Update chain building function.

Don't clear verification errors from the error queue unless
SSL_BUILD_CHAIN_FLAG_CLEAR_ERROR is set.

If errors occur during verification and SSL_BUILD_CHAIN_FLAG_IGNORE_ERROR
is set return 2 so applications can issue warnings.
(cherry picked from commit 2dd6976f6d02f98b30c376951ac38f780a86b3b5)

doc/ssl/SSL_CTX_add1_chain_cert.pod

@@ -60,7 +60,9 @@ existing chain certificates as untrusted CAs, B<SSL_BUILD_CHAIN_FLAG_NO_ROOT>
 to omit the root CA from the built chain, B<SSL_BUILD_CHAIN_FLAG_CHECK> to
 use all existing chain certificates only to build the chain (effectively
 sanity checking and rearranging them if necessary), the flag
-B<SSL_BUILD_CHAIN_FLAG_IGNORE_ERROR> ignores any errors during verification.
+B<SSL_BUILD_CHAIN_FLAG_IGNORE_ERROR> ignores any errors during verification:
+if flag B<SSL_BUILD_CHAIN_FLAG_CLEAR_ERROR> is also set verification errors
+are cleared from the error queue.
 
 Each of these functions operates on the I<current> end entity
 (i.e. server or client) certificate. This is the last certificate loaded or
@@ -113,6 +115,10 @@ For example an application can add any set of certificates using
 SSL_CTX_use_certificate_chain_file() then call SSL_CTX_build_cert_chain()
 with the option B<SSL_BUILD_CHAIN_FLAG_CHECK> to check and reorder them.
 
+Applications can issue non fatal warnings when checking chains by setting
+the flag B<SSL_BUILD_CHAIN_FLAG_IGNORE_ERRORS> and checking the return
+value.
+
 Calling SSL_CTX_build_cert_chain() or SSL_build_cert_chain() is more
 efficient than the automatic chain building as it is only performed once.
 Automatic chain building is performed on each new session.
@@ -126,6 +132,10 @@ SSL_set_current_cert() with B<SSL_CERT_SET_SERVER> return 1 for success, 2 if
 no server certificate is used because the ciphersuites is anonymous and 0
 for failure.
 
+SSL_CTX_build_cert_chain() and SSL_build_cert_chain() return 1 for success
+and 0 for failure. If the flag B<SSL_BUILD_CHAIN_FLAG_IGNORE_ERROR> and
+a verification error occurs then 2 is returned.
+
 All other functions return 1 for success and 0 for failure.
 
 =head1 SEE ALSO

ssl/ssl.h

@@ -791,6 +791,8 @@ struct ssl_session_st
 #define SSL_BUILD_CHAIN_FLAG_CHECK		0x4
 /* Ignore verification errors */
 #define SSL_BUILD_CHAIN_FLAG_IGNORE_ERROR	0x8
+/* Clear verification errors from queue */
+#define SSL_BUILD_CHAIN_FLAG_CLEAR_ERROR	0x10
 
 /* Flags returned by SSL_check_chain */
 /* Certificate can be used with this session */

ssl/ssl_cert.c

@@ -1247,8 +1247,10 @@ int ssl_build_cert_chain(CERT *c, X509_STORE *chain_store, int flags)
 	i = X509_verify_cert(&xs_ctx);
 	if (i <= 0 && flags & SSL_BUILD_CHAIN_FLAG_IGNORE_ERROR)
 		{
-		ERR_clear_error();
+		if (flags & SSL_BUILD_CHAIN_FLAG_CLEAR_ERROR)
+			ERR_clear_error();
 		i = 1;
+		rv = 2;
 		}
 	if (i > 0)
 		chain = X509_STORE_CTX_get1_chain(&xs_ctx);
@@ -1283,7 +1285,8 @@ int ssl_build_cert_chain(CERT *c, X509_STORE *chain_store, int flags)
 			}
 		}
 	cpk->chain = chain;
-	rv = 1;
+	if (rv == 0)
+		rv = 1;
 	err:
 	if (flags & SSL_BUILD_CHAIN_FLAG_CHECK)
 		X509_STORE_free(chain_store);