Make git fetching in gerrit-git-prep more robust.

gerrit-git-prep would fail if a single git fetch was unable to get
what it requested from Gerrit. These errors occur due to server side
issues and gerrit-git-prep should simply try again. This patch has
gerrit-git-prep attempt each git fetch at least three times with a
random sleep interval between 30 and 90 seconds between each fetch.

Change-Id: Ib7f9292ec6e55ee7e6d3a5fe832c302660a1aa00

modules/jenkins_slave/files/slave_scripts/gerrit-git-prep.sh

@@ -38,12 +38,32 @@ fi
 function merge_change {
     PROJECT=$1
     REFSPEC=$2
+    MAX_ATTEMPTS=${3:-3}
+    COUNT=0
 
-    git fetch https://$SITE/p/$PROJECT $REFSPEC
+    until git fetch https://$SITE/p/$PROJECT $REFSPEC
+    do
+        COUNT=$(($COUNT + 1))
+        if [ $COUNT -eq $MAX_ATTEMPTS ]
+        then
+            break
+        fi
+        SLEEP_TIME=$((30 + $RANDOM % 60))
+        sleep $SLEEP_TIME
+    done
+
+    if [ $COUNT -lt $MAX_ATTEMPTS ]
+    then
         # This should be equivalent to what gerrit does if a repo is
         # set to "merge commits when necessary" and "automatically resolve
         # conflicts" is set to true:
         git merge -s resolve FETCH_HEAD
+    else
+        # Failed to fetch too many times. Notify jenkins of the failure.
+        # This is necessary because set -e does not apply to the condition of
+        # until.
+        exit 1
+    fi
 }
 
 function merge_changes {