Fix intermittent authentication failures

Sometimes the authentication handler got called again just after the
search operation has been started, immediately failing because nothing
has been found yet. Added an extra safety check for these cases.

ngx_http_auth_ldap_module.c

@@ -110,6 +110,7 @@ typedef struct {
 
     struct ngx_http_auth_ldap_connection *c;
     ngx_queue_t queue;
+    int replied;
     int error_code;
     ngx_str_t error_msg;
     ngx_str_t dn;
@@ -960,6 +961,7 @@ ngx_http_auth_ldap_get_connection(ngx_http_auth_ldap_ctx_t *ctx)
         c = ngx_queue_data(q, ngx_http_auth_ldap_connection_t, queue);
         c->rctx = ctx;
         ctx->c = c;
+        ctx->replied = 0;
         return 1;
     }
 
@@ -999,6 +1001,7 @@ ngx_http_auth_ldap_reply_connection(ngx_http_auth_ldap_connection_t *c, int erro
     ngx_log_debug1(NGX_LOG_DEBUG_HTTP, c->log, 0, "http_auth_ldap: LDAP request to \"%V\" has finished",
         &c->server->alias);
 
+    ctx->replied = 1;
     ctx->error_code = error_code;
     if (error_msg) {
         ctx->error_msg.len = ngx_strlen(error_msg);
@@ -1401,6 +1404,11 @@ ngx_http_auth_ldap_authenticate(ngx_http_request_t *r, ngx_http_auth_ldap_ctx_t
         return NGX_ERROR;
     }
 
+    if (!ctx->replied && ctx->phase != PHASE_START) {
+        ngx_log_debug0(NGX_LOG_DEBUG_HTTP, r->connection->log, 0, "http_auth_ldap: The LDAP operation did not finish yet");
+        return NGX_AGAIN;
+    }
+
     for (;;) {
         ngx_log_debug2(NGX_LOG_DEBUG_HTTP, r->connection->log, 0, "http_auth_ldap: Authentication loop (phase=%d, iteration=%d)",
             ctx->phase, ctx->iteration);