diff --git a/frontend/ui/uimanager.lua b/frontend/ui/uimanager.lua
index b2c0e1541..a0f63d0c8 100644
--- a/frontend/ui/uimanager.lua
+++ b/frontend/ui/uimanager.lua
@@ -21,6 +21,9 @@ local UIManager = {
         G_reader_settings:readSetting("full_refresh_count") or DRCOUNTMAX,
     refresh_count = 0,
 
+    -- How long to wait between ZMQ wakeups: 50ms.
+    ZMQ_TIMEOUT = 50 * 1000,
+
     event_handlers = nil,
 
     _running = true,
@@ -660,6 +663,27 @@ function UIManager:resetInputTimeout()
     self.INPUT_TIMEOUT = nil
 end
 
+function UIManager:handleInputEvent(input_event)
+    if input_event.handler ~= "onInputError" then
+        self.event_hook:execute("InputEvent", input_event)
+    end
+    local handler = self.event_handlers[input_event]
+    if handler then
+        handler(input_event)
+    else
+        self.event_handlers["__default__"](input_event)
+    end
+end
+
+-- Process all pending events on all registered ZMQs.
+function UIManager:processZMQs()
+    for _, zeromq in ipairs(self._zeromqs) do
+        for input_event in zeromq.waitEvent,zeromq do
+            self:handleInputEvent(input_event)
+        end
+    end
+end
+
 function UIManager:handleInput()
     local wait_until, now
     -- run this in a loop, so that paints can trigger events
@@ -683,42 +707,32 @@ function UIManager:handleInput()
         self:_repaint()
     until not self._task_queue_dirty
 
-    -- wait for next event
-    -- note that we will skip that if we have tasks that are ready to run
-    local input_event = nil
-    if not wait_until then
-        if #self._zeromqs > 0 then
-            -- pending message queue, wait 100ms for input
-            input_event = Input:waitEvent(1000*100)
-            if not input_event or input_event.handler == "onInputError" then
-                for _, zeromq in ipairs(self._zeromqs) do
-                    input_event = zeromq:waitEvent()
-                    if input_event then break end
-                end
-            end
-        else
-            -- no pending task, wait without timeout
-            input_event = Input:waitEvent(self.INPUT_TIMEOUT)
-        end
-    elseif wait_until[1] > now[1]
-    or wait_until[1] == now[1] and wait_until[2] > now[2] then
-        -- wait until next task is pending
-        local wait_us = (wait_until[1] - now[1]) * MILLION
-                        + (wait_until[2] - now[2])
-        input_event = Input:waitEvent(wait_us)
+    -- run ZMQs if any
+    self:processZMQs()
+
+    -- Figure out how long to wait.
+    -- Default to INPUT_TIMEOUT (which may be nil, i.e. block until an event happens).
+    local wait_us = self.INPUT_TIMEOUT
+
+    -- If there's a timed event pending, that puts an upper bound on how long to wait.
+    if wait_until then
+        wait_us = math.min(
+            wait_us or math.huge,
+            (wait_until[1] - now[1]) * MILLION
+            + (wait_until[2] - now[2]))
+    end
+
+    -- If we have any ZMQs registered, ZMQ_TIMEOUT is another upper bound.
+    if #self._zeromqs > 0 then
+        wait_us = math.min(wait_us or math.huge, self.ZMQ_TIMEOUT)
     end
 
+    -- wait for next event
+    local input_event = Input:waitEvent(wait_us)
+
     -- delegate input_event to handler
     if input_event then
-        if input_event.handler ~= "onInputError" then
-            self.event_hook:execute("InputEvent", input_event)
-        end
-        local handler = self.event_handlers[input_event]
-        if handler then
-            handler(input_event)
-        else
-            self.event_handlers["__default__"](input_event)
-        end
+        self:handleInputEvent(input_event)
     end
 
     if self.looper then
diff --git a/frontend/util.lua b/frontend/util.lua
index 9737b17e3..c3fd55230 100644
--- a/frontend/util.lua
+++ b/frontend/util.lua
@@ -304,6 +304,32 @@ function util.isEmptyDir(path)
     return true
 end
 
+--- Checks if the given path exists. Doesn't care if it's a file or directory.
+---- @string path
+---- @treturn bool
+function util.pathExists(path)
+    local lfs = require("libs/libkoreader-lfs")
+    return lfs.attributes(path, "mode") ~= nil
+end
+
+--- As `mkdir -p`.
+--- Unlike lfs.mkdir(), does not error if the directory already exists, and
+--- creates intermediate directories as needed.
+---- @string path the directory to create
+---- @treturn bool true on success; nil, err_message on error
+function util.makePath(path)
+    path = path:gsub("/+$", "")
+    if util.pathExists(path) then return true end
+
+    local success, err = util.makePath((util.splitFilePathName(path)))
+    if not success then
+        return nil, err.." (creating "..path..")"
+    end
+
+    local lfs = require("libs/libkoreader-lfs")
+    return lfs.mkdir(path)
+end
+
 --- Replaces characters that are invalid filenames.
 --
 -- Replaces the characters <code>\/:*?"<>|</code> with an <code>_</code>.
@@ -326,7 +352,9 @@ function util.replaceSlashChar(str)
     end
 end
 
---- Splits a file into its path and name
+--- Splits a file into its directory path and file name.
+--- If the given path has a trailing /, returns the entire path as the directory
+--- path and "" as the file name.
 ---- @string file
 ---- @treturn