Improve Git URI handling

Git URI can also support scp style links similar to git itself.

This change augments the function fixGitURL to better handle the scp
style urls through a minimal parser rather than regex which has been
found to be brittle.

* Support for IPV6 added
* New test cases added for fixGitURL
* Clearer documentation on purpose and goal of function
* More `std::string_view` for performance
* A few more URL tests

Fixes #5958

src/libutil-tests/url.cc

@@ -12,6 +12,64 @@ namespace nix {
 using Authority = ParsedURL::Authority;
 using HostType = Authority::HostType;
 
+struct FixGitURLParam
+{
+    std::string_view input;
+    std::string_view expected;
+};
+
+std::ostream & operator<<(std::ostream & os, const FixGitURLParam & param)
+{
+    return os << "Input: \"" << param.input << "\", Expected: \"" << param.expected << "\"";
+}
+
+class FixGitURLTestSuite : public ::testing::TestWithParam<FixGitURLParam>
+{};
+
+INSTANTIATE_TEST_SUITE_P(
+    FixGitURLs,
+    FixGitURLTestSuite,
+    ::testing::Values(
+        // https://github.com/NixOS/nix/issues/5958
+        // Already proper URL with git+ssh
+        FixGitURLParam{"git+ssh://user@domain:1234/path", "git+ssh://user@domain:1234/path"},
+        // SCP-like URL (rewritten to ssh://)
+        FixGitURLParam{"git@github.com:owner/repo.git", "ssh://git@github.com/owner/repo.git"},
+        // SCP-like URL (no user)
+        FixGitURLParam{"github.com:owner/repo.git", "ssh://github.com/owner/repo.git"},
+        // SCP-like URL (leading slash)
+        FixGitURLParam{"github.com:/owner/repo.git", "ssh://github.com/owner/repo.git"},
+        // Absolute path (becomes file:)
+        FixGitURLParam{"/home/me/repo", "file:///home/me/repo"},
+        // Relative path (becomes file:// absolute)
+        FixGitURLParam{"relative/repo", "file:///relative/repo"},
+        // Already file: scheme
+        // NOTE: This is not valid technically as it's not absolute
+        FixGitURLParam{"file:/var/repos/x", "file:/var/repos/x"},
+        // IPV6 test case
+        FixGitURLParam{"user@[2001:db8:1::2]:/home/file", "ssh://user@[2001:db8:1::2]/home/file"}));
+
+TEST_P(FixGitURLTestSuite, parsesVariedGitUrls)
+{
+    auto & p = GetParam();
+    const auto actual = fixGitURL(p.input).to_string();
+    EXPECT_EQ(actual, p.expected);
+}
+
+TEST_P(FixGitURLTestSuite, fixGitIsIdempotent)
+{
+    auto & p = GetParam();
+    const auto actual = fixGitURL(p.expected).to_string();
+    EXPECT_EQ(actual, p.expected);
+}
+
+TEST_P(FixGitURLTestSuite, fixGitOutputParses)
+{
+    auto & p = GetParam();
+    const auto parsed = fixGitURL(p.expected);
+    EXPECT_EQ(parseURL(parsed.to_string()), parsed);
+}
+
 TEST(parseURL, parsesSimpleHttpUrl)
 {
     auto s = "http://www.example.org/file.tar.gz";

src/libutil/include/nix/util/url.hh

@@ -327,10 +327,23 @@ struct ParsedUrlScheme
 
 ParsedUrlScheme parseUrlScheme(std::string_view scheme);
 
-/* Detects scp-style uris (e.g. git@github.com:NixOS/nix) and fixes
-   them by removing the `:` and assuming a scheme of `ssh://`. Also
-   changes absolute paths into file:// URLs. */
-ParsedURL fixGitURL(const std::string & url);
+/**
+ * Normalize a Git remote string from various styles into a URL-like form.
+ * Input forms handled:
+ *   1) SCP-style SSH syntax:    "[user@]host:path"      -> "ssh://user@host/path"
+ *   2) Already "file:" URLs:    "file:/abs/or/rel"      -> unchanged
+ *   3) Bare paths / filenames:  "src/repo" or "/abs"    -> "file:src/repo" or "file:/abs"
+ *   4) Anything with "://":     treated as a proper URL -> unchanged
+ *
+ *  Note: for the scp-style, as they are converted to ssh-form, all paths are assumed to
+ *  then be absolute whereas in programs like git, they retain the scp form which allows
+ *  relative paths.
+ *
+ *  Additionally, if no url can be determined, it is returned as a file:// URI.
+ *  If the url does not start with a leading slash, one will be added since there are no
+ *  relative path URIs.
+ */
+ParsedURL fixGitURL(std::string_view url);
 
 /**
  * Whether a string is valid as RFC 3986 scheme name.

src/libutil/url.cc

@@ -408,21 +408,77 @@ ParsedUrlScheme parseUrlScheme(std::string_view scheme)
     };
 }
 
-ParsedURL fixGitURL(const std::string & url)
+struct ScpLike
 {
-    std::regex scpRegex("([^/]*)@(.*):(.*)");
-    if (!hasPrefix(url, "/") && std::regex_match(url, scpRegex))
-        return parseURL(std::regex_replace(url, scpRegex, "ssh://$1@$2/$3"));
-    if (hasPrefix(url, "file:"))
-        return parseURL(url);
-    if (url.find("://") == std::string::npos) {
+    ParsedURL::Authority authority;
+    std::string_view path;
+};
+
+/**
+ * Parse a scp url. This is a helper struct for fixGitURL.
+ * This is needed since we support scp-style urls for git urls.
+ * https://git-scm.com/book/ms/v2/Git-on-the-Server-The-Protocols
+ *
+ * A good reference is libgit2 also allows scp style
+ * https://github.com/libgit2/libgit2/blob/58d9363f02f1fa39e46d49b604f27008e75b72f2/src/util/net.c#L806
+ */
+static std::optional<ScpLike> parseScp(const std::string_view s) noexcept
+{
+    if (s.empty() || s.front() == '/')
+        return std::nullopt;
+
+    // Find the colon that separates host from path.
+    // Find the right-most since ipv6 has colons
+    const auto colon = s.rfind(':');
+    if (colon == std::string_view::npos)
+        return std::nullopt;
+
+    // Split head:[path]
+    const auto head = s.substr(0, colon);
+    const auto path = s.substr(colon + 1);
+
+    if (head.empty())
+        return std::nullopt;
+
+    return ScpLike{
+        .authority = ParsedURL::Authority::parse(head),
+        .path = path,
+    };
+}
+
+ParsedURL fixGitURL(const std::string_view url)
+{
+    try {
+        if (auto parsed = parseURL(url); parsed.scheme == "file" || parsed.authority)
+            return parsed;
+    } catch (BadURL &) {
+    }
+
+    // if the url does not start with forward slash, add one
+    auto splitMakeAbs = [&](std::string_view pathS) {
+        std::vector<std::string> path;
+
+        if (!hasPrefix(pathS, "/")) {
+            path.emplace_back("");
+        }
+        splitStringInto(path, pathS, "/");
+
+        return path;
+    };
+
+    if (auto scp = parseScp(url)) {
         return ParsedURL{
-            .scheme = "file",
-            .authority = ParsedURL::Authority{},
-            .path = splitString<std::vector<std::string>>(url, "/"),
+            .scheme = "ssh",
+            .authority = std::move(scp->authority),
+            .path = splitMakeAbs(scp->path),
         };
     }
-    return parseURL(url);
+
+    return ParsedURL{
+        .scheme = "file",
+        .authority = ParsedURL::Authority{},
+        .path = splitMakeAbs(url),
+    };
 }
 
 // https://www.rfc-editor.org/rfc/rfc3986#section-3.1