groonga Relicensing to LGPL-2.1-or-later from LGPL-2.1-only

What is your problem?

Groonga is licensed under LGPL-2.1-only.

I want to relicense Groonga to LGPL-2.1-or-later from LGPL-2.1-only to improve Apache-2.0 compatibility.

Incompatibility between GPL-2.0 and Apache-2.0 is well known because both of FSF and ASF explain it:

https://www.gnu.org/licenses/license-list.html.en#apache2
https://www.apache.org/licenses/GPL-compatibility.html

Compatibility between LGPL-2.1 and Apache-2.0 isn't explained there but they may be incompatible too because LGPL-2.1 is based on GPL-2.0.

If LGPL-2.1 and Apache-2.0 are incompatible, LGPL-2.1-only licensed Groonga can't link with Apache-2.0 libraries such as Apache Arrow. LGPL-2.1-only licensed software can be also used as GPL-2.0-or-later licensed software because LGPL-2.1 has the following sentence:

You may opt to apply the terms of the ordinary GNU General Public License instead of this License to a given copy of the Library. To do this, you must alter all the notices that refer to this License, so that they refer to the ordinary GNU General Public License, version 2, instead of to this License. (If a newer version than version 2 of the ordinary GNU General Public License has appeared, then you can specify that version instead if you wish.) Do not make any other change in these notices.

So we can link with Apache-2.0 libraries by using Groonga as GPL-3.0-or-later licensed software. But this has a problem. If we use Groonga as GPL-3.0-or-later licensed software, we can't use Groonga with GPL-2.0-only licensed software such as MySQL.

If Groonga uses LGPL-2.1-or-later not LGPL-2.1-only, Groonga can link with Apache-2.0 libraries by using LGPL-3.0-or-later (LGPL-2.1-or-later includes it).

We need to ask all patch authors of Groonga to relicense Groonga to LGPL-2.1-or-later from LGPL-2.1-only.

Here are patch authors of Groonga:

$ git shortlog -sn
 11409	Kouhei Sutou
  3485	Sutou Kouhei
  1818	HAYASHI Kentaro
  1078	Daijiro MORI
   647	Kentaro Hayashi
   619	Susumu Yata
   449	Tasuku SUENAGA a.k.a. gunyarakun
   408	Horimoto Yasuhiro
   371	susumu.yata
   304	Yasuhiro Horimoto
   267	mori
   262	Ryo Onodera
   254	Masafumi Yokoyama
   162	naoa
   120	t2001
   110	Takashi Hashida
    94	Naoya Murakami
    93	cosmo0920
    86	Abe Tomoaki
    66	Nobuyoshi Nakada
    55	YUKI Hiroshi
    49	Haruka Yoshihara
    40	HorimotoYasuhiro
    39	Yoji SHIDARA
    36	Hiroshi Hatake
    35	Yuto Hayamizu
    34	whombx
    30	yoshihara haruka
    26	Toshihisa
    22	takuya kodama
    19	khayashi
    18	Yutaro Shimamura
    18	wanabe
    12	Hiroyuki Sato
    12	IWAI, Masaharu
    10	dependabot[bot]
     6	Kenji Okimoto
     6	jacob16bit
     5	orangain
     4	Hiroshi Ohkubo
     4	Motoi Washida
     3	Daiki Ueno
     3	OBATA Akio
     3	Tetsuro IKEDA
     3	do-aki
     3	mori daijiro
     3	tamano
     3	yagisumi
     2	Jun Kuriyama
     2	Noritada Kobayashi
     2	Tetsuharu OHZEKI
     2	YUKI "Piro" Hiroshi
     2	brazil
     2	kou
     2	tashiro
     2	unknown
     2	wi24rd
     1	Akinori MUSHA
     1	Anthony M. Cook
     1	Ayumu
     1	Dylan Golow
     1	Genki Takiuchi
     1	HashidaTKS
     1	Jasper Siepkes
     1	NOKUBI Takatsugu
     1	Natanael Copa
     1	SATOH Fumiyasu
     1	SHIDARA Yoji
     1	SHIMODA Hiroshi
     1	Sebastian Wiedenroth
     1	Shimomura Tatsuya
     1	Sho Minagawa
     1	Tasuku SUENAGA
     1	Tim Gates
     1	Yosuke Sekikawa
     1	Yosuke Shiro
     1	Yu.Shimamura
     1	Yuya TAMANO
     1	cafedomancer
     1	mooz
     1	ryo-pinus
     1	tSU-RooT
     1	tSU_RooT
     1	vagrant
     1	yu
     1	yutaro
     1	zunda

