[libc++] use copy_file_range for fs::copy_file

opportunistically use copy_file_range (Linux, FreeBSD) where possible.
This allows for fast copies via reflinks,
and server side copies for network filesystems.
Fall back to sendfile if not supported.

Signed-off-by: Jannik Glückert <[email protected]>

Author: Jannik2099

libcxx/src/CMakeLists.txt

@@ -173,6 +173,12 @@ if (APPLE AND LLVM_USE_SANITIZER)
   endif()
 endif()
 
+include(CheckCXXSymbolExists)
+check_cxx_symbol_exists("copy_file_range" "unistd.h" LIBCXX_USE_COPY_FILE_RANGE)
+if(LIBCXX_USE_COPY_FILE_RANGE)
+  list(APPEND LIBCXX_COMPILE_FLAGS "-D_LIBCPP_FILESYSTEM_USE_COPY_FILE_RANGE")
+endif()
+
 split_list(LIBCXX_COMPILE_FLAGS)
 split_list(LIBCXX_LINK_FLAGS)
 

libcxx/src/filesystem/operations.cpp

@@ -32,6 +32,7 @@
 #  include <dirent.h>
 #  include <sys/stat.h>
 #  include <sys/statvfs.h>
+#  include <sys/types.h>
 #  include <unistd.h>
 #endif
 #include <fcntl.h> /* values for fchmodat */
@@ -178,8 +179,35 @@ void __copy(const path& from, const path& to, copy_options options, error_code*
 namespace detail {
 namespace {
 
+#if defined(_LIBCPP_FILESYSTEM_USE_COPY_FILE_RANGE)
+bool copy_file_impl_copy_file_range(FileDescriptor& read_fd, FileDescriptor& write_fd, error_code& ec) {
+  size_t count = read_fd.get_stat().st_size;
+  // a zero-length file is either empty, or not copyable by this syscall
+  // return early to avoid the syscall cost
+  if (count == 0) {
+    ec = {EINVAL, generic_category()};
+    return false;
+  }
+  // do not modify the fd positions as copy_file_impl_sendfile may be called after a partial copy
+  off_t off_in  = 0;
+  off_t off_out = 0;
+  do {
+    ssize_t res;
+
+    if ((res = ::copy_file_range(read_fd.fd, &off_in, write_fd.fd, &off_out, count, 0)) == -1) {
+      ec = capture_errno();
+      return false;
+    }
+    count -= res;
+  } while (count > 0);
+
+  ec.clear();
+
+  return true;
+}
+#endif
 #if defined(_LIBCPP_FILESYSTEM_USE_SENDFILE)
-bool copy_file_impl(FileDescriptor& read_fd, FileDescriptor& write_fd, error_code& ec) {
+bool copy_file_impl_sendfile(FileDescriptor& read_fd, FileDescriptor& write_fd, error_code& ec) {
   size_t count = read_fd.get_stat().st_size;
   do {
     ssize_t res;
@@ -194,6 +222,34 @@ bool copy_file_impl(FileDescriptor& read_fd, FileDescriptor& write_fd, error_cod
 
   return true;
 }
+#endif
+
+#if defined(_LIBCPP_FILESYSTEM_USE_SENDFILE)
+bool copy_file_impl(FileDescriptor& read_fd, FileDescriptor& write_fd, error_code& ec) {
+#  if defined(_LIBCPP_FILESYSTEM_USE_COPY_FILE_RANGE)
+  if (copy_file_impl_copy_file_range(read_fd, write_fd, ec)) {
+    return true;
+  }
+  // EINVAL: src and dst are the same file (this is not cheaply
+  // detectable from userspace)
+  // EINVAL: copy_file_range is unsupported for this file type by the
+  // underlying filesystem
+  // ENOTSUP: undocumented, can arise with old kernels and NFS
+  // EOPNOTSUPP: filesystem does not implement copy_file_range
+  // ETXTBSY: src or dst is an active swapfile (nonsensical, but allowed
+  // with normal copying)
+  // EXDEV: src and dst are on different filesystems that do not support
+  // cross-fs copy_file_range
+  // ENOENT: undocumented, can arise with CIFS
+  // ENOSYS: unsupported by kernel or blocked by seccomp
+  if (ec.value() != EINVAL && ec.value() != ENOTSUP && ec.value() != EOPNOTSUPP && ec.value() != ETXTBSY &&
+      ec.value() != EXDEV && ec.value() != ENOENT && ec.value() != ENOSYS) {
+    return false;
+  }
+  ec.clear();
+#  endif
+  return copy_file_impl_sendfile(read_fd, write_fd, ec);
+}
 #elif defined(_LIBCPP_FILESYSTEM_USE_COPYFILE)
 bool copy_file_impl(FileDescriptor& read_fd, FileDescriptor& write_fd, error_code& ec) {
   struct CopyFileState {