Store a reverse mapping when wiring together graph, detect unused nodes.

The connectivity checking code was previously unable to detect the
following error case:

  local input = nn.Identity()()
  local usedOutput = nn.Linear(20, 10)(input)
  local unusedOutput = nn.Linear(20, 10)(input)
  local gmod = nn.gModule({input}, {usedOutput})

With this fix, when gModule is called it will throw an error, because of
unusedOutput.

This is a backwards incompatible change, but I feel that the current
flexibility is error prone, and I can't see any advantage to it. We have
flushed out a couple of bugs in internal code with this change.

3 files changed, 57 insertions, 0 deletions

diff --git a/gmodule.lua b/gmodule.lua
index 99698c0..910e2c4 100644
--- a/gmodule.lua
+++ b/gmodule.lua
@@ -130,6 +130,29 @@ function gModule:__init(inputs,outputs)
                              debugLabel))
       end
 
+      -- Check whether any nodes were defined as taking this node as an input,
+      -- but then left dangling and don't connect to the output. If this is
+      -- the case, then they won't be present in forwardnodes, so error out.
+      for successor, _ in pairs(node.data.reverseMap) do
+         local successorIsInGraph = false
+
+         -- Only need to the part of forwardnodes from i onwards, topological
+         -- sort guarantees it cannot be in the first part.
+         for j = i+1, #self.forwardnodes do
+            -- Compare equality of data tables, as new Node objects have been
+            -- created by processes such as topoological sort, but the
+            -- underlying .data table is shared.
+            if self.forwardnodes[j].data == successor.data then
+               successorIsInGraph = true
+               break
+            end
+         end
+         local errStr =
+            "node declared on %s does not connect to gmodule output"
+         assert(successorIsInGraph,
+                string.format(errStr, successor.data.annotations._debugLabel))
+      end
+
       -- set data.forwardNodeId for node:label() output
       node.data.forwardNodeId = node.id
 
diff --git a/node.lua b/node.lua
index a55aa48..93f35bc 100644
--- a/node.lua
+++ b/node.lua
@@ -11,6 +11,7 @@ function nnNode:__init(data)
    parent.__init(self,data)
    self.data.annotations = self.data.annotations or {}
    self.data.mapindex = self.data.mapindex or {}
+   self.data.reverseMap = self.data.reverseMap or {}
    if not self.data.annotations._debugLabel then
       self:_makeDebugLabel(debug.getinfo(6, 'Sl'))
    end
@@ -39,6 +40,13 @@ function nnNode:add(child,domap)
       assert(not mapindex[data], "Don't pass the same input twice.")
       table.insert(mapindex,data)
       mapindex[data] = #mapindex
+
+      -- The "child" that is added here actually represents the input node,
+      -- so we write into that node to indicate that we are downstream of it.
+      -- This enables dangling pointer detection.
+      local revMap = child.data.reverseMap
+      assert(not revMap[self], 'this connection has already been made!')
+      revMap[self] = true
    end
 end
 
diff --git a/test/test_connectivity.lua b/test/test_connectivity.lua
new file mode 100644
index 0000000..99b2539
--- /dev/null
+++ b/test/test_connectivity.lua
@@ -0,0 +1,26 @@
+local totem = require 'totem'
+require 'nngraph'
+local tests = totem.TestSuite()
+local tester = totem.Tester()
+
+function tests.connectivity()
+  -- Store debug info here, need to call debug.getinfo on same line as the
+  -- dangling pointer is declared.
+  local dInfo
+  local input = nn.Identity()()
+  local lin = nn.Linear(20, 10)(input)
+  -- The Sigmoid does not connect to the output, so should cause an error
+  -- when we call gModule.
+  local dangling = nn.Sigmoid()(lin); dInfo = debug.getinfo(1, 'Sl')
+  local actualOutput = nn.Tanh()(lin)
+  local errStr = string.format(
+      'node declared on %%[%s%%]:%d_ does not connect to gmodule output',
+      dInfo.short_src, dInfo.currentline)
+  tester:assertErrorPattern(
+      function()
+        return nn.gModule({input}, {actualOutput})
+      end,
+      errStr)
+end
+
+return tester:add(tests):run()