Preserve input maps when concatenating source maps

CHANGELOG.md

@@ -1,5 +1,7 @@
 **Master**
 
+- Preserve input maps when concatenating source maps. [Oskar Eichler](https://github.com/OskarEichler)
+
 Get upgrade notes from Sprockets 3.x to 4.x at https://github.com/rails/sprockets/blob/master/UPGRADING.md
 
 # 4.4.1

README.md

@@ -714,3 +714,5 @@ Please see the [CHANGELOG](https://github.com/rails/sprockets/tree/master/CHANGE
 
 ## License
 Sprockets is released under the [MIT License](MIT-LICENSE).
+
+Source map concatenation preserves both inputs so repeated concatenation retains the same line offsets.

lib/sprockets/source_map_utils.rb

@@ -72,12 +72,16 @@ def format_source_map(map, input)
     # Returns a new source map hash.
     def concat_source_maps(a, b)
       return a || b unless a && b
-      a = make_index_map(a)
+      a = make_index_map(a).dup
+      a["sections"] = a["sections"].dup
       b = make_index_map(b)
 
       offset = 0
       if a["sections"].count != 0 && !a["sections"].last["map"]["mappings"].empty?
-        last_line_count = a["sections"].last["map"].delete("x_sprockets_linecount")
+        last_section = a["sections"].last.dup
+        last_section["map"] = last_section["map"].dup
+        a["sections"][-1] = last_section
+        last_line_count = last_section["map"].delete("x_sprockets_linecount")
         offset += last_line_count || 1
 
         last_offset = a["sections"].last["offset"]["line"]