Fix File.Stream Enumerable.count for files without trailing newline (#15147)

The optimized count implementation was counting newline characters
instead of actual lines, returning incorrect results for files that
don't end with a newline (which is common for source code files).

For example, a file with content "line1\nline2\nline3" (3 lines)
was returning 2 instead of 3.

The fix tracks the last byte read and adds 1 at EOF if the file
has content but doesn't end with a newline.

Author: pnezis

lib/elixir/lib/file/stream.ex

@@ -119,7 +119,7 @@ defmodule File.Stream do
 
       counter = fn device ->
         device = skip_bom_and_offset(device, raw, modes)
-        count_lines(device, path, pattern, read_function(stream), 0)
+        count_lines(device, path, pattern, read_function(stream), 0, :empty)
       end
 
       {:ok, open!(stream, modes, counter)}
@@ -229,21 +229,28 @@ defmodule File.Stream do
       for mode <- modes, mode not in [:write, :append, :trim_bom], do: mode
     end
 
-    defp count_lines(device, path, pattern, read, count) do
+    defp count_lines(device, path, pattern, read, count, last_byte) do
       case read.(device) do
+        data when is_binary(data) and byte_size(data) > 0 ->
+          newlines = length(:binary.matches(data, pattern))
+          last = :binary.last(data)
+          count_lines(device, path, pattern, read, count + newlines, last)
+
         data when is_binary(data) ->
-          count_lines(device, path, pattern, read, count + count_lines(data, pattern))
+          count_lines(device, path, pattern, read, count, last_byte)
 
         :eof ->
-          count
+          case last_byte do
+            :empty -> 0
+            ?\n -> count
+            _ -> count + 1
+          end
 
         {:error, reason} ->
           raise File.Error, reason: reason, action: "stream", path: path
       end
     end
 
-    defp count_lines(data, pattern), do: length(:binary.matches(data, pattern))
-
     defp read_function(%{raw: true}), do: &IO.binread(&1, @read_ahead_size)
     defp read_function(%{raw: false}), do: &IO.read(&1, @read_ahead_size)
   end

lib/elixir/test/elixir/file/stream_test.exs

@@ -65,6 +65,34 @@ defmodule File.StreamTest do
         assert Enum.count(stream) == 2
       end
 
+      test "counts lines without trailing newline" do
+        no_trailing = tmp_path("no_trailing.txt")
+        single_line = tmp_path("single_line.txt")
+        empty_file = tmp_path("empty.txt")
+
+        try do
+          File.write!(no_trailing, "line1\nline2\nline3")
+          File.write!(single_line, "hello")
+          File.write!(empty_file, "")
+
+          # 3 lines, no trailing newline
+          stream = stream!(@node, no_trailing)
+          assert Enum.count(stream) == 3
+
+          # 1 line, no newline at all
+          stream = stream!(@node, single_line)
+          assert Enum.count(stream) == 1
+
+          # empty file
+          stream = stream!(@node, empty_file)
+          assert Enum.count(stream) == 0
+        after
+          File.rm(no_trailing)
+          File.rm(single_line)
+          File.rm(empty_file)
+        end
+      end
+
       test "reads and writes lines" do
         src = fixture_path("file.txt")
         dest = tmp_path("tmp_test.txt")