From fbd4e3bf99afbc80012d6dc029be19e1bd8e6890 Mon Sep 17 00:00:00 2001
From: Steve Kemp <steve@steve.org.uk>
Date: Mon, 18 Jan 2016 14:13:24 +0200
Subject: Updated the queue-handling.

We now use a zset to store our pending tests.  This means that
jobs are only in the queue once - no duplicates are allowed.

This closes #12428.
---
 bin/custodian-enqueue  | 11 +++++++-
 lib/custodian/queue.rb | 69 ++++++++++++++++++++++++++++++++++++++++++++------
 2 files changed, 71 insertions(+), 9 deletions(-)

diff --git a/bin/custodian-enqueue b/bin/custodian-enqueue
index 7dd864b..f4f9f0e 100755
--- a/bin/custodian-enqueue
+++ b/bin/custodian-enqueue
@@ -85,16 +85,25 @@ if __FILE__ == $PROGRAM_NAME
     exit(1)
   end
 
+  #
+  #  Did we fail to add any tests to the queue?
+  #
+  failed = false
+
   mon.jobs.each do |test|
     if  ENV['TEST']
       # nop
     elsif  ENV['DUMP']
       puts test
     else
-      queue.add(test.to_s)
+      failed = true unless queue.add(test.to_s)
     end
   end
 
+  if ( failed )
+    puts "We failed to add at least one job to the queue."
+    exit( 1 )
+  end
 end
 
 
diff --git a/lib/custodian/queue.rb b/lib/custodian/queue.rb
index d1f4a2c..7caad1a 100644
--- a/lib/custodian/queue.rb
+++ b/lib/custodian/queue.rb
@@ -81,12 +81,15 @@ module Custodian
       job = nil
 
       loop do
+        job = @redis.zrange('zset', '0', '0')
 
-        # Get a random job
-        job = @redis.spop('custodian_queue')
+        if !job.empty?
+          # We only have one entry in our array
+          job = job[0]
+
+          # Remove from the queue
+          @redis.zrem('zset', job );
 
-        # If that worked return it
-        if !job.nil?
           return job
         else
           sleep(timeout)
@@ -96,13 +99,63 @@ module Custodian
 
 
     #
-    #  Add a new job to the queue.
+    #  Add a new job to the queue - this can stall for the case where the
+    # job is already pending.
     #
     def add(test)
 
-      # Add unless already present
-      @redis.sadd('custodian_queue', test) unless
-        ( @redis.sismember( 'custodian_queue', test ) )
+        @redis.watch('zset')
+
+        #
+        # Count the number of times we attempt to add the test
+        #
+        attempts = 0
+        added    = false
+
+
+        #
+        # This is run in a loop, as we have to wait until both
+        #
+        # (a) the score is missing
+        # (b) the zadd function succeeds
+        #
+        while( attempts < 40 ) do
+
+          #
+          # Only update if no score is set
+          #
+          if !@redis.zscore("zset", test)
+
+            #
+            # If MULTI returns nil, the transaction failed, so we need to try
+            # again.
+            #
+            break unless @redis.multi do |r|
+              @redis.zadd('zset', Time.now.to_f, test)
+              added = true
+            end.nil?
+          end
+
+          #
+          # This could be tighter..
+          #
+          sleep 0.1
+
+          #
+          # Bump the count of attempts.
+          #
+          attempts = attempts + 1
+        end
+
+        #
+        # Do we need to unwatch here?
+        #
+        @redis.unwatch
+
+        #
+        # Return the success/fail
+        #
+        return added
     end
 
 
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