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diff --git a/mod/external-handler.hxx b/mod/external-handler.hxx
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+++ b/mod/external-handler.hxx
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+// file      : mod/external-handler.hxx -*- C++ -*-
+// copyright : Copyright (c) 2014-2018 Code Synthesis Ltd
+// license   : MIT; see accompanying LICENSE file
+
+#ifndef MOD_EXTERNAL_HANDLER_HXX
+#define MOD_EXTERNAL_HANDLER_HXX
+
+#include <libbutl/manifest-parser.mxx>
+
+#include <libbrep/types.hxx>
+#include <libbrep/utility.hxx>
+
+#include <mod/diagnostics.hxx>
+
+namespace brep
+{
+  // Utility for running external handler programs.
+  //
+  namespace external_handler
+  {
+    // Run an external handler program and, if it exited normally with the
+    // zero exit status, return the result manifest it is expected to write to
+    // stdout, containing at least the HTTP status value. Otherwise, log an
+    // error and return nullopt. Redirect the program stderr to the web server
+    // error log.
+    //
+    // If the timeout (in seconds) is not zero and the handler program does
+    // not exit in the allotted time, then it is killed and its termination is
+    // treated as abnormal.
+    //
+    // Note that warnings can be logged regardless of the program success. If
+    // the trace argument is not NULL, then trace records are also logged.
+    //
+    struct result_manifest
+    {
+      uint16_t status;
+      vector<butl::manifest_name_value> values; // Note: all values, including
+                                                // status.
+    };
+
+    optional<result_manifest>
+    run (const path& handler,
+         const strings& args,
+         const dir_path& data_dir,
+         size_t timeout,
+         const basic_mark& error,
+         const basic_mark& warn,
+         const basic_mark* trace);
+  }
+}
+
+#endif // MOD_EXTERNAL_HANDLER_HXX