flakes: for detected git repository now assume shallow clones by default

Shallow clones are faster to access because we don't have to compute the
revCount, which in sparse checkouts might not even exists.

This is especially useful in combination with lazy trees in mind on large
repository such as nixpkgs.

Author: Mic92

src/libflake/flakeref.cc

@@ -168,8 +168,10 @@ std::pair<FlakeRef, std::string> parsePathFlakeRefWithFragment(
                         parsedURL.query.insert_or_assign("dir", subdir);
                     }
 
-                    if (pathExists(flakeRoot + "/.git/shallow"))
-                        parsedURL.query.insert_or_assign("shallow", "1");
+                    if (!parsedURL.query.count("shallow")) {
+                       // We assume shallow by default, so we don't need to compute the revCount, which is an expensive operation.
+                       parsedURL.query.insert_or_assign("shallow", "1");
+                    }
 
                     return fromParsedURL(fetchSettings, std::move(parsedURL), isFlake);
                 }

tests/functional/fetchGitShallow.sh

@@ -14,8 +14,9 @@ git -C "$TEST_ROOT/shallow-parent" config user.email "foobar@example.com"
 git -C "$TEST_ROOT/shallow-parent" config user.name "Foobar"
 
 # Add several commits to have history
-echo "first" > "$TEST_ROOT/shallow-parent/file.txt"
-git -C "$TEST_ROOT/shallow-parent" add file.txt
+echo "{ outputs = _: {}; }" > "$TEST_ROOT/shallow-parent/flake.nix"
+echo "" > "$TEST_ROOT/shallow-parent/file.txt"
+git -C "$TEST_ROOT/shallow-parent" add file.txt flake.nix
 git -C "$TEST_ROOT/shallow-parent" commit -m "First commit"
 
 echo "second" > "$TEST_ROOT/shallow-parent/file.txt"
@@ -52,16 +53,23 @@ expectStderr 1 nix eval --expr 'builtins.fetchTree { type = "git"; url = "file:/
 git -C "$TEST_ROOT/shallow-clone" worktree add "$TEST_ROOT/shallow-worktree"
 
 # Prior to the fix, this would error out because of the shallow clone's
-# inability to find parent commits. Now it should return a partial revCount.
-revCount=$(nix eval --impure --expr "(builtins.fetchGit { url = \"file://$TEST_ROOT/shallow-worktree\"; }).revCount")
-
-# Verify we got a valid revCount (will be 1 since it's a depth=1 clone)
-[[ "${revCount}" -eq 1 ]]
-
-# Also verify that we can get the revCount with fetchTree
-revCount=$(nix eval --impure --expr "(builtins.fetchTree { type = \"git\"; url = \"file://$TEST_ROOT/shallow-worktree\"; }).revCount")
-[[ "${revCount}" -eq 1 ]]
-
-# Verify that we can get other attributes too
-rev=$(nix eval --impure --raw --expr "(builtins.fetchGit { url = \"file://$TEST_ROOT/shallow-worktree\"; }).rev")
-[[ "${rev}" != "0000000000000000000000000000000000000000" ]]
+# inability to find parent commits. Now it should return an error.
+if nix eval --impure --expr "(builtins.fetchGit { url = \"file://$TEST_ROOT/shallow-worktree\"; }).revCount" 2>/dev/null; then
+    echo "fetchGit unexpectedly succeeded on shallow clone" >&2
+    exit 1
+fi
+
+# Also verify that fetchTree fails similarly
+if nix eval --impure --expr "(builtins.fetchTree { type = \"git\"; url = \"file://$TEST_ROOT/shallow-worktree\"; }).revCount" 2>/dev/null; then
+    echo "fetchTree unexpectedly succeeded on shallow clone" >&2
+    exit 1
+fi
+
+# Verify that we can shallow fetch the worktree
+git -C "$TEST_ROOT/shallow-worktree" rev-list --count HEAD >/dev/null
+nix eval --impure --raw --expr "(builtins.fetchGit { url = \"file://$TEST_ROOT/shallow-worktree\"; shallow = true; }).rev"
+
+# Normal flake operation work because they use shallow by default
+pushd "$TEST_ROOT/shallow-worktree"
+nix flake metadata
+popd