add DirectoryIterator to re-throw std::filesystem::filesystem_error

Co-authored-by: Sergei Zimmerman <145775305+xokdvium@users.noreply.github.com>

src/libutil-tests/file-system.cc

@@ -297,4 +297,25 @@ TEST(chmodIfNeeded, nonexistent)
     ASSERT_THROW(chmodIfNeeded("/schnitzel/darmstadt/pommes", 0755), SysError);
 }
 
+/* ----------------------------------------------------------------------------
+ * DirectoryIterator
+ * --------------------------------------------------------------------------*/
+
+TEST(DirectoryIterator, works)
+{
+    auto tmpDir = nix::createTempDir();
+    nix::AutoDelete delTmpDir(tmpDir, true);
+
+    nix::writeFile(tmpDir + "/somefile", "");
+
+    for (auto path : DirectoryIterator(tmpDir)) {
+        ASSERT_EQ(path.path().string(), tmpDir + "/somefile");
+    }
+}
+
+TEST(DirectoryIterator, nonexistent)
+{
+    ASSERT_THROW(DirectoryIterator("/schnitzel/darmstadt/pommes"), SysError);
+}
+
 }

src/libutil/file-system.cc

@@ -43,6 +43,34 @@ namespace fs {
   }
 }
 
+DirectoryIterator::DirectoryIterator(const std::filesystem::path& p) {
+    try {
+        // **Attempt to create the underlying directory_iterator**
+        it_ = std::filesystem::directory_iterator(p);
+    } catch (const std::filesystem::filesystem_error& e) {
+        // **Catch filesystem_error and throw SysError**
+        // Adapt the error message as needed for SysError
+        throw SysError("cannot read directory %s", p);
+    }
+}
+
+DirectoryIterator& DirectoryIterator::operator++() {
+    // **Attempt to increment the underlying iterator**
+    std::error_code ec;
+    it_.increment(ec);
+    if (ec) {
+        // Try to get path info if possible, might fail if iterator is bad
+        try {
+            if (it_ != std::filesystem::directory_iterator{}) {
+                throw SysError("cannot read directory past %s: %s", it_->path(), ec.message());
+            }
+        } catch (...) {
+            throw SysError("cannot read directory");
+        }
+    }
+    return *this;
+}
+
 bool isAbsolute(PathView path)
 {
     return fs::path { path }.is_absolute();

src/libutil/include/nix/util/file-system.hh

@@ -376,4 +376,64 @@ extern PathFilter defaultPathFilter;
  */
 bool chmodIfNeeded(const std::filesystem::path & path, mode_t mode, mode_t mask = S_IRWXU | S_IRWXG | S_IRWXO);
 
+/**
+  * @brief A directory iterator that can be used to iterate over the
+  * contents of a directory. It is similar to std::filesystem::directory_iterator
+  * but throws NixError on failure instead of std::filesystem::filesystem_error.
+  */
+class DirectoryIterator {
+public:
+    // --- Iterator Traits ---
+    using iterator_category = std::input_iterator_tag;
+    using value_type        = std::filesystem::directory_entry;
+    using difference_type   = std::ptrdiff_t;
+    using pointer           = const std::filesystem::directory_entry*;
+    using reference         = const std::filesystem::directory_entry&;
+
+    // Default constructor (represents end iterator)
+    DirectoryIterator() noexcept = default;
+
+    // Constructor taking a path
+    explicit DirectoryIterator(const std::filesystem::path& p);
+
+    reference operator*() const {
+        // Accessing the value itself doesn't typically throw filesystem_error
+        // after successful construction/increment, but underlying operations might.
+        // If directory_entry methods called via -> could throw, add try-catch there.
+        return *it_;
+    }
+
+    pointer operator->() const {
+        return &(*it_);
+    }
+
+
+    DirectoryIterator& operator++();
+
+    // Postfix increment operator
+    DirectoryIterator operator++(int) {
+        DirectoryIterator temp = *this;
+        ++(*this); // Uses the prefix increment's try-catch logic
+        return temp;
+    }
+
+    // Equality comparison
+    friend bool operator==(const DirectoryIterator& a, const DirectoryIterator& b) noexcept {
+        return a.it_ == b.it_;
+    }
+
+    // Inequality comparison
+    friend bool operator!=(const DirectoryIterator& a, const DirectoryIterator& b) noexcept {
+        return !(a == b);
+    }
+
+    // Allow direct use in range-based for loops if iterating over an instance
+    DirectoryIterator begin() const { return *this; }
+    DirectoryIterator end() const { return DirectoryIterator{}; }
+
+
+private:
+    std::filesystem::directory_iterator it_;
+};
+
 }