libpq_system_ca_fix_v3.diff

diff --git a/src/interfaces/libpq/fe-secure-openssl.c b/src/interfaces/libpq/fe-secure-openssl.c
index 00b203cbfa..a2cc612f4b 100644
--- a/src/interfaces/libpq/fe-secure-openssl.c
+++ b/src/interfaces/libpq/fe-secure-openssl.c
@@ -1489,10 +1489,12 @@ open_client_SSL(PGconn *conn)
 {
 	int			r;
 
+	SOCK_ERRNO_SET(0);
 	ERR_clear_error();
 	r = SSL_connect(conn->ssl);
 	if (r <= 0)
 	{
+		int			save_errno = SOCK_ERRNO;
 		int			err = SSL_get_error(conn->ssl, r);
 		unsigned long ecode;
 
@@ -1508,10 +1510,22 @@ open_client_SSL(PGconn *conn)
 			case SSL_ERROR_SYSCALL:
 				{
 					char		sebuf[PG_STRERROR_R_BUFLEN];
-
-					if (r == -1)
+					unsigned long vcode;
+
+					vcode = SSL_get_verify_result(conn->ssl);
+
+					/*
+					 * If we get an X509 error here without an error in the
+					 * socket layer it means that verification failed without
+					 * it being a protocol error. A common cause is trying to
+					 * a default system CA which is missing or broken.
+					 */
+					if (!save_errno && vcode != X509_V_OK)
+						libpq_append_conn_error(conn, "SSL error: certificate verify failed: %s",
+												X509_verify_cert_error_string(ecode));
+					else if (r == -1)
 						libpq_append_conn_error(conn, "SSL SYSCALL error: %s",
-										  SOCK_STRERROR(SOCK_ERRNO, sebuf, sizeof(sebuf)));
+										  SOCK_STRERROR(save_errno, sebuf, sizeof(sebuf)));
 					else
 						libpq_append_conn_error(conn, "SSL SYSCALL error: EOF detected");
 					pgtls_close(conn);