gppylib: refactor SegmentPair to not support multiple mirrors

Long ago, we thought we might need to support multiple mirrors. But we
don't, and don't forsee it coming soon. Simplify the code to only ever
have one mirror, but still allow for the possibility of no mirrors

Author: Shoaib Lari <slari@pivotal.io>
Author: C.J. Jameson <cjameson@pivotal.io>

gpMgmt/bin/gpexpand

@@ -742,13 +742,13 @@ class SegmentTemplate:
     def _select_src_segment(self):
         """Gets a segment to use as a source for pg_hba.conf
         and postgresql.conf files"""
-        seg = self.gparray.segmentPairs[0]
-        if seg.primaryDB.valid:
-            self.srcSegHostname = seg.primaryDB.getSegmentHostName()
-            self.srcSegDataDir = seg.primaryDB.getSegmentDataDirectory()
-        elif seg.mirrorDBs[0] is not None and seg.mirrorDBs[0].valid:
-            self.srcSegHostname = seg.mirrorDBs[0].getSegmentHostName()
-            self.srcSegDataDir = seg.mirrorDBs[0].getSegmentDataDirectory()
+        segPair = self.gparray.segmentPairs[0]
+        if segPair.primaryDB.valid:
+            self.srcSegHostname = segPair.primaryDB.getSegmentHostName()
+            self.srcSegDataDir = segPair.primaryDB.getSegmentDataDirectory()
+        elif segPair.mirrorDB and segPair.mirrorDB.valid:
+            self.srcSegHostname = segPair.mirrorDB.getSegmentHostName()
+            self.srcSegDataDir = segPair.mirrorDB.getSegmentDataDirectory()
         else:
             raise SegmentTemplateError("no valid segdb for content=0 to use as a template")
 

gpMgmt/bin/gppylib/test/unit/test_unit_gparray.py

@@ -224,7 +224,7 @@ class GpArrayTestCase(GpTestCase):
         lastport = 0
         for seg in gparray.segmentPairs:
             prim = seg.primaryDB            
-            mir = seg.mirrorDBs[0]
+            mir = seg.mirrorDB
             self.assertNotEqual(prim.hostname, mir.hostname)
             if prim.port not in portdict:
                 portdict[prim.port] = 1