fetchTree: shallow git fetching by default

Motivation:
make git fetching more efficient for most repos by default

doc/manual/rl-next/shallow-git-fetching-by-default.md

@@ -0,0 +1,12 @@
+---
+synopsis: "`fetchTree` now fetches git repositories shallowly by default"
+prs: 10028
+---
+
+`builtins.fetchTree` now clones git repositories shallowly by default, which reduces network traffic and disk usage significantly in many cases.
+
+Previously, the default behavior was to clone the full history of a specific tag or branch (eg. `ref`) and only afterwards extract the files of one specific revision.
+
+From now on, the `ref` and `allRefs` arguments will be ignored, except if shallow cloning is disabled by setting `shallow = false`.
+
+The defaults for `builtins.fetchGit` remain unchanged. Here, shallow cloning has to be enabled manually by passing `shallow = true`.

src/libexpr/primops/fetchTree.cc

@@ -138,6 +138,11 @@ static void fetchTree(
             attrs.emplace("exportIgnore", Explicit<bool>{true});
         }
 
+        // fetchTree should fetch git repos with shallow = true by default
+        if (type == "git" && !params.isFetchGit && !attrs.contains("shallow")) {
+            attrs.emplace("shallow", Explicit<bool>{true});
+        }
+
         if (!params.allowNameArgument)
             if (auto nameIter = attrs.find("name"); nameIter != attrs.end())
                 state.error<EvalError>(
@@ -321,6 +326,8 @@ static RegisterPrimOp primop_fetchTree({
 
         - `ref` (String, optional)
 
+          By default, this has no effect. This becomes relevant only once `shallow` cloning is disabled.
+
           A [Git reference](https://git-scm.com/book/en/v2/Git-Internals-Git-References), such as a branch or tag name.
 
           Default: `"HEAD"`
@@ -334,8 +341,9 @@ static RegisterPrimOp primop_fetchTree({
         - `shallow` (Bool, optional)
 
           Make a shallow clone when fetching the Git tree.
+          When this is enabled, the options `ref` and `allRefs` have no effect anymore.
 
-          Default: `false`
+          Default: `true`
 
         - `submodules` (Bool, optional)
 
@@ -345,8 +353,11 @@ static RegisterPrimOp primop_fetchTree({
 
         - `allRefs` (Bool, optional)
 
-          If set to `true`, always fetch the entire repository, even if the latest commit is still in the cache.
-          Otherwise, only the latest commit is fetched if it is not already cached.
+          By default, this has no effect. This becomes relevant only once `shallow` cloning is disabled.
+
+          Whether to fetch all references (eg. branches and tags) of the repository.
+          With this argument being true, it's possible to load a `rev` from *any* `ref`.
+          (Without setting this option, only `rev`s from the specified `ref` are supported).
 
           Default: `false`
 
@@ -600,6 +611,8 @@ static RegisterPrimOp primop_fetchGit({
 
         [Git reference]: https://git-scm.com/book/en/v2/Git-Internals-Git-References
 
+        This option has no effect once `shallow` cloning is enabled.
+
         By default, the `ref` value is prefixed with `refs/heads/`.
         As of 2.3.0, Nix will not prefix `refs/heads/` if `ref` starts with `refs/`.
 
@@ -617,13 +630,15 @@ static RegisterPrimOp primop_fetchGit({
       - `shallow` (default: `false`)
 
         Make a shallow clone when fetching the Git tree.
-
+        When this is enabled, the options `ref` and `allRefs` have no effect anymore.
       - `allRefs`
 
-        Whether to fetch all references of the repository.
-        With this argument being true, it's possible to load a `rev` from *any* `ref`
+        Whether to fetch all references (eg. branches and tags) of the repository.
+        With this argument being true, it's possible to load a `rev` from *any* `ref`.
         (by default only `rev`s from the specified `ref` are supported).
 
+        This option has no effect once `shallow` cloning is enabled.
+
       - `verifyCommit` (default: `true` if `publicKey` or `publicKeys` are provided, otherwise `false`)
 
         Whether to check `rev` for a signature matching `publicKey` or `publicKeys`.

tests/nixos/fetch-git/test-cases/fetchTree-shallow/default.nix

@@ -0,0 +1,45 @@
+{
+  description = "fetchTree fetches git repos shallowly by default";
+  script = ''
+    # purge nix git cache to make sure we start with a clean slate
+    client.succeed("rm -rf ~/.cache/nix")
+
+    # add two commits to the repo:
+    #   - one with a large file (2M)
+    #   - another one making the file small again
+    client.succeed(f"""
+      dd if=/dev/urandom of={repo.path}/thailand bs=1M count=2 \
+      && {repo.git} add thailand \
+      && {repo.git} commit -m 'commit1' \
+      && echo 'ThaigerSprint' > {repo.path}/thailand \
+      && {repo.git} add thailand \
+      && {repo.git} commit -m 'commit2' \
+      && {repo.git} push origin main
+    """)
+
+    # memoize the revision
+    commit2_rev = client.succeed(f"""
+      {repo.git} rev-parse HEAD
+    """).strip()
+
+    # construct the fetcher call
+    fetchGit_expr = f"""
+      builtins.fetchTree {{
+        type = "git";
+        url = "{repo.remote}";
+        rev = "{commit2_rev}";
+      }}
+    """
+
+    # fetch the repo via nix
+    fetched1 = client.succeed(f"""
+      nix eval --impure --raw --expr '({fetchGit_expr}).outPath'
+    """)
+
+    # check that the size of ~/.cache/nix is less than 1M
+    cache_size = client.succeed("""
+      du -s ~/.cache/nix
+    """).strip().split()[0]
+    assert int(cache_size) < 1024, f"cache size is {cache_size}K which is larger than 1M"
+  '';
+}