Do not fingerprint if filename contains a valid digest

CHANGELOG.md

@@ -4,6 +4,7 @@ Get upgrade notes from Sprockets 3.x to 4.x at https://github.com/rails/sprocket
 
 ## Master
 
+- Do not fingerprint files that already contain a valid digest in their name
 - Remove remaining support for Ruby < 2.4.[#672](https://github.com/rails/sprockets/pull/672)
 
 ## 4.0.2

lib/sprockets/asset.rb

@@ -64,7 +64,11 @@ def to_hash
     #
     # Returns String.
     def digest_path
-      logical_path.sub(/\.(\w+)$/) { |ext| "-#{etag}#{ext}" }
+      if DigestUtils.already_digested?(@name)
+        logical_path
+      else
+        logical_path.sub(/\.(\w+)$/) { |ext| "-#{etag}#{ext}" }
+      end
     end
 
     # Public: Return load path + logical path with digest spliced in.

lib/sprockets/digest_utils.rb

@@ -179,6 +179,19 @@ def hexdigest_integrity_uri(hexdigest)
       integrity_uri(unpack_hexdigest(hexdigest))
     end
 
+    # Internal: Checks an asset name for a valid digest
+    #
+    # name - The name of the asset
+    #
+    # Returns true if the name contains a digest recognized by one of the valid digest classes
+    def already_digested?(name)
+      return if name.nil?
+
+      if hexdigest = name.scan(/[a-fA-F0-9]+\z/).last
+        detect_digest_class(unpack_hexdigest(hexdigest))
+      end
+    end
+
     private
       def build_digest(obj)
         digest = digest_class.new

test/test_asset.rb

@@ -1124,6 +1124,32 @@ def read(logical_path)
   end
 end
 
+class PreDigestedAssetTest < Sprockets::TestCase
+  def setup
+    @env = Sprockets::Environment.new
+    @env.append_path(fixture_path('asset'))
+    @env.cache = {}
+
+    @pipeline = nil
+  end
+
+  test "digest path" do
+    path     = File.expand_path("test/fixtures/asset/application")
+    original = "#{path}.js"
+    digested = "#{path}-d41d8cd98f00b204e9800998ecf8427e.js"
+    FileUtils.cp(original, digested)
+
+    assert_equal "application-d41d8cd98f00b204e9800998ecf8427e.js",
+      asset("application-d41d8cd98f00b204e9800998ecf8427e.js").digest_path
+  ensure
+    FileUtils.rm(digested) if File.exists?(digested)
+  end
+
+  def asset(logical_path, options = {})
+    @env.find_asset(logical_path, **{pipeline: @pipeline}.merge(options))
+  end
+end
+
 
 class DebugAssetTest < Sprockets::TestCase
   def setup

test/test_digest_utils.rb

@@ -94,4 +94,17 @@ def test_hexdigest_integrity_uri
     assert_equal "sha512-+uuYUxxe7oWIShQrWEmMn/fixz/rxDP4qcAZddXLDM3nN8/tpk1ZC2jXQk6N+mXE65jwfzNVUJL/qjA3y9KbuQ==",
       hexdigest_integrity_uri(sha512)
   end
+
+  def test_already_digested
+    refute already_digested?(nil)
+    refute already_digested?("application-d41d8cd98f00b204e9800998ecf8427z")
+    refute already_digested?("application-d41d8cd98f00b204e9800998ecf8427ef")
+    refute already_digested?("application-d41d8cd98f00b204e9800998ecf8427e-")
+
+    assert already_digested?("application-" + Digest::MD5.new.hexdigest)
+    assert already_digested?("application-" + Digest::SHA1.new.hexdigest)
+    assert already_digested?("application-" + Digest::SHA256.new.hexdigest)
+    assert already_digested?("application-" + Digest::SHA384.new.hexdigest)
+    assert already_digested?("application-" + Digest::SHA512.new.hexdigest)
+  end
 end