Plug Tests : Test _precomputedHash more robustly

Our hash caching is now so aggressive that even if the precomputed hash was ignored, we'd still be pulling the hash out of the cache. So we now clear the cache to be sure that if the precomputed hash was ignored, that would trigger another call to `ComputeNode::hash()`.

python/GafferTest/NumericPlugTest.py

@@ -450,17 +450,14 @@ def testPrecomputedHash( self ) :
 		self.assertEqual( n.numComputeCalls, 1 )
 
 		h = n["sum"].hash()
-		numHashCalls = n.numHashCalls
-		# Accept either 1 or 2 - it would be reasonable for the ValuePlug
-		# to have either cached the hash or not, but that's not what we're
-		# testing here.
-		self.assertTrue( numHashCalls == 1 or numHashCalls == 2 )
+		self.assertEqual( n.numHashCalls, 1 )
 		self.assertEqual( n.numComputeCalls, 1 )
 
-		# What we care about is that calling getValue() with a precomputed hash
-		# definitely doesn't recompute the hash again.
+		# Calling `getValue()`` with a precomputed hash shouldn't recompute the
+		# hash again, even if it has been cleared from the cache.
+		Gaffer.ValuePlug.clearHashCache()
 		self.assertEqual( n["sum"].getValue( _precomputedHash = h ), 30 )
-		self.assertEqual( n.numHashCalls, numHashCalls )
+		self.assertEqual( n.numHashCalls, 1 )
 		self.assertEqual( n.numComputeCalls, 1 )
 
 	def testIsSetToDefault( self ) :

python/GafferTest/TypedPlugTest.py

@@ -208,17 +208,14 @@ def testPrecomputedHash( self ) :
 		self.assertEqual( n.numComputeCalls, 1 )
 
 		h = n["out"].hash()
-		numHashCalls = n.numHashCalls
-		# Accept either 1 or 2 - it would be reasonable for the ValuePlug
-		# to have either cached the hash or not, but that's not what we're
-		# testing here.
-		self.assertTrue( numHashCalls == 1 or numHashCalls == 2 )
+		self.assertEqual( n.numHashCalls, 1 )
 		self.assertEqual( n.numComputeCalls, 1 )
 
-		# What we care about is that calling getValue() with a precomputed hash
-		# definitely doesn't recompute the hash again.
+		# Calling `getValue()` with a precomputed hash shouldn't recompute the
+		# hash again, even if it has been cleared from the cache.
+		Gaffer.ValuePlug.clearHashCache()
 		self.assertEqual( n["out"].getValue( _precomputedHash = h ), "hi" )
-		self.assertEqual( n.numHashCalls, numHashCalls )
+		self.assertEqual( n.numHashCalls, 1 )
 		self.assertEqual( n.numComputeCalls, 1 )
 
 	def testBoolPlugStringConnections( self ) :

python/GafferTest/ValuePlugTest.py

@@ -286,22 +286,18 @@ def testPrecomputedHashOptimisation( self ) :
 		self.assertEqual( a1, IECore.StringData( "a" ) )
 		self.assertEqual( n.numHashCalls, 1 )
 
-		# We apply some leeway in our test for how many hash calls are
-		# made - a good ValuePlug implementation will probably avoid
-		# unecessary repeated calls in most cases, but it's not
-		# what this unit test is about.
 		a2 = n["out"].getValue( _copy = False )
 		self.assertTrue( a2.isSame( a1 ) )
-		self.assertTrue( n.numHashCalls == 1 or n.numHashCalls == 2 )
+		self.assertEqual( n.numHashCalls, 1 )
 
 		h = n["out"].hash()
-		self.assertTrue( n.numHashCalls >= 1 and n.numHashCalls <= 3 )
-		numHashCalls = n.numHashCalls
+		self.assertEqual( n.numHashCalls, 1 )
 
-		# What we care about is that calling getValue() with a precomputed hash
-		# definitely doesn't recompute the hash again.
+		# Calling `getValue()` with a precomputed hash shouldn't recompute the
+		# hash again, even if it has been cleared from the cache.
+		Gaffer.ValuePlug.clearHashCache()
 		a3 = n["out"].getValue( _copy = False, _precomputedHash = h )
-		self.assertEqual( n.numHashCalls, numHashCalls )
+		self.assertEqual( n.numHashCalls, 1 )
 		self.assertTrue( a3.isSame( a1 ) )
 
 	def testSerialisationOfChildValues( self ) :