Bug 1164618 - Start Xvfb in a loop in docker images; r=dustin

Sometimes the Xvfb sanity check can fail due to slow startup times,
this introduces a simple retry system, with sleeps, to ensure that failures are
due to another cause.

testing/docker/desktop-build/bin/build.sh

@@ -80,9 +80,21 @@ if $NEED_XVFB; then
     xvfb_pid=$!
     # Only error code 255 matters, because it signifies that no
     # display could be opened. As long as we can open the display
-    # tests should work.
-    sleep 2 # we need to sleep so that Xvfb has time to startup
-    xvinfo || if [ $? == 255 ]; then exit 255; fi
+    # tests should work. We'll retry a few times with a sleep before
+    # failing.
+    retry_count=0
+    max_retries=2
+    xvfb_test=0
+    until [ $retry_count -gt $max_retries ]; do
+        xvinfo || xvfb_test=$?
+        if [ $xvfb_test != 255 ]; then
+            retry_count=$(($max_retries + 1))
+        else
+            retry_count=$(($retry_count + 1))
+            echo "Failed to start Xvfb, retry: $retry_count"
+            sleep 2
+        fi done
+    if [ $xvfb_test == 255 ]; then exit 255; fi
 fi
 
 # set up mozharness configuration, via command line, env, etc.