https://git.ruby-lang.org/ruby.git/commit/?id=5e0479f26a

From 5e0479f26afe1505afd9014ea96a206a88845828 Mon Sep 17 00:00:00 2001
From: Nobuyoshi Nakada <nobu@r...>
Date: Tue, 1 Oct 2019 12:23:25 +0900
Subject: ENV.update should not call block on existing keys

[Bug #16192]

diff --git a/hash.c b/hash.c
index e11afb0..6b3f844 100644
--- a/hash.c
+++ b/hash.c
@@ -5931,8 +5931,16 @@ env_replace(VALUE env, VALUE hash) https://github.com/ruby/ruby/blob/trunk/hash.c#L5931
 static int
 env_update_i(VALUE key, VALUE val, VALUE _)
 {
-    if (rb_block_given_p()) {
-	val = rb_yield_values(3, key, rb_f_getenv(Qnil, key), val);
+    env_aset(key, val);
+    return ST_CONTINUE;
+}
+
+static int
+env_update_block_i(VALUE key, VALUE val, VALUE _)
+{
+    VALUE oldval = rb_f_getenv(Qnil, key);
+    if (!NIL_P(oldval)) {
+	val = rb_yield_values(3, key, oldval, val);
     }
     env_aset(key, val);
     return ST_CONTINUE;
@@ -5955,7 +5963,9 @@ env_update(VALUE env, VALUE hash) https://github.com/ruby/ruby/blob/trunk/hash.c#L5963
 {
     if (env == hash) return env;
     hash = to_hash(hash);
-    rb_hash_foreach(hash, env_update_i, 0);
+    rb_foreach_func *func = rb_block_given_p() ?
+        env_update_block_i : env_update_i;
+    rb_hash_foreach(hash, func, 0);
     return env;
 }
 
diff --git a/test/ruby/test_env.rb b/test/ruby/test_env.rb
index d9301ff..ffce9b7 100644
--- a/test/ruby/test_env.rb
+++ b/test/ruby/test_env.rb
@@ -445,7 +445,7 @@ class TestEnv < Test::Unit::TestCase https://github.com/ruby/ruby/blob/trunk/test/ruby/test_env.rb#L445
     ENV.clear
     ENV["foo"] = "bar"
     ENV["baz"] = "qux"
-    ENV.update({"baz"=>"quux","a"=>"b"}) {|k, v1, v2| v1 ? k + "_" + v1 + "_" + v2 : v2 }
+    ENV.update({"baz"=>"quux","a"=>"b"}) {|k, v1, v2| k + "_" + v1 + "_" + v2 }
     check(ENV.to_hash.to_a, [%w(foo bar), %w(baz baz_qux_quux), %w(a b)])
   end
 
-- 
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