Merge branch 'ps/fetch-atomic'

"git fetch" learns to treat ref updates atomically in all-or-none
fashion, just like "git push" does, with the new "--atomic" option.

* ps/fetch-atomic:
  fetch: implement support for atomic reference updates
  fetch: allow passing a transaction to `s_update_ref()`
  fetch: refactor `s_update_ref` to use common exit path
  fetch: use strbuf to format FETCH_HEAD updates
  fetch: extract writing to FETCH_HEAD

diff --combined t/t5510-fetch.sh
index b11e04d59d2ff4476d222ed71f3c6856eb9203f8,109d15be98ed039846b32406db89bc18393e402f..8da907e770d837c52092293bfc8d1f3d1500dc12
--- 1/t/t5510-fetch.sh
--- 2/t/t5510-fetch.sh
+++ b/t/t5510-fetch.sh
@@@ -5,9 -5,6 +5,9 @@@ test_description='Per branch config var
  
  '
  
 +GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main
 +export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
 +
  . ./test-lib.sh
  
  D=$(pwd)
@@@ -179,6 -176,174 +179,174 @@@ test_expect_success 'fetch --prune --ta
        git rev-parse sometag
  '
  
+ test_expect_success 'fetch --atomic works with a single branch' '
+       test_when_finished "rm -rf \"$D\"/atomic" &&
+ 
+       cd "$D" &&
+       git clone . atomic &&
+       git branch atomic-branch &&
+       oid=$(git rev-parse atomic-branch) &&
+       echo "$oid" >expected &&
+ 
+       git -C atomic fetch --atomic origin &&
+       git -C atomic rev-parse origin/atomic-branch >actual &&
+       test_cmp expected actual &&
+       test $oid = "$(git -C atomic rev-parse --verify FETCH_HEAD)"
+ '
+ 
+ test_expect_success 'fetch --atomic works with multiple branches' '
+       test_when_finished "rm -rf \"$D\"/atomic" &&
+ 
+       cd "$D" &&
+       git clone . atomic &&
+       git branch atomic-branch-1 &&
+       git branch atomic-branch-2 &&
+       git branch atomic-branch-3 &&
+       git rev-parse refs/heads/atomic-branch-1 refs/heads/atomic-branch-2 refs/heads/atomic-branch-3 >actual &&
+ 
+       git -C atomic fetch --atomic origin &&
+       git -C atomic rev-parse refs/remotes/origin/atomic-branch-1 refs/remotes/origin/atomic-branch-2 refs/remotes/origin/atomic-branch-3 >expected &&
+       test_cmp expected actual
+ '
+ 
+ test_expect_success 'fetch --atomic works with mixed branches and tags' '
+       test_when_finished "rm -rf \"$D\"/atomic" &&
+ 
+       cd "$D" &&
+       git clone . atomic &&
+       git branch atomic-mixed-branch &&
+       git tag atomic-mixed-tag &&
+       git rev-parse refs/heads/atomic-mixed-branch refs/tags/atomic-mixed-tag >actual &&
+ 
+       git -C atomic fetch --tags --atomic origin &&
+       git -C atomic rev-parse refs/remotes/origin/atomic-mixed-branch refs/tags/atomic-mixed-tag >expected &&
+       test_cmp expected actual
+ '
+ 
+ test_expect_success 'fetch --atomic prunes references' '
+       test_when_finished "rm -rf \"$D\"/atomic" &&
+ 
+       cd "$D" &&
+       git branch atomic-prune-delete &&
+       git clone . atomic &&
+       git branch --delete atomic-prune-delete &&
+       git branch atomic-prune-create &&
+       git rev-parse refs/heads/atomic-prune-create >actual &&
+ 
+       git -C atomic fetch --prune --atomic origin &&
+       test_must_fail git -C atomic rev-parse refs/remotes/origin/atomic-prune-delete &&
+       git -C atomic rev-parse refs/remotes/origin/atomic-prune-create >expected &&
+       test_cmp expected actual
+ '
+ 
+ test_expect_success 'fetch --atomic aborts with non-fast-forward update' '
+       test_when_finished "rm -rf \"$D\"/atomic" &&
+ 
+       cd "$D" &&
+       git branch atomic-non-ff &&
+       git clone . atomic &&
+       git rev-parse HEAD >actual &&
+ 
+       git branch atomic-new-branch &&
+       parent_commit=$(git rev-parse atomic-non-ff~) &&
+       git update-ref refs/heads/atomic-non-ff $parent_commit &&
+ 
+       test_must_fail git -C atomic fetch --atomic origin refs/heads/*:refs/remotes/origin/* &&
