[PATCH] clone: --filter=tree:0 implies fetch.recurseSubmodules=no

From: Derrick Stolee <dstolee@microsoft.com>

The partial clone feature has several modes, but only a few are quick for a server to process using reachability bitmaps:

* Blobless: --filter=blob:none downloads all commits and trees and
  fetches necessary blobs on-demand.

* Treeless: --filter=tree:0 downloads all commits and fetches necessary
  trees and blobs on demand.

This treeles mode is most similar to a shallow clone in the total size (it only adds the commit objects for the full history). This makes treeless clones an interesting replacement for shallow clones. A user can run more commands in a treeless clone than in a shallow clone, especially 'git log' (no pathspec).

In particular, servers can still serve 'git fetch' requests quickly by calculating the difference between commit wants and haves using bitmaps.

I was testing this feature with this in mind, and I knew that some trees would be downloaded multiple times when checking out a new branch, but I did not expect to discover a significant issue with 'git fetch', at least in repostiories with submodules.

I was testing these commands:

	$ git clone --filter=tree:0 --single-branch --branch=master \
	  https://github.com/git/git
	$ git -C git fetch origin "+refs/heads/*:refs/remotes/origin/*"

This fetch command started downloading several pack-files of trees before completing the command. I never let it finish since I got so impatient with the repeated downloads. During debugging, I found that the stack triggering promisor_remote_get_direct() was going through fetch_populated_submodules(). Notice that I did not recurse my submodules in the original clone, so the sha1collisiondetection submodule is not initialized. Even so, my 'git fetch' was scanning commits for updates to submodules.

I decided that even if I did populate the submodules, the nature of treeless clones makes me not want to care about the contents of commits other than those that I am explicitly navigating to.

This loop of tree fetches can be avoided by adding --no-recurse-submodules to the 'git fetch' command or setting fetch.recurseSubmodules=no.

To make this as painless as possible for future users of treeless clones, automatically set fetch.recurseSubmodules=no at clone time.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
---
    clone: --filter=tree:0 implies fetch.recurseSubmodules=no
    
    While testing different partial clone options, I stumbled across this
    one. My initial thought was that we were parsing commits and loading
    their root trees unnecessarily, but I see that doesn't happen after this
    change.
    
    Here are some recent discussions about using --filter=tree:0:
    
    [1] 
    https://lore.kernel.org/git/aa7b89ee-08aa-7943-6a00-28dcf344426e@syntevo.com/
    [2] https://lore.kernel.org/git/cover.1588633810.git.me@ttaylorr.com/[3] 
    https://lore.kernel.org/git/58274817-7ac6-b6ae-0d10-22485dfe5e0e@syntevo.com/
    
    Thanks, -Stolee

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-797%2Fderrickstolee%2Ftree-0-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-797/derrickstolee/tree-0-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/797

 list-objects-filter-options.c | 4 ++++
 t/t5616-partial-clone.sh      | 6 ++++++
 2 files changed, 10 insertions(+)