them, I think an error should be raised, rather than silently giving the user back something other than what they requested. I think the expectation in #2266 agrees with me.

I've created #9766 as an alternative.

[binggao1230]

You're right about the explicit case — if the caller names sizes we can't provide, raising beats silently handing back something else. #9766 is the better shape.

One edge case in it though: the default path raises too, when one dimension is under 16 and the other is over 256.

Image.new("RGBA", (300, 8)).save("out.ico")   # ValueError: All sizes too large for image

sizes becomes [im.size] = [(300, 8)], which the size[0] > min(256, im.width) filter then drops, leaving frames empty. Same for (8, 300) and (260, 10). Not a regression — those write the 6-byte empty file on main today — but a plain save() with no sizes argument probably shouldn't raise.

Happy to close this in favour of #9766.

[radarhere]

So you think 300x8 should save as 256x7? In that scenario, I wonder if it should also save as 128x3, 64x2 and 32x1.

[radarhere]

#9766 has been merged.