escape .git for tarball cache

Fix for https://github.com/NixOS/nix/issues/10575

src/libfetchers/git-utils.cc

@@ -46,6 +46,7 @@ template<> struct hash<git_oid>
 
 }
 
+
 std::ostream & operator << (std::ostream & str, const git_oid & oid)
 {
     str << git_oid_tostr_s(&oid);
@@ -57,6 +58,78 @@ bool operator == (const git_oid & oid1, const git_oid & oid2)
     return git_oid_equal(&oid1, &oid2);
 }
 
+namespace {
+
+int matchesDotPlusGit(const std::string& str) {
+    // String must have at least 4 characters (at least one '.' plus "git")
+    if (str.size() < 4) {
+        return 0;
+    }
+
+    // Count consecutive dots at the beginning
+    size_t dotCount = 0;
+    while (dotCount < str.size() && str[dotCount] == '.') {
+        dotCount++;
+    }
+
+    // Must have at least one dot
+    if (dotCount == 0) {
+        return 0;
+    }
+
+    // After the dots, check if the remaining string is exactly "git"
+    if ((str.size() == dotCount + 3) &&
+           (str[dotCount] == 'g') &&
+           (str[dotCount + 1] == 'i') &&
+           (str[dotCount + 2] == 't')) {
+        return dotCount;
+    }
+    return 0;
+}
+
+std::string escapeDotGit(const std::string& filename) {
+    // Check if this filename matches the pattern of dots followed by "git"
+    int dotCount = matchesDotPlusGit(filename);
+    if (dotCount == 0) {
+        // Not a dot+git pattern, return as is
+        return filename;
+    }
+
+    std::string result(dotCount * 2, '.'); // String with 2*dotCount dots
+    result += "git";
+
+    return result;
+}
+
+std::string unescapeDotGit(const std::string filename) {
+    // Check if this filename matches the pattern of dots followed by "git"
+    int dotCount = matchesDotPlusGit(filename);
+    // Ensure dots are even for unescaping (must be divisible by 2)
+    if (dotCount == 0 || dotCount % 2 != 0) {
+        // Can't unescape an odd number of dots, return as is
+        return filename;
+    }
+
+    // Create a new string with half the dots plus "git"
+    std::string result(dotCount / 2, '.'); // String with dotCount/2 dots
+    result += "git";
+
+    return result;
+}
+
+const git_tree_entry* gitTreebuilderGet(git_treebuilder *bld, std::string name)
+{
+    auto escapedName = escapeDotGit(name);
+    return git_treebuilder_get(bld, escapedName.c_str());
+}
+
+const std::string gitTreeEntryName(const git_tree_entry *entry)
+{
+    auto escapedName = git_tree_entry_name(entry);
+    return unescapeDotGit(escapedName);
+}
+}
+
 namespace nix {
 
 struct GitSourceAccessor;
@@ -740,7 +813,7 @@ struct GitSourceAccessor : SourceAccessor
                 for (size_t n = 0; n < count; ++n) {
                     auto entry = git_tree_entry_byindex(tree.get(), n);
                     // FIXME: add to cache
-                    res.emplace(std::string(git_tree_entry_name(entry)), DirEntry{});
+                    res.emplace(std::string(gitTreeEntryName(entry)), DirEntry{});
                 }
 
                 return res;
@@ -799,7 +872,7 @@ struct GitSourceAccessor : SourceAccessor
             if (git_tree_entry_dup(Setter(copy), entry))
                 throw Error("dupping tree entry: %s", git_error_last()->message);
 
-            auto entryName = std::string_view(git_tree_entry_name(entry));
+            auto entryName = gitTreeEntryName(entry);
 
             if (entryName == name)
                 res = copy.get();
@@ -970,6 +1043,7 @@ struct GitExportIgnoreSourceAccessor : CachingFilteringSourceAccessor {
         return !isExportIgnored(path);
     }
 
+
 };
 
 struct GitFileSystemObjectSinkImpl : GitFileSystemObjectSink
@@ -990,7 +1064,7 @@ struct GitFileSystemObjectSinkImpl : GitFileSystemObjectSink
         Tree prevTree = nullptr;
 
         if (!pendingDirs.empty() &&
-            (entry = git_treebuilder_get(pendingDirs.back().builder.get(), name.c_str())))
+            (entry = gitTreebuilderGet(pendingDirs.back().builder.get(), name)))
         {
             /* Clone a tree that we've already finished. This happens
                if a tarball has directory entries that are not
@@ -1028,7 +1102,8 @@ struct GitFileSystemObjectSinkImpl : GitFileSystemObjectSink
     {
         assert(!pendingDirs.empty());
         auto & pending = pendingDirs.back();
-        if (git_treebuilder_insert(nullptr, pending.builder.get(), name.c_str(), &oid, mode))
+        auto escapedName = escapeDotGit(name);
+        if (git_treebuilder_insert(nullptr, pending.builder.get(), escapedName.c_str(), &oid, mode))
             throw Error("adding a file to a tree builder: %s", git_error_last()->message);
     };
 
@@ -1159,7 +1234,7 @@ struct GitFileSystemObjectSinkImpl : GitFileSystemObjectSink
         for (auto & c : CanonPath(relTargetLeft)) {
             if (auto builder = std::get_if<git_treebuilder *>(&curDir)) {
                 assert(*builder);
-                if (!(entry = git_treebuilder_get(*builder, std::string(c).c_str())))
+                if (!(entry = gitTreebuilderGet(*builder, std::string(c))))
                     throw Error("cannot find hard link target '%s' for path '%s'", target, path);
                 curDir = *git_tree_entry_id(entry);
             } else if (auto oid = std::get_if<git_oid>(&curDir)) {