core.app: assert that links are connected only once

This reveals bugs in some test cases:

In apps.lwaft.V4V6, the same source is connected to different apps,
which does not work as intended.

In apps.test.match, fix bug in config reuse the code re-uses the same
app config object for each test. In the penultimate case, the config
contains the linkspec "src.output -> sink.rx". The last case adds the
specs "src.output -> join.src" and "join.output -> sink.rx",
i.e. there are now two links connected to sink.rx.  The hash-part of
the app's input table only contains the second of these links while
the array part contains both.

Re-use of an app config object only works in an additive fashion as
there is no method to remove apps or links from it.  The bug is fixed
by creating a new config object and adding all elements from scratch.

program.snabbvmx.tests.end-to-end.selftest.sh also fails.  This
appears to be a bug in program.snabbvmx.lwaftr.setup, which creates
the link specs

vm_v4v6.v6 -> nh_fwd6.vm
vm_v4v6.v4 -> nh_fwd6.vm

The latter should probably be

vm_v4v6.v4 -> nh_fwd4.vm

src/apps/lwaftr/V4V6.lua

@@ -189,18 +189,19 @@ local function test_join ()
    engine.configure(config.new()) -- Clean up engine.
 
    local c = config.new()
-   config.app(c, 'source', basic_apps.Join)
+   config.app(c, 'source4', basic_apps.Join)
+   config.app(c, 'source6', basic_apps.Join)
    config.app(c, 'v4v6', V4V6)
    config.app(c, 'sink', basic_apps.Sink)
 
-   config.link(c, 'source.output -> v4v6.v4')
-   config.link(c, 'source.output -> v4v6.v6')
+   config.link(c, 'source4.output -> v4v6.v4')
+   config.link(c, 'source6.output -> v4v6.v6')
    config.link(c, 'v4v6.output -> sink.input')
 
    engine.configure(c)
-   link.transmit(engine.app_table.source.output.output, arp_pkt())
-   link.transmit(engine.app_table.source.output.output, ipv4_pkt())
-   link.transmit(engine.app_table.source.output.output, ipv6_pkt())
+   link.transmit(engine.app_table.source4.output.output, arp_pkt())
+   link.transmit(engine.app_table.source4.output.output, ipv4_pkt())
+   link.transmit(engine.app_table.source6.output.output, ipv6_pkt())
    engine.main({duration = 0.1, noreport = true})
 
    assert(link.stats(engine.app_table.sink.input.input).rxpackets == 3)

src/apps/test/match.lua

@@ -84,14 +84,16 @@ function selftest()
    engine.main({duration=0.0001})
    assert(#engine.app_table.sink:errors() > 0)
 
-   engine.configure(config.new())
+   c = config.new()
    config.app(c, "sink", Match, {fuzzy=true})
+   config.app(c, "src", basic_apps.Source, 8)
    config.app(c, "comparator", basic_apps.Source, 8)
    config.app(c, "garbage", basic_apps.Source, 12)
    config.app(c, "join", basic_apps.Join)
    config.link(c, "src.output -> join.src")
    config.link(c, "garbage.output -> join.garbage")
    config.link(c, "join.output -> sink.rx")
+   config.link(c, "comparator.output -> sink.comparator")
    engine.configure(c)
    engine.main({duration=0.0001})
    assert(#engine.app_table.sink:errors() == 0)

src/core/app.lua

@@ -326,13 +326,17 @@ function apply_config_actions (actions)
    function ops.link_output (appname, linkname, linkspec)
       local app = app_table[appname]
       local link = assert(link_table[linkspec])
+      assert(not app.output[linkname],
+             appname..": duplicate output link "..linkname)
       app.output[linkname] = link
       table.insert(app.output, link)
       if app.link then app:link() end
    end
    function ops.link_input (appname, linkname, linkspec)
       local app = app_table[appname]
       local link = assert(link_table[linkspec])
+      assert(not app.input[linkname],
+             appname..": duplicate input link "..linkname)
       app.input[linkname] = link
       table.insert(app.input, link)
       if app.link then app:link() end