Intern column names so we always get the same string

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 +364,21 @@ 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);
+    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 +397,7 @@ 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 +48,23 @@ 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_predicate columns[i], :frozen?
+        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(?)")