Fix the parsing of the sourcehut refs file

Since a26be9f3b8, the same parser is used
to parse the result of sourcehut’s `HEAD` endpoint (coming from [git
dumb protocol]) and the output of `git ls-remote`. However, they are very
slightly different (the former doesn’t specify the current reference
since it’s implied to be `HEAD`).

Unify both, and make the parser a bit more robust and understandable (by
making it more typed and adding tests for it)

[git dumb protocol]: https://git-scm.com/book/en/v2/Git-Internals-Transfer-Protocols#_the_dumb_protocol

src/libfetchers/git-utils.cc

@@ -1,27 +0,0 @@
-#include "git-utils.hh"
-
-#include <regex>
-
-std::optional<std::string> parseListReferenceHeadRef(std::string_view line)
-{
-    const static std::regex head_ref_regex("^ref: ([^\\s]+)\\t+HEAD$");
-    std::match_results<std::string_view::const_iterator> match;
-    if (std::regex_match(line.cbegin(), line.cend(), match, head_ref_regex)) {
-        return match[1];
-    } else {
-        return std::nullopt;
-    }
-}
-
-std::optional<std::string> parseListReferenceForRev(std::string_view rev, std::string_view line)
-{
-    const static std::regex rev_regex("^([^\\t]+)\\t+(.*)$");
-    std::match_results<std::string_view::const_iterator> match;
-    if (!std::regex_match(line.cbegin(), line.cend(), match, rev_regex)) {
-        return std::nullopt;
-    }
-    if (rev != match[2].str()) {
-        return std::nullopt;
-    }
-    return match[1];
-}

src/libfetchers/git-utils.hh

@@ -1,23 +0,0 @@
-#pragma once
-
-#include <string>
-#include <string_view>
-#include <optional>
-
-// Parses the HEAD ref as reported by `git ls-remote --symref`
-//
-// Returns the head branch name as reported by `git ls-remote --symref`, e.g., if
-// ls-remote returns the output below, "main" is returned based on the ref line.
-//
-//   ref: refs/heads/main       HEAD
-//
-// If the repository is in 'detached head' state (HEAD is pointing to a rev
-// instead of a branch), parseListReferenceForRev("HEAD") may be used instead.
-std::optional<std::string> parseListReferenceHeadRef(std::string_view line);
-
-// Parses a reference line from `git ls-remote --symref`, e.g.,
-// parseListReferenceForRev("refs/heads/master", line) will return 6926...
-// given the line below.
-//
-// 6926beab444c33fb57b21819b6642d032016bb1e	refs/heads/master
-std::optional<std::string> parseListReferenceForRev(std::string_view rev, std::string_view line);

src/libfetchers/git.cc

@@ -6,7 +6,7 @@
 #include "url-parts.hh"
 #include "pathlocks.hh"
 #include "util.hh"
-#include "git-utils.hh"
+#include "git.hh"
 
 #include "fetch-settings.hh"
 
@@ -72,13 +72,16 @@ std::optional<std::string> readHead(const Path & path)
 
     std::string_view line = output;
     line = line.substr(0, line.find("\n"));
-    if (const auto ref = parseListReferenceHeadRef(line); ref) {
-        debug("resolved HEAD ref '%s' for repo '%s'", *ref, path);
-        return *ref;
+    if (const auto parseResult = git::parseLsRemoteLine(line)) {
+        switch (parseResult->kind) {
+            case git::LsRemoteRefLine::Kind::Symbolic:
+                debug("resolved HEAD ref '%s' for repo '%s'", parseResult->target, path);
+                break;
+            case git::LsRemoteRefLine::Kind::Object:
+                debug("resolved HEAD rev '%s' for repo '%s'", parseResult->target, path);
+                break;
         }
-    if (const auto rev = parseListReferenceForRev("HEAD", line); rev) {
-        debug("resolved HEAD rev '%s' for repo '%s'", *rev, path);
-        return *rev;
+        return parseResult->target;
     }
     return std::nullopt;
 }

src/libfetchers/github.cc

@@ -4,7 +4,7 @@
 #include "store-api.hh"
 #include "types.hh"
 #include "url-parts.hh"
-#include "git-utils.hh"
+#include "git.hh"
 #include "fetchers.hh"
 #include "fetch-settings.hh"
 
@@ -383,11 +383,11 @@ struct SourceHutInputScheme : GitArchiveInputScheme
             std::string line;
             getline(is, line);
 
