sequence search : when child process returns empty file and status 0 …

…(OK), return empty result to client

feature.js : dnaSequenceSearch() : searchDataOut() : if ! chunk, reply OK with [].
child-process.js : childProcess() : child.on(close ) : if status OK and ! child.killed, dataOutCb(null ).

backend/common/models/feature.js

@@ -76,9 +76,13 @@ module.exports = function(Feature) {
 
     /** Receive the results from the Blast.
      * @param chunk is a Buffer
+     * null / undefined indicates child process closed with status 0 (OK) and sent no output.
      * @param cb is cbWrap of cb passed to dnaSequenceSearch().
      */
     let searchDataOut = (chunk, cb) => {
+      if (! chunk) {
+        cb(null, []);
+      } else
       if (chunk.asciiSlice(0,6) === 'Error:') {
         cb(new Error(chunk.toString()));
       } else {

backend/common/utilities/child-process.js

@@ -17,6 +17,8 @@ var fs = require('fs');
  * @param moreParams array of params to pass as command-line params to
  * child process, after [fileName, useFile]
  * @param dataOutCb (Buffer chunk, cb) {}
+ * If child process closes with status 0 (OK) and sent no output, then
+ * dataOutCb will be called with chunk === null
  * @param cb  response node callback
  * @param progressive true means pass received data back directly to
  * dataOutCb, otherwise catenate it and call dataOutCb just once when
@@ -138,7 +140,12 @@ exports.childProcess = (scriptName, postData, useFile, fileName, moreParams, dat
       const message = 'Processed file ' + fileName;
       if (child.killed) {
         cb(null, message, true);
-      } // else check again after timeout
+      } else { //  return empty data result
+        dataOutCb(null, cb);
+        /* it would be equivalent to do : cb(null, [], true);
+         * dataOutCb() may have completion actions.
+         */
+      }
     }
   });