[PATCH] rely on String#-@ (str_uminus) to dedupe headers

[Eric Wong <e@80x24.org>]

At least for Ruby 2.5.0 (due Dec 2017), this will reduce memory
overhead when dealing with common headers.  There is a small
behavior change for people using Arrays, as they get deduplicate
strings; so we'll need a version bump.
---
 ext/kcar/extconf.rb | 11 +++++++++++
 ext/kcar/kcar.rl    | 17 +++++++++++++----
 test/test_parser.rb | 17 +++++++++++++++++
 3 files changed, 41 insertions(+), 4 deletions(-)

diff --git a/ext/kcar/extconf.rb b/ext/kcar/extconf.rb
index 218c2e3..89ac586 100644
--- a/ext/kcar/extconf.rb
+++ b/ext/kcar/extconf.rb
@@ -6,4 +6,15 @@ dir_config("kcar")
 have_macro("SIZEOF_OFF_T", "ruby.h") or check_sizeof("off_t", "sys/types.h")
 have_macro("SIZEOF_LONG", "ruby.h") or check_sizeof("long", "sys/types.h")
 
+uminus_dedupe = false
+begin
+  # oddly, opt_str_freeze is not always effective:
+  # https://bugs.ruby-lang.org/issues/13282
+  a = -(%w(t e s t).join)
+  b = -(%w(t e s t).join)
+  uminus_dedupe = a.object_id == b.object_id
+rescue NoMethodError
+end
+$CFLAGS += " -DSTR_UMINUS_DEDUPE=#{uminus_dedupe ? 1 : 0}"
+
 create_makefile("kcar_ext")
diff --git a/ext/kcar/kcar.rl b/ext/kcar/kcar.rl
index e015bdb..cbffb7e 100644
--- a/ext/kcar/kcar.rl
+++ b/ext/kcar/kcar.rl
@@ -14,7 +14,7 @@
 #include "c_util.h"
 
 static VALUE eParserError;
-static ID id_sq, id_sq_set;
+static ID id_uminus, id_sq, id_sq_set;
 
 /** Defines common length and error messages for input length validation. */
 #define DEF_MAX_LENGTH(N, length) \
@@ -95,6 +95,15 @@ static int is_lws(char c)
   return (c == ' ' || c == '\t');
 }
 
+/* this will dedupe under Ruby 2.5+ (December 2017) */
+static VALUE str_dd_freeze(VALUE str)
+{
+  if (STR_UMINUS_DEDUPE)
+    return rb_funcall(str, id_uminus, 0);
+  OBJ_FREEZE(str);
+  return str;
+}
+
 static VALUE stripped_str_new(const char *str, long len)
 {
   long end;
@@ -150,7 +159,7 @@ status_phrase(struct http_parser *hp, VALUE hdr, const char *ptr, size_t len)
 {
   long nr;
 
-  hp->status = rb_str_new(ptr, len);
+  hp->status = str_dd_freeze(rb_str_new(ptr, len));
 
   /* RSTRING_PTR is null terminated, ptr is not */
   nr = strtol(RSTRING_PTR(hp->status), NULL, 10);
@@ -227,7 +236,7 @@ static void write_value(VALUE hdr, struct http_parser *hp,
   vlen = LEN(mark, p);
   VALIDATE_MAX_LENGTH(vlen, FIELD_VALUE);
   VALIDATE_MAX_LENGTH(flen, FIELD_NAME);
-  f = rb_str_new(fptr, (long)flen);
+  f = str_dd_freeze(rb_str_new(fptr, (long)flen));
   v = stripped_str_new(vptr, (long)vlen);
 
   /* needs more tests for error-checking here */
@@ -286,7 +295,6 @@ static void write_value(VALUE hdr, struct http_parser *hp,
 
     if (NIL_P(e)) {
       /* new value, freeze it since it speeds up MRI slightly */
-      OBJ_FREEZE(f);
 
       if (hclass == rb_cHash)
         rb_hash_aset(hdr, f, v);
@@ -740,4 +748,5 @@ void Init_kcar_ext(void)
   rb_define_const(cParser, "LENGTH_MAX", OFFT2NUM(UH_OFF_T_MAX));
   id_sq = rb_intern("[]");
   id_sq_set = rb_intern("[]=");
+  id_uminus = rb_intern("-@");
 }
diff --git a/test/test_parser.rb b/test/test_parser.rb
index c0a05f1..3768808 100644
--- a/test/test_parser.rb
+++ b/test/test_parser.rb
@@ -303,4 +303,21 @@ class TestParser < Test::Unit::TestCase
   rescue LoadError
     warn 'ObjectSpace not available'
   end
+
+  def test_uminus_dd
+    # oddly, opt_str_freeze is not always effective:
+    # https://bugs.ruby-lang.org/issues/13282
+    a = -(%w(H o s t).join)
+    b = -(%w(H o s t).join)
+    if a.object_id == b.object_id
+      resp = "HTTP/1.1 200 OK\r\nHost: example.com\r\n\r\n"
+      assert @hp.headers(e = {}, resp.dup)
+      @hp.reset
+      assert @hp.headers(f = {}, resp.dup)
+      assert_same e.keys[0], f.keys[0]
+      assert_same a, e.keys[0]
+    else
+      warn "String#-@ does not dedupe with #{RUBY_ENGINE}-#{RUBY_VERSION}"
+    end
+  end
 end
-- 
EW