Re: [RFC PATCH] merge-recursive: create new files with O_EXCL

On Thu, Mar 11, 2021 at 1:58 AM Ephrim Khong [off-list ref] wrote:

On 11.03.2021 00:01, Junio C Hamano wrote:

quoted

Ephrim Khong [off-list ref] writes:

As a side-note, the strace on the affected file also shows that git
writes that file twice during the merge, with the same content. There
might be some potential to further optimize merges to avoid such
double-writes. A small example to reproduce, note how "b" is opened
twice during the merge:

  git init
  echo "foo" > a
  git add a
  git commit -m "Initial commit"

  git mv a b
  git commit -m "File moved"

  git checkout -b other_branch HEAD~
  touch c && git add c && git commit -m "Some other commit"
  strace -otrace git merge master -m "merge message"
  grep '"b"' trace

Yeah, this is somewhat fundamental to merge-recursive's implementation
design; fixing it essentially requires a rewrite.  That rewrite is
nearing completion; so this double-write issue will be fixed when
merge-ort is complete (or for anyone who applies the patches and tries
it out now).

In fact, if you don't see the permissions problems when switching
branches (because branch switching uses O_EXCL?), then merge-ort
almost certainly incidentally fixes that problem for you as well.