git/revCount: improve error message when we have incomplete git history

src/libfetchers/git-utils.cc

@@ -322,8 +322,17 @@ struct GitRepoImpl : GitRepo, std::enable_shared_from_this<GitRepoImpl>
 
             for (size_t n = 0; n < git_commit_parentcount(commit->get()); ++n) {
                 git_commit * parent;
-                if (git_commit_parent(&parent, commit->get(), n))
+                if (git_commit_parent(&parent, commit->get(), n)) {
-                    throw Error("getting parent of Git commit '%s': %s", *git_commit_id(commit->get()), git_error_last()->message);
+                    throw Error(
+                        "Failed to retrieve the parent of Git commit '%s': %s. "
+                        "This may be due to an incomplete repository history. "
+                        "To resolve this, either enable the shallow parameter in your flake URL (?shallow=1) "
+                        "or add set the shallow parameter to true in builtins.fetchGit, "
+                        "or fetch the complete history for this branch.",
+                        *git_commit_id(commit->get()),
+                        git_error_last()->message
+                    );
+                }
                 todo.push(Commit(parent));
             }
         }

tests/functional/fetchGit.sh

@@ -12,7 +12,7 @@ repo=$TEST_ROOT/./git
 
 export _NIX_FORCE_HTTP=1
 
-rm -rf $repo ${repo}-tmp $TEST_HOME/.cache/nix $TEST_ROOT/worktree $TEST_ROOT/shallow $TEST_ROOT/minimal
+rm -rf $repo ${repo}-tmp $TEST_HOME/.cache/nix $TEST_ROOT/worktree $TEST_ROOT/minimal
 
 git init $repo
 git -C $repo config user.email "foobar@example.com"
@@ -216,18 +216,6 @@ git -C $TEST_ROOT/minimal fetch $repo $rev2
 git -C $TEST_ROOT/minimal checkout $rev2
 [[ $(nix eval --impure --raw --expr "(builtins.fetchGit { url = $TEST_ROOT/minimal; }).rev") = $rev2 ]]
 
-# Fetching a shallow repo shouldn't work by default, because we can't
-# return a revCount.
-git clone --depth 1 file://$repo $TEST_ROOT/shallow
-(! nix eval --impure --raw --expr "(builtins.fetchGit { url = $TEST_ROOT/shallow; ref = \"dev\"; }).outPath")
-
-# But you can request a shallow clone, which won't return a revCount.
-path6=$(nix eval --impure --raw --expr "(builtins.fetchTree { type = \"git\"; url = \"file://$TEST_ROOT/shallow\"; ref = \"dev\"; shallow = true; }).outPath")
-[[ $path3 = $path6 ]]
-[[ $(nix eval --impure --expr "(builtins.fetchTree { type = \"git\"; url = \"file://$TEST_ROOT/shallow\"; ref = \"dev\"; shallow = true; }).revCount or 123") == 123 ]]
-
-expectStderr 1 nix eval --expr 'builtins.fetchTree { type = "git"; url = "file:///foo"; }' | grepQuiet "'fetchTree' will not fetch unlocked input"
-
 # Explicit ref = "HEAD" should work, and produce the same outPath as without ref
 path7=$(nix eval --impure --raw --expr "(builtins.fetchGit { url = \"file://$repo\"; ref = \"HEAD\"; }).outPath")
 path8=$(nix eval --impure --raw --expr "(builtins.fetchGit { url = \"file://$repo\"; }).outPath")

tests/functional/fetchGitShallow.sh

@@ -0,0 +1,67 @@
+#!/usr/bin/env bash
+
+# shellcheck source=common.sh
+source common.sh
+
+requireGit
+
+# Create a test repo with multiple commits for all our tests
+git init "$TEST_ROOT/shallow-parent"
+git -C "$TEST_ROOT/shallow-parent" config user.email "foobar@example.com"
+git -C "$TEST_ROOT/shallow-parent" config user.name "Foobar"
+
+# Add several commits to have history
+echo "{ outputs = _: {}; }" > "$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"
+git -C "$TEST_ROOT/shallow-parent" commit -m "Second commit" -a
+
+echo "third" > "$TEST_ROOT/shallow-parent/file.txt"
+git -C "$TEST_ROOT/shallow-parent" commit -m "Third commit" -a
+
+# Add a branch for testing ref fetching
+git -C "$TEST_ROOT/shallow-parent" checkout -b dev
+echo "branch content" > "$TEST_ROOT/shallow-parent/branch-file.txt"
+git -C "$TEST_ROOT/shallow-parent" add branch-file.txt
+git -C "$TEST_ROOT/shallow-parent" commit -m "Branch commit"
+
+# Make a shallow clone (depth=1)
+git clone --depth 1 "file://$TEST_ROOT/shallow-parent" "$TEST_ROOT/shallow-clone"
+
+# Test 1: Fetching a shallow repo shouldn't work by default, because we can't
+# return a revCount.
+(! nix eval --impure --raw --expr "(builtins.fetchGit { url = \"$TEST_ROOT/shallow-clone\"; ref = \"dev\"; }).outPath")
+
+# Test 2: But you can request a shallow clone, which won't return a revCount.
+path=$(nix eval --impure --raw --expr "(builtins.fetchTree { type = \"git\"; url = \"file://$TEST_ROOT/shallow-clone\"; ref = \"dev\"; shallow = true; }).outPath")
+# Verify file from dev branch exists
+[[ -f "$path/branch-file.txt" ]]
+# Verify revCount is missing
+[[ $(nix eval --impure --expr "(builtins.fetchTree { type = \"git\"; url = \"file://$TEST_ROOT/shallow-clone\"; ref = \"dev\"; shallow = true; }).revCount or 123") == 123 ]]
+
+# Test 3: Check unlocked input error message
+expectStderr 1 nix eval --expr 'builtins.fetchTree { type = "git"; url = "file:///foo"; }' | grepQuiet "'fetchTree' will not fetch unlocked input"
+
+# Test 4: Regression test for revCount in worktrees derived from shallow clones
+# Add a worktree to the shallow clone
+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 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"

tests/functional/meson.build

@@ -73,6 +73,7 @@ suites = [
       'gc-runtime.sh',
       'tarball.sh',
       'fetchGit.sh',
+      'fetchGitShallow.sh',
       'fetchurl.sh',
       'fetchPath.sh',
       'fetchTree-file.sh',