core.counter: Fixes for double-buffering extension

Added a method 'counter.delete(name)' to unmap and unlink a
counter. This is needed because the value returned by 'counter.open()'
cannot be manipulated directly by the shm module anymore.

Updated core.link to use counter.delete(name). This fixes breakage in
the core.link selftest function.

Currently have a broken core.counter selftest. The new semantics are not
consistent with the original test case. selftest must be updated (TODO).

src/core/counter.lua

@@ -39,15 +39,31 @@ local counter_t = ffi.typeof("struct counter")
 -- to set-associative CPU cache). See SnabbCo/snabbswitch#558.
 local public  = {}
 local private = {}
+local numbers = {} -- name -> number
 
 function open (name, readonly)
-   public[#public+1] = shm.map(name, counter_t, readonly)
+   if numbers[name] then error("counter already opened: " .. name) end
+   local n = #public+1
+   numbers[name] = n
+   public[n] = shm.map(name, counter_t, readonly)
    if readonly then
-      private[#private+1] = public[#public] -- use counter directly
+      private[n] = public[#public] -- use counter directly
    else
-      private[#private+1] = ffi.new(counter_t)
+      private[n] = ffi.new(counter_t)
    end
-   return private[#private]
+   return private[n]
+end
+
+function delete (name)
+   local ptr = public[numbers[name]]
+   if not ptr then error("counter not found for deletion: " .. name) end
+   -- Free shm object
+   shm.unmap(ptr)
+   shm.unlink(name)
+   -- Free local state
+   numbers[name] = false
+   public[ptr] = false
+   private[ptr] = false
 end
 
 -- Copy counter private counter values to public shared memory.

src/core/link.lua

@@ -19,21 +19,20 @@ local band = require("bit").band
 local size = C.LINK_RING_SIZE         -- NB: Huge slow-down if this is not local
 max        = C.LINK_MAX_PACKETS
 
+local counternames = {"rxpackets", "txpackets", "rxbytes", "txbytes", "txdrop"}
+
 function new (name)
    local r = shm.map("links/"..name, "struct link")
-   for _, c
-   in ipairs({"rxpackets", "txpackets", "rxbytes", "txbytes", "txdrop"}) do
+   for _, c in ipairs(counternames) do
       r.stats[c] = counter.open("counters/"..name.."/"..c)
    end
    return r
 end
 
 function free (r, name)
-   for _, c
-   in ipairs({"rxpackets", "txpackets", "rxbytes", "txbytes", "txdrop"}) do
-      shm.unmap(r.stats[c])
+   for _, c in ipairs(counternames) do
+      counter.delete("counters/"..name.."/"..c)
    end
-   shm.unlink("counters/"..name)
    shm.unmap(r)
    shm.unlink("links/"..name)
 end
@@ -92,7 +91,7 @@ end
 function stats (r)
    local stats = {}
    for _, c
-   in ipairs({"rxpackets", "txpackets", "rxbytes", "txbytes", "txdrop"}) do
+   in ipairs(counternames) do
       stats[c] = tonumber(counter.read(r.stats[c]))
    end
    return stats