ossl_pem_passwd_cb: handle nil from the block explicitly

There is code that returns nil in the passphrase block on purpose (to
prevent OpenSSL from prompting on stdin):

	OpenSSL::PKey.read(File.read("file.pem")) { nil }

This is working just by chance because the TypeError from StringValue()
is silently ignored. Let's short circuit in that case and save raising
a needless exception, as this pattern has become too common.

Author: rhenium

ext/openssl/ossl.c

@@ -148,11 +148,10 @@ ossl_pem_passwd_value(VALUE pass)
 static VALUE
 ossl_pem_passwd_cb0(VALUE flag)
 {
-    VALUE pass;
-
-    pass = rb_yield(flag);
+    VALUE pass = rb_yield(flag);
+    if (NIL_P(pass))
+	return Qnil;
     StringValue(pass);
-
     return pass;
 }
 
@@ -195,6 +194,8 @@ ossl_pem_passwd_cb(char *buf, int max_len, int flag, void *pwd_)
 	    rb_set_errinfo(Qnil);
 	    return -1;
 	}
+	if (NIL_P(pass))
+	    return -1;
 	len = RSTRING_LEN(pass);
 	if (len > max_len) {
 	    rb_warning("password must not be longer than %d bytes", max_len);

test/test_pkey_rsa.rb

@@ -248,6 +248,9 @@ def test_pem_passwd
     assert_match (/ENCRYPTED/), pem3c
     assert_equal key.to_der, OpenSSL::PKey.read(pem3c, "key").to_der
     assert_equal key.to_der, OpenSSL::PKey.read(pem3c) { "key" }.to_der
+    assert_raise(OpenSSL::PKey::PKeyError) {
+      OpenSSL::PKey.read(pem3c) { nil }
+    }
   end
 
   def test_dup