Re: [PATCH/RFC v3 07/12] pack-objects: move in_pack out of struct object_entry

Nguyễn Thái Ngọc Duy  [off-list ref] writes:

Instead of using 8 bytes (on 64 bit arch) to store a pointer to a
pack. Use an index isntead since the number of packs should be
relatively small.

This limits the number of packs we can handle to 16k. For now if you hit
16k pack files limit, pack-objects will simply fail [1].

This technically saves 7 bytes. But we don't see any of that in
practice due to padding. The saving becomes real when we pack this
struct tighter later.

Somehow 7 and 16k do not add up.

We use 8 bytes in the original code, and a solution that potentially
saves 7 bytes would use only 1 byte instead of the original 8, which
would allow us to index/identify 1<<8 == 256 packs, but for some reason
we can handle up to 16k.

[1] The escape hatch is .keep file to limit the non-kept pack files
    below 16k limit. Then you can go for another pack-objects run to
    combine another 16k pack files. Repeat until you're satisfied.

;-)

+static inline unsigned int oe_add_pack(struct packing_data *pack,
+				       struct packed_git *p)
+{
+	if (pack->in_pack_count >= (1 << OE_IN_PACK_BITS))
+		die(_("too many packs to handle in one go. "
+		      "Please add .keep files to exclude\n"
+		      "some pack files and keep the number "
+		      "of non-kept files below %d."),
+		    1 << OE_IN_PACK_BITS);

OK.