ケチなのでAppleのiCloud（世界最悪の同期サービス）をObsidian Second Brainの同期に使っているのだが、最近この問題に頻繁に遭遇している：

$ git status
fatal: bad object HEAD
$ git log
fatal: bad object refs/heads/main

Stack Overflowで一番上に来る回答は10年前のもので、rm -rf .gitしろと言うもので、もちろん正しく直したいならやりたいことではない。

何度か直す羽目になり、つい最近ChatGPTと一緒に解決策を見つけた。後世のためにここに残しておく：

$ git fetch origin
# THIS USUALLY DOES IT FOR ME - then i can commit and merge and push normally

# hopefully this is good enough for git status to start working again
$ git pull

通常より盛大にぶっ壊れているとき

# confirm fuckedness

$ git fsck
bad sha1 file: .git/objects/00/084ad8182051c580009e8ccc22e224d97a4823 2
bad sha1 file: .git/objects/02/8027ce244a5390478159b510bd6767d648e31f 2
bad sha1 file: .git/objects/03/8c235cd6cc3e93596b25d7fb0ab14d9a999ec7 2
bad sha1 file: .git/objects/05/72e5490252d8fd25230669000747a7032a14df 2
bad sha1 file: .git/objects/07/ecafe1981e587022b562562ce02fd0adc45beb 2
bad sha1 file: .git/objects/09/e5b5f51f4f9683155f7a81d4fad1321e4085ee 2
bad sha1 file: .git/objects/0a/47019b78b8fd69a1b1a2d96b2be442fbceaf49 2
bad sha1 file: .git/objects/0c/b9400cec84abac87519225f7e5ceca987e7f66 2
bad sha1 file: .git/objects/10/2a21b02d2f08c777bfe0066de28afe0df3290c 2
bad sha1 file: .git/objects/14/e8edad8d4898bc04c4a00ce0df268cf359df63 2
bad sha1 file: .git/objects/14/65567afb6ee576ceb3e3061ffdf9c4ccd47095 2
bad sha1 file: .git/objects/1a/ac8cf84058d474b8845fe4ff410a2a1bc5a25a 2
bad sha1 file: .git/objects/1b/5cf76bb3a880fcf70083fda0aa4d0faccb5864 2

# etc etc etc

「2」が大量に表示されたら、iCloudにやられている証拠だ。以下で直せる

#  remove all files in .git with a space and then a number in their name, likely duplicates created by icloud
find .git -type f -name '* [0-9]*' -delete

そのあと再びgit fetch originすればいい。

もし

$ git log
fatal: bad object refs/heads/main

となっているなら、以下を実行する

git update-ref -d refs/heads/main

その他のメモ

# NOTE: THIS WILL DESTROY ANY UNCOMMITTED/UNSAVED CHANGES - take care before running it
$ git reset --hard origin/main

こんな感じでbad objectが表示されることがある：

remote: Enumerating objects: 3899, done.
remote: Counting objects: 100% (888/888), done.
remote: Compressing objects: 100% (364/364), done.
remote: Total 3899 (delta 554), reused 597 (delta 523), pack-reused 3011
Receiving objects: 100% (3899/3899), 1.32 MiB | 9.92 MiB/s, done.
error: corrupt loose object 'dc3711cb3ba71eefe40c8e49cc9c9fb311d29553'
fatal: loose object dc3711cb3ba71eefe40c8e49cc9c9fb311d29553 (stored in .git/objects/dc/3711cb3ba71eefe40c8e49cc9c9fb311d29553) is corrupt
fatal: fetch-pack: invalid index-pack output
fatal: Could not parse object 'origin/main'.

なので、修正方法はこれだ：

rm -f .git/objects/dc/3711cb3ba71eefe40c8e49cc9c9fb311d29553
git fetch origin

（7月31日追記）corrupt loose object

今日もこの問題が起きた：

$ git fsck

error: corrupt loose object '25726bffc0e97532098679d0ed04095710452d29'
error: unable to unpack contents of .git/objects/25/726bffc0e97532098679d0ed04095710452d29
error: 25726bffc0e97532098679d0ed04095710452d29: object corrupt or missing: .git/objects/25/726bffc0e97532098679d0ed04095710452d29
error: corrupt loose object '5b4e2ac94c33715545674545e501a705ea20924b'
error: unable to unpack contents of .git/objects/5b/4e2ac94c33715545674545e501a705ea20924b
error: 5b4e2ac94c33715545674545e501a705ea20924b: object corrupt or missing: .git/objects/5b/4e2ac94c33715545674545e501a705ea20924b
error: corrupt loose object 'ad7ceee5b187af409cbd2219bc4b578faf425471'
error: unable to unpack contents of .git/objects/ad/7ceee5b187af409cbd2219bc4b578faf425471
error: ad7ceee5b187af409cbd2219bc4b578faf425471: object corrupt or missing: .git/objects/ad/7ceee5b187af409cbd2219bc4b578faf425471
error: corrupt loose object 'fdacf1b391398e4548fa3a9f09b0f6714881907c'
error: unable to unpack contents of .git/objects/fd/acf1b391398e4548fa3a9f09b0f6714881907c
error: fdacf1b391398e4548fa3a9f09b0f6714881907c: object corrupt or missing: .git/objects/fd/acf1b391398e4548fa3a9f09b0f6714881907c
Checking object directories: 100% (256/256), done.
Checking objects: 100% (27396/27396), done.
missing blob 60e0633917e44a958f890039cff7297f99d8e8eb
dangling commit d885e9e214a1f84ce5feb8fdcdf51a27249e689e
dangling commit a38bc134bc0ec9741e7abbfa8ee200a0c82253d6
missing blob fdacf1b391398e4548fa3a9f09b0f6714881907c
missing blob 5b4e2ac94c33715545674545e501a705ea20924b
missing blob 25726bffc0e97532098679d0ed04095710452d29
missing blob f2f68aa9deda03f256574d468395e840144d9d05
missing blob ad7ceee5b187af409cbd2219bc4b578faf425471

いろいろと奇抜なオプションを勧められたが、結局効いたのはこれだった

git reset <last good remote commit> # NOT hard!
git stash # to store stuff that has been changed since last good remote commit
git fetch origin # pull down everything since last commit
git stash pop # put back the new stuff
git add . && git commit -m "add back new stuff" # new stuff
git push origin main # done!