index: Make sure that pool is safely closed and joined.

Make sure we call close and join methods of Pool object 
properly in any case. This fixes hundreds of processes
appeard in buildfarm.

pisi/index.py

@@ -113,16 +113,19 @@ class Index(xmlfile.XmlFile):
         # have
         pool = multiprocessing.Pool()
 
+        # Before calling pool.map check if list is empty or not: python#12157
+        if specs:
             try:
                 # Add source packages to index using a process pool
                 self.specs = pool.map(add_spec, specs)
             except:
-            # If an exception occurs (like a keyboard interrupt), immediately
-            # terminate worker processes and propagate exception. (CLI honors
-            # KeyboardInterrupt exception, if you're not using CLI, you must
-            # handle KeyboardException yourself)
+                # If an exception occurs (like a keyboard interrupt),
+                # immediately terminate worker processes and propagate
+                # exception. (CLI honors KeyboardInterrupt exception, if you're
+                # not using CLI, you must handle KeyboardException yourself)
 
                 pool.terminate()
+                pool.join()
                 raise
 
         try:
@@ -144,13 +147,19 @@ class Index(xmlfile.XmlFile):
 
                 latest_packages.append((pkg, deltas, repo_uri))
 
+        # Before calling pool.map check if list is empty or not: python#12157
+        if latest_packages:
             try:
                 # Add binary packages to index using a process pool
                 self.packages = pool.map(add_package, latest_packages)
             except:
                 pool.terminate()
+                pool.join()
                 raise
-        else:
+
+        pool.close()
+        pool.join()
+
         # Clean up output
         ctx.ui.info("\r%-80.80s" % (_('Done.')))