Revert "Should return "." for File.extname("file.") also on Windows"

We want to introduce consistency and better compatibility with unixen,
but the Windows APIs doues not have consistency fundamentally and
we can not found any logical way...

This reverts commit 61aff0cd18.

file.c

@@ -4711,26 +4711,13 @@ ruby_enc_find_extname(const char *name, long *len, rb_encoding *enc)
     while (*p) {
 	if (*p == '.' || istrailinggarbage(*p)) {
 #if USE_NTFS
-	    const char *first = 0, *last, *dot;
-	    if (*p == '.') first = p;
-	    last = p++;
-	    dot = last;
+	    const char *last = p++, *dot = last;
 	    while (istrailinggarbage(*p)) {
-		if (*p == '.') {
-		    dot = p;
-		    if (!first) {
-			first = p;
-		    }
-		}
+		if (*p == '.') dot = p;
 		p++;
 	    }
 	    if (!*p || isADS(*p)) {
-		if (first == dot && e == 0) {
-		    e = first;
-		}
-		else {
-		    p = last;
-		}
+		p = last;
 		break;
 	    }
 	    if (*last == '.' || dot > last) e = dot;
@@ -4779,7 +4766,8 @@ ruby_enc_find_extname(const char *name, long *len, rb_encoding *enc)
  *     File.extname("test.rb")         #=> ".rb"
  *     File.extname("a/b/d/test.rb")   #=> ".rb"
  *     File.extname(".a/b/d/test.rb")  #=> ".rb"
- *     File.extname("foo.")            #=> "."
+ *     File.extname("foo.")            #=> "" on Windows
+ *     File.extname("foo.")            #=> "." on non-Windows
  *     File.extname("test")            #=> ""
  *     File.extname(".profile")        #=> ""
  *     File.extname(".profile.sh")     #=> ".sh"

spec/ruby/core/file/extname_spec.rb

@@ -23,14 +23,14 @@ describe "File.extname" do
   end
 
   describe "for a filename ending with a dot" do
-    ruby_version_is ""..."2.7" do
+    guard -> { platform_is :windows or ruby_version_is ""..."2.7" } do
       it "returns ''" do
         File.extname(".foo.").should == ""
         File.extname("foo.").should == ""
       end
     end
 
-    ruby_version_is "2.7" do
+    guard -> { platform_is_not :windows and ruby_version_is "2.7" } do
       it "returns '.'" do
         File.extname(".foo.").should == "."
         File.extname("foo.").should == "."

test/ruby/test_file_exhaustive.rb

@@ -1268,19 +1268,19 @@ class TestFileExhaustive < Test::Unit::TestCase
     infixes2 = infixes + [".ext "]
     appendixes = [""]
     if NTFS
-      appendixes << " " << [".", ".", ""] << "::$DATA" << "::$DATA.bar"
+      appendixes << " " << "." << "::$DATA" << "::$DATA.bar"
     else
       appendixes << [".", "."]
     end
     prefixes.each do |prefix|
-      appendixes.each do |appendix, ext = "", ext2 = ext|
+      appendixes.each do |appendix, ext = ""|
         infixes.each do |infix|
           path = "#{prefix}foo#{infix}#{appendix}"
           assert_equal(ext, File.extname(path), "File.extname(#{path.inspect})")
         end
         infixes2.each do |infix|
           path = "#{prefix}foo#{infix}.ext#{appendix}"
-          assert_equal(ext2.empty? ? ".ext" : appendix, File.extname(path), "File.extname(#{path.inspect})")
+          assert_equal(ext.empty? ? ".ext" : appendix, File.extname(path), "File.extname(#{path.inspect})")
         end
       end
     end

test/ruby/test_path.rb

@@ -239,7 +239,11 @@ class TestPath < Test::Unit::TestCase
       ext = '.'
     end
     assert_equal(ext, File.extname('a.rb.'))
-    assert_equal('.', File.extname('a.'))
+    if /mswin|bccwin|mingw/ =~ RUBY_PLATFORM
+      # trailing spaces and dots are ignored on NTFS.
+      ext = ''
+    end
+    assert_equal(ext, File.extname('a.'))
     assert_equal('', File.extname('.x'))
     assert_equal('', File.extname('..x'))
   end