bin.js: exit on dead sbot connection instead of wedging

ssb-viewer holds one muxrpc handle captured in a closure and has no
reconnect path. When sbot is OOM-killed and restarted by run-server.sh,
the viewer kept port 8807 open while holding a dead connection and never
answered again -- the silent wedge.

Listen for muxrpc closed and exit(1) so the while-loop in
run-ssb-viewer.sh restarts with a fresh connection. Also poll whoami to
catch half-open connections where closed never fires.

Verified: killing sbot now self-heals in ~4s instead of hanging.

bin.js

@@ -1,6 +1,49 @@
 #!/usr/bin/env node
 
+// ssb-viewer connects to sbot exactly once and hands the handle to index.js,
+// which captures it in a closure behind http.createServer().listen(). There is
+// no reconnect. So when sbot dies -- which on this box means "gets OOM-killed",
+// after which run-server.sh restarts it -- the viewer keeps port 8807 open
+// while holding a dead muxrpc connection, and never answers another request.
+//
+// That is the silent wedge. The fix is not to reconnect (index.js can't take a
+// new handle) but to exit loudly, so the `while true` loop in run-ssb-viewer.sh
+// restarts us with a fresh connection.
+
+var PING_INTERVAL_MS = 60000
+var PING_TIMEOUT_MS = 20000
+
+function bail (why) {
+  console.error('[viewer] ' + new Date().toISOString() + ' ' + why + ' — exiting so the run loop restarts us')
+  process.exit(1)
+}
+
 require('ssb-client')(function (err, sbot, config) {
-  if (err) throw err
+  // ssb-client already wraps this as "could not connect to sbot", so don't
+  // prefix it again.
+  if (err) return bail(String(err.message || err))
+
+  // muxrpc emits 'closed' when the underlying stream ends
+  // (see node_modules/muxrpc/index.js). This is the fast path.
+  sbot.on('closed', function () {
+    bail('sbot connection closed')
+  })
+
+  // Belt and braces: a half-open TCP connection can leave 'closed' unfired
+  // while calls silently hang forever. Poll a cheap RPC to catch that case.
+  if (typeof sbot.whoami === 'function') {
+    var timer = setInterval(function () {
+      var timeout = setTimeout(function () {
+        bail('sbot did not answer whoami within ' + (PING_TIMEOUT_MS / 1000) + 's')
+      }, PING_TIMEOUT_MS)
+      sbot.whoami(function (err) {
+        clearTimeout(timeout)
+        if (err) bail('sbot whoami failed: ' + (err.message || err))
+      })
+    }, PING_INTERVAL_MS)
+    // Don't hold the event loop open on our account.
+    timer.unref()
+  }
+
   require('.').init(sbot, config)
 })