Improve ref validity checking in fetchGit

The previous regex was too strict and did not match what git was allowing. It
could lead to `fetchGit` not accepting valid branch names, even though they
exist in a repository (for example, branch names containing `/`, which are
pretty standard, like `release/1.0` branches).

The new regex defines what a branch name should **NOT** contain. It takes the
definitions from `refs.c` in https://github.com/git/git and `git help
check-ref-format` pages.

This change also introduces a test for ref name validity checking, which
compares the result from Nix with the result of `git check-ref-format --branch`.

src/libfetchers/git.cc

@@ -418,7 +418,7 @@ struct GitInputScheme : InputScheme
 
         auto input = std::make_unique<GitInput>(parseURL(getStrAttr(attrs, "url")));
         if (auto ref = maybeGetStrAttr(attrs, "ref")) {
-            if (!std::regex_match(*ref, refRegex))
+            if (std::regex_search(*ref, badGitRefRegex))
                 throw BadURL("invalid Git branch/tag name '%s'", *ref);
             input->ref = *ref;
         }

src/libutil/url.cc

@@ -4,6 +4,7 @@
 namespace nix {
 
 std::regex refRegex(refRegexS, std::regex::ECMAScript);
+std::regex badGitRefRegex(badGitRefRegexS, std::regex::ECMAScript);
 std::regex revRegex(revRegexS, std::regex::ECMAScript);
 std::regex flakeIdRegex(flakeIdRegexS, std::regex::ECMAScript);
 

src/libutil/url.hh

@@ -49,6 +49,12 @@ const static std::string pathRegex = "(?:" + segmentRegex + "(?:/" + segmentRege
 const static std::string refRegexS = "[a-zA-Z0-9][a-zA-Z0-9_.-]*"; // FIXME: check
 extern std::regex refRegex;
 
+// Instead of defining what a good Git Ref is, we define what a bad Git Ref is
+// This is because of the definition of a ref in refs.c in https://github.com/git/git
+// See tests/fetchGitRefs.sh for the full definition
+const static std::string badGitRefRegexS = "//|^[./]|/\\.|\\.\\.|[[:cntrl:][:space:]:?^~\[]|\\\\|\\*|\\.lock$|\\.lock/|@\\{|[/.]$|^@$|^$";
+extern std::regex badGitRefRegex;
+
 // A Git revision (a SHA-1 commit hash).
 const static std::string revRegexS = "[0-9a-fA-F]{40}";
 extern std::regex revRegex;