+       test_must_fail git -C atomic rev-parse refs/remotes/origin/atomic-new-branch &&
+       git -C atomic rev-parse refs/remotes/origin/atomic-non-ff >expected &&
+       test_cmp expected actual &&
+       test_must_be_empty atomic/.git/FETCH_HEAD
+ '
+ 
+ test_expect_success 'fetch --atomic executes a single reference transaction only' '
+       test_when_finished "rm -rf \"$D\"/atomic" &&
+ 
+       cd "$D" &&
+       git clone . atomic &&
+       git branch atomic-hooks-1 &&
+       git branch atomic-hooks-2 &&
+       head_oid=$(git rev-parse HEAD) &&
+ 
+       cat >expected <<-EOF &&
+               prepared
+               $ZERO_OID $head_oid refs/remotes/origin/atomic-hooks-1
+               $ZERO_OID $head_oid refs/remotes/origin/atomic-hooks-2
+               committed
+               $ZERO_OID $head_oid refs/remotes/origin/atomic-hooks-1
+               $ZERO_OID $head_oid refs/remotes/origin/atomic-hooks-2
+       EOF
+ 
+       rm -f atomic/actual &&
+       write_script atomic/.git/hooks/reference-transaction <<-\EOF &&
+               ( echo "$*" && cat ) >>actual
+       EOF
+ 
+       git -C atomic fetch --atomic origin &&
+       test_cmp expected atomic/actual
+ '
+ 
+ test_expect_success 'fetch --atomic aborts all reference updates if hook aborts' '
+       test_when_finished "rm -rf \"$D\"/atomic" &&
+ 
+       cd "$D" &&
+       git clone . atomic &&
+       git branch atomic-hooks-abort-1 &&
+       git branch atomic-hooks-abort-2 &&
+       git branch atomic-hooks-abort-3 &&
+       git tag atomic-hooks-abort &&
+       head_oid=$(git rev-parse HEAD) &&
+ 
+       cat >expected <<-EOF &&
+               prepared
+               $ZERO_OID $head_oid refs/remotes/origin/atomic-hooks-abort-1
+               $ZERO_OID $head_oid refs/remotes/origin/atomic-hooks-abort-2
+               $ZERO_OID $head_oid refs/remotes/origin/atomic-hooks-abort-3
+               $ZERO_OID $head_oid refs/tags/atomic-hooks-abort
+               aborted
+               $ZERO_OID $head_oid refs/remotes/origin/atomic-hooks-abort-1
+               $ZERO_OID $head_oid refs/remotes/origin/atomic-hooks-abort-2
+               $ZERO_OID $head_oid refs/remotes/origin/atomic-hooks-abort-3
+               $ZERO_OID $head_oid refs/tags/atomic-hooks-abort
+       EOF
+ 
+       rm -f atomic/actual &&
+       write_script atomic/.git/hooks/reference-transaction <<-\EOF &&
+               ( echo "$*" && cat ) >>actual
+               exit 1
+       EOF
+ 
+       git -C atomic for-each-ref >expected-refs &&
+       test_must_fail git -C atomic fetch --tags --atomic origin &&
+       git -C atomic for-each-ref >actual-refs &&
+       test_cmp expected-refs actual-refs &&
+       test_must_be_empty atomic/.git/FETCH_HEAD
+ '
+ 
+ test_expect_success 'fetch --atomic --append appends to FETCH_HEAD' '
+       test_when_finished "rm -rf \"$D\"/atomic" &&
+ 
+       cd "$D" &&
+       git clone . atomic &&
+       oid=$(git rev-parse HEAD) &&
+ 
+       git branch atomic-fetch-head-1 &&
+       git -C atomic fetch --atomic origin atomic-fetch-head-1 &&
+       test_line_count = 1 atomic/.git/FETCH_HEAD &&
+ 
+       git branch atomic-fetch-head-2 &&
+       git -C atomic fetch --atomic --append origin atomic-fetch-head-2 &&
+       test_line_count = 2 atomic/.git/FETCH_HEAD &&
+       cp atomic/.git/FETCH_HEAD expected &&
+ 
+       write_script atomic/.git/hooks/reference-transaction <<-\EOF &&
+               exit 1
+       EOF
+ 
+       git branch atomic-fetch-head-3 &&
+       test_must_fail git -C atomic fetch --atomic --append origin atomic-fetch-head-3 &&
+       test_cmp expected atomic/.git/FETCH_HEAD
+ '
+ 
  test_expect_success '--refmap="" ignores configured refspec' '
        cd "$TRASH_DIRECTORY" &&
        git clone "$D" remote-refs &&