Here are distinct authors (except trivial patch authors):

[x] @kou
[x] @kenhys
[x] @daijiro
[x] @s-yata
[x] @gunyarakun
[x] @komainu8
[x] @ryoqun
[x] @myokoym
[x] @naoa
[x] @t2001
[x] @HashidaTKS
[x] @cosmo0920
[x] @abetomo
[x] @nobu
[x] @piroor
[x] @yoshihara
[x] @darashi
[x] @hayamiz
[x] @yu
[x] @otegami
[x] @wanabe
[x] @hiroyuki-sato
[x] @iwaim
[x] @orangain
[ ] @ohkubo
[x] @wm3
[x] @ueno
[x] @obache
[x] @ikdttr
[x] @do-aki
[x] @tamano
[ ] @yagisumi
[x] @kuriyama
[x] @noritada
[x] @tetsuharuohzeki
[x] @wi24rd
[x] @knu
[x] @genki
[x] @siepkes
[x] @knok
[ ] @ncopa
[x] @wiedi
[ ] @msh5
[ ] @OffGao
[ ] @shiro615
[x] @ryo-pinus

If you agree to this license change, could you add the following comment to this issue?

I agree to apply LGPL-2.1-or-later to my commits.

If you have any questions or object this license change, please let us know.

Trivial patch authors:

Kenji Okimoto: Typo fixes
- a0e3de50e45916daa3d2313a1a4e6128f4fc20fb
- 666f22af0a6d187fab67559f587f091fa7431e8f
- fcccbde106bb3b58cb8b4215b94e98aefdb6480f
- 1a81014950b422a207fba06211e41008facd9a7b
- adcda8b944193cbbd7e0558fa7cb4b207c0cdfb9
- db42c1007b43698721da91e668b5bf543bdfabd2
jacob16bit: groonga-httpd was extracted to https://github.com/groonga/groonga-nginx and it doesn't have RPM package
- https://github.com/groonga/groonga/pull/357
- https://github.com/groonga/groonga/pull/351
Ayumu: Remove unused code
- b518d63baab084dab9dac15c7cc5363f8a4fd337
Dylan Golow: Typo fix
- b827b5ef0810ca31f50136e33619109c0e094fcb
SATOH Fumiyasu: Remove redundant configurations
- ebdd2f6d1651a7b0ffa5b13a03dbe69142e76f6c
Tim Gates: Typo fix
- 26623b13f01dbcc1ea763cd4a20eb5671d6c52d5
cafedomancer: Typo fix
- 3b0daad671d30beee0486c1b077a2d6ffa878856
mooz: Typo fix
- 8f248d91235d597332f4fdf044dac615ea1e2088
tSU-RooT: Typo fixes
- b73eeb36a07b0d166f17612433bcbb3118766d37
- aaeff8e92bb2a1263b69b17ded5f5012998eb6f9
zunda: Typo fix
- 39b3788a2a638d0c571b57843d2114c919af0323

Patch authors who can't be contacted on GitHub:

[x] @t-shm
- #47
[ ] @whombx

PRs by @whombx (all of them are moving document location)

#100
#99
#98
#97
#96
#95
#93
#92
#91
#90
#89
#88
#87
#86
#85
#84
#83
#82
#81
#80
#79
#78
#75
#74
#73
#72
#71
#70
#69
#68
#67
#66
#65
#64