tests/fetchGitRefs.sh

@@ -0,0 +1,111 @@
+source common.sh
+
+if [[ -z $(type -p git) ]]; then
+    echo "Git not installed; skipping Git tests"
+    exit 99
+fi
+
+clearStore
+
+repo="$TEST_ROOT/git"
+
+rm -rf "$repo" "${repo}-tmp" "$TEST_HOME/.cache/nix"
+
+git init "$repo"
+git -C "$repo" config user.email "foobar@example.com"
+git -C "$repo" config user.name "Foobar"
+
+echo utrecht > "$repo"/hello
+git -C "$repo" add hello
+git -C "$repo" commit -m 'Bla1'
+
+path=$(nix eval --raw "(builtins.fetchGit { url = $repo; ref = \"master\"; }).outPath")
+
+# Test various combinations of ref names
+# (taken from the git project)
+
+# git help check-ref-format
+#       Git imposes the following rules on how references are named:
+#
+#        1. They can include slash / for hierarchical (directory) grouping, but no slash-separated component can begin with a dot .  or end with the sequence .lock.
+#        2. They must contain at least one /. This enforces the presence of a category like heads/, tags/ etc. but the actual names are not restricted. If the --allow-onelevel option is used, this rule is waived.
+#        3. They cannot have two consecutive dots ..  anywhere.
+#        4. They cannot have ASCII control characters (i.e. bytes whose values are lower than \040, or \177 DEL), space, tilde ~, caret ^, or colon : anywhere.
+#        5. They cannot have question-mark ?, asterisk *, or open bracket [ anywhere. See the --refspec-pattern option below for an exception to this rule.
+#        6. They cannot begin or end with a slash / or contain multiple consecutive slashes (see the --normalize option below for an exception to this rule)
+#        7. They cannot end with a dot ..
+#        8. They cannot contain a sequence @{.
+#        9. They cannot be the single character @.
+#       10. They cannot contain a \.
+
+valid_ref() {
+    { set +x; printf >&2 '\n>>>>>>>>>> valid_ref %s\b <<<<<<<<<<\n' $(printf %s "$1" | sed -n -e l); set -x; }
+    git check-ref-format --branch "$1" >/dev/null
+    git -C "$repo" branch "$1" master >/dev/null
+    path1=$(nix eval --raw "(builtins.fetchGit { url = $repo; ref = ''$1''; }).outPath")
+    [[ $path1 = $path ]]
+    git -C "$repo" branch -D "$1" >/dev/null
+}
+
+invalid_ref() {
+    { set +x; printf >&2 '\n>>>>>>>>>> invalid_ref %s\b <<<<<<<<<<\n' $(printf %s "$1" | sed -n -e l); set -x; }
+    # special case for a sole @:
+    # --branch @ will try to interpret @ as a branch reference and not fail. Thus we need --allow-onelevel
+    if [ "$1" = "@" ]; then
+        (! git check-ref-format --allow-onelevel "$1" >/dev/null 2>&1)
+    else
+        (! git check-ref-format --branch "$1" >/dev/null 2>&1)
+    fi
+    nix --debug eval --raw "(builtins.fetchGit { url = $repo; ref = ''$1''; }).outPath" 2>&1 | grep 'error: invalid Git branch/tag name' >/dev/null
+}
+
+
+valid_ref 'foox'
+valid_ref '1337'
+valid_ref 'foo.baz'
+valid_ref 'foo/bar/baz'
+valid_ref 'foo./bar'
+valid_ref 'heads/foo@bar'
+valid_ref "$(printf 'heads/fu\303\237')"
+valid_ref 'foo-bar-baz'
+valid_ref '$1'
+valid_ref 'foo.locke'
+
+invalid_ref 'refs///heads/foo'
+invalid_ref 'heads/foo/'
+invalid_ref '///heads/foo'
+invalid_ref '.foo'
+invalid_ref './foo'
+invalid_ref './foo/bar'
+invalid_ref 'foo/./bar'
+invalid_ref 'foo/bar/.'
+invalid_ref 'foo bar'
+invalid_ref 'foo?bar'
+invalid_ref 'foo^bar'
+invalid_ref 'foo~bar'
+invalid_ref 'foo:bar'
+invalid_ref 'foo[bar'
+invalid_ref 'foo/bar/.'
+invalid_ref '.refs/foo'
+invalid_ref 'refs/heads/foo.'
+invalid_ref 'heads/foo..bar'
+invalid_ref 'heads/foo?bar'
+invalid_ref 'heads/foo.lock'
+invalid_ref 'heads///foo.lock'
+invalid_ref 'foo.lock/bar'
+invalid_ref 'foo.lock///bar'
+invalid_ref 'heads/v@{ation'
+invalid_ref 'heads/foo\.ar' # should fail due to \
+invalid_ref 'heads/foo\bar' # should fail due to \
+invalid_ref "$(printf 'heads/foo\t')" # should fail because it has a TAB
+invalid_ref "$(printf 'heads/foo\177')"
+invalid_ref '@'
+
+invalid_ref 'foo/*'
+invalid_ref '*/foo'
+invalid_ref 'foo/*/bar'
+invalid_ref '*'
+invalid_ref 'foo/*/*'
+invalid_ref '*/foo/*'
+invalid_ref '/foo'
+invalid_ref ''

tests/local.mk

@@ -18,6 +18,7 @@ nix_tests = \
   nar-access.sh \
   structured-attrs.sh \
   fetchGit.sh \
+  fetchGitRefs.sh \
   fetchGitSubmodules.sh \
   fetchMercurial.sh \
   signing.sh \