-            auto r = parseListReferenceHeadRef(line);
-            if (!r) {
+            auto remoteLine = git::parseLsRemoteLine(line);
+            if (!remoteLine) {
                 throw BadURL("in '%d', couldn't resolve HEAD ref '%d'", input.to_string(), ref);
             }
-            ref_uri = *r;
+            ref_uri = remoteLine->target;
         } else {
             ref_uri = fmt("refs/(heads|tags)/%s", ref);
         }
@@ -399,7 +399,9 @@ struct SourceHutInputScheme : GitArchiveInputScheme
         std::string line;
         std::optional<std::string> id;
         while(!id && getline(is, line)) {
-            id = parseListReferenceForRev(ref_uri, line);
+            auto parsedLine = git::parseLsRemoteLine(line);
+            if (parsedLine && parsedLine->reference == ref_uri)
+                id = parsedLine->target;
         }
 
         if(!id)

src/libutil/git.cc

@@ -0,0 +1,25 @@
+#include "git.hh"
+
+#include <regex>
+
+namespace nix {
+namespace git {
+
+std::optional<LsRemoteRefLine> parseLsRemoteLine(std::string_view line)
+{
+    const static std::regex line_regex("^(ref: *)?([^\\s]+)(?:\\t+(.*))?$");
+    std::match_results<std::string_view::const_iterator> match;
+    if (!std::regex_match(line.cbegin(), line.cend(), match, line_regex))
+        return std::nullopt;
+
+    return LsRemoteRefLine {
+        .kind = match[1].length() == 0
+            ? LsRemoteRefLine::Kind::Object
+            : LsRemoteRefLine::Kind::Symbolic,
+        .target = match[2],
+        .reference = match[3].length() == 0 ? std::nullopt : std::optional<std::string>{ match[3] }
+    };
+}
+
+}
+}

src/libutil/git.hh

@@ -0,0 +1,40 @@
+#pragma once
+
+#include <string>
+#include <string_view>
+#include <optional>
+
+namespace nix {
+
+namespace git {
+
+// A line from the output of `git ls-remote --symref`.
+//
+// These can be of two kinds:
+//
+// - Symbolic references of the form
+//
+//      ref: {target}	{reference}
+//
+//    where {target} is itself a reference and {reference} is optional
+//
+// - Object references of the form
+//
+//      {target}	{reference}
+//
+//    where {target} is a commit id and {reference} is mandatory
+struct LsRemoteRefLine {
+    enum struct Kind {
+        Symbolic,
+        Object
+    };
+    Kind kind;
+    std::string target;
+    std::optional<std::string> reference;
+};
+
+std::optional<LsRemoteRefLine> parseLsRemoteLine(std::string_view line);
+
+}
+
+}

src/libutil/tests/git.cc

@@ -0,0 +1,33 @@
+#include "git.hh"
+#include <gtest/gtest.h>
+
+namespace nix {
+
+    TEST(GitLsRemote, parseSymrefLineWithReference) {
+        auto line = "ref: refs/head/main	HEAD";
+        auto res = git::parseLsRemoteLine(line);
+        ASSERT_TRUE(res.has_value());
+        ASSERT_EQ(res->kind, git::LsRemoteRefLine::Kind::Symbolic);
+        ASSERT_EQ(res->target, "refs/head/main");
+        ASSERT_EQ(res->reference, "HEAD");
+    }
+
+    TEST(GitLsRemote, parseSymrefLineWithNoReference) {
+        auto line = "ref: refs/head/main";
+        auto res = git::parseLsRemoteLine(line);
+        ASSERT_TRUE(res.has_value());
+        ASSERT_EQ(res->kind, git::LsRemoteRefLine::Kind::Symbolic);
+        ASSERT_EQ(res->target, "refs/head/main");
+        ASSERT_EQ(res->reference, std::nullopt);
+    }
+
+    TEST(GitLsRemote, parseObjectRefLine) {
+        auto line = "abc123	refs/head/main";
+        auto res = git::parseLsRemoteLine(line);
+        ASSERT_TRUE(res.has_value());
+        ASSERT_EQ(res->kind, git::LsRemoteRefLine::Kind::Object);
+        ASSERT_EQ(res->target, "abc123");
+        ASSERT_EQ(res->reference, "refs/head/main");
+    }
+}
+