Upstream: fixed passwords support for dynamic certificates.

Passwords were not preserved in optimized SSL contexts, the bug had
appeared in d791b4aab (1.23.1), as in the following configuration:

    server {
        proxy_ssl_password_file password;
        proxy_ssl_certificate $ssl_server_name.crt;
        proxy_ssl_certificate_key $ssl_server_name.key;

        location /original/ {
            proxy_pass https://u1/;
        }

        location /optimized/ {
            proxy_pass https://u2/;
        }
    }

The fix is to always preserve passwords, by copying to the configuration
pool, if dynamic certificates are used.  This is done as part of merging
"ssl_passwords" configuration.

To minimize the number of copies, a preserved version is then used for
inheritance.  A notable exception is inheritance of preserved empty
passwords to the context with statically configured certificates:

    server {
        proxy_ssl_certificate $ssl_server_name.crt;
        proxy_ssl_certificate_key $ssl_server_name.key;

        location / {
            proxy_pass ...;

            proxy_ssl_certificate example.com.crt;
            proxy_ssl_certificate_key example.com.key;
        }
    }

In this case, an unmodified version (NULL) of empty passwords is set,
to allow reading them from the password prompt on nginx startup.

As an additional optimization, a preserved instance of inherited
configured passwords is set to the previous level, to inherit it
to other contexts:

    server {
        proxy_ssl_password_file password;

        location /1/ {
            proxy_pass https://u1/;
            proxy_ssl_certificate $ssl_server_name.crt;
            proxy_ssl_certificate_key $ssl_server_name.key;
        }

        location /2/ {
            proxy_pass https://u2/;
            proxy_ssl_certificate $ssl_server_name.crt;
            proxy_ssl_certificate_key $ssl_server_name.key;
        }
    }

src/stream/ngx_stream_proxy_module.c

@@ -108,6 +108,8 @@ static ngx_int_t ngx_stream_proxy_ssl_name(ngx_stream_session_t *s);
 static ngx_int_t ngx_stream_proxy_ssl_certificate(ngx_stream_session_t *s);
 static ngx_int_t ngx_stream_proxy_merge_ssl(ngx_conf_t *cf,
     ngx_stream_proxy_srv_conf_t *conf, ngx_stream_proxy_srv_conf_t *prev);
+static ngx_int_t ngx_stream_proxy_merge_ssl_passwords(ngx_conf_t *cf,
+    ngx_stream_proxy_srv_conf_t *conf, ngx_stream_proxy_srv_conf_t *prev);
 static ngx_int_t ngx_stream_proxy_set_ssl(ngx_conf_t *cf,
     ngx_stream_proxy_srv_conf_t *pscf);
 
@@ -2315,7 +2317,9 @@ ngx_stream_proxy_merge_srv_conf(ngx_conf_t *cf, void *parent, void *child)
     ngx_conf_merge_ptr_value(conf->ssl_certificate_cache,
                               prev->ssl_certificate_cache, NULL);
 
-    ngx_conf_merge_ptr_value(conf->ssl_passwords, prev->ssl_passwords, NULL);
+    if (ngx_stream_proxy_merge_ssl_passwords(cf, conf, prev) != NGX_OK) {
+        return NGX_CONF_ERROR;
+    }
 
     ngx_conf_merge_ptr_value(conf->ssl_conf_commands,
                               prev->ssl_conf_commands, NULL);
@@ -2381,6 +2385,57 @@ ngx_stream_proxy_merge_ssl(ngx_conf_t *cf, ngx_stream_proxy_srv_conf_t *conf,
 }
 
 
+static ngx_int_t
+ngx_stream_proxy_merge_ssl_passwords(ngx_conf_t *cf,
+    ngx_stream_proxy_srv_conf_t *conf, ngx_stream_proxy_srv_conf_t *prev)
+{
+    ngx_uint_t  preserve;
+
+    ngx_conf_merge_ptr_value(conf->ssl_passwords, prev->ssl_passwords, NULL);
+
+    if (conf->ssl_certificate == NULL
+        || conf->ssl_certificate->value.len == 0
+        || conf->ssl_certificate_key == NULL)
+    {
+        return NGX_OK;
+    }
+
+    if (conf->ssl_certificate->lengths == NULL
+        && conf->ssl_certificate_key->lengths == NULL)
+    {
+        if (conf->ssl_passwords && conf->ssl_passwords->pool == NULL) {
+            /* un-preserve empty password list */
+            conf->ssl_passwords = NULL;
+        }
+
+        return NGX_OK;
+    }
+
+    if (conf->ssl_passwords && conf->ssl_passwords->pool != cf->temp_pool) {
+        /* already preserved */
+        return NGX_OK;
+    }
+
+    preserve = (conf->ssl_passwords == prev->ssl_passwords) ? 1 : 0;
+
+    conf->ssl_passwords = ngx_ssl_preserve_passwords(cf, conf->ssl_passwords);
+    if (conf->ssl_passwords == NULL) {
+        return NGX_ERROR;
+    }
+
+    /*
+     * special handling to keep a preserved ssl_passwords copy
+     * in the previous configuration to inherit it to all children
+     */
+
+    if (preserve) {
+        prev->ssl_passwords = conf->ssl_passwords;
+    }
+
+    return NGX_OK;
+}
+
+
 static ngx_int_t
 ngx_stream_proxy_set_ssl(ngx_conf_t *cf, ngx_stream_proxy_srv_conf_t *pscf)
 {
@@ -2418,16 +2473,9 @@ ngx_stream_proxy_set_ssl(ngx_conf_t *cf, ngx_stream_proxy_srv_conf_t *pscf)
             return NGX_ERROR;
         }
 
-        if (pscf->ssl_certificate->lengths
-            || pscf->ssl_certificate_key->lengths)
+        if (pscf->ssl_certificate->lengths == NULL
+            && pscf->ssl_certificate_key->lengths == NULL)
         {
-            pscf->ssl_passwords =
-                           ngx_ssl_preserve_passwords(cf, pscf->ssl_passwords);
-            if (pscf->ssl_passwords == NULL) {
-                return NGX_ERROR;
-            }
-
-        } else {
             if (ngx_ssl_certificate(cf, pscf->ssl,
                                     &pscf->ssl_certificate->value,
                                     &pscf->ssl_certificate_key->value,