Intern column names so we always get the same string

When we get column names back from the database, it's very common to
always return the same strings. This patch uses Ruby's "interned string"
API so that we're always getting the same string objects back from the database.

Fixes: #155

ext/sqlite3/extconf.rb

@@ -109,6 +109,10 @@ def configure_extension
 
         abort_could_not_find(libname) unless find_library(libname, "sqlite3_libversion_number", "sqlite3.h")
 
+        # Truffle Ruby doesn't support this yet:
+        # https://github.com/oracle/truffleruby/issues/3408
+        have_func("rb_enc_interned_str_cstr")
+
         # Functions defined in 1.9 but not 1.8
         have_func("rb_proc_arity")
 

ext/sqlite3/statement.c

@@ -364,6 +364,22 @@ column_count(VALUE self)
     return INT2NUM(sqlite3_column_count(ctx->st));
 }
 
+#if HAVE_RB_ENC_INTERNED_STR_CSTR
+static VALUE
+interned_utf8_cstr(const char * str)
+{
+    return rb_enc_interned_str_cstr(str, rb_utf8_encoding());
+}
+#else
+static VALUE
+interned_utf8_cstr(const char * str)
+{
+    VALUE rb_str = rb_utf8_str_new_cstr(str);
+    rb_obj_freeze(rb_str);
+    return rb_funcall(rb_str, rb_intern("-@"), 0);
+}
+#endif
+
 /* call-seq: stmt.column_name(index)
  *
  * Get the column name at +index+.  0 based.
@@ -382,8 +398,7 @@ column_name(VALUE self, VALUE index)
     VALUE ret = Qnil;
 
     if (name) {
-        ret = SQLITE3_UTF8_STR_NEW2(name);
-        rb_obj_freeze(ret);
+        ret = interned_utf8_cstr(name);
     }
     return ret;
 }

test/test_statement.rb

@@ -48,6 +48,22 @@ def test_raises_type_error
       end
     end
 
+    def test_column_names_are_deduped
+      @db.execute "CREATE TABLE 'things' ('float' float, 'int' int, 'text' blob, 'string' string, 'nil' string)"
+      stmt = @db.prepare "SELECT float, int, text, string, nil FROM things"
+      assert_equal ["float", "int", "text", "string", "nil"], stmt.columns
+      columns = stmt.columns
+      stmt.close
+
+      stmt = @db.prepare "SELECT float, int, text, string, nil FROM things"
+      # Make sure this new statement returns the same interned strings
+      stmt.columns.each_with_index do |str, i|
+        assert_same columns[i], str
+      end
+    ensure
+      stmt&.close
+    end
+
     def test_insert_duplicate_records
       @db.execute 'CREATE TABLE "things" ("name" varchar(20) CONSTRAINT "index_things_on_name" UNIQUE)'
       stmt = @db.prepare("INSERT INTO things(name) VALUES(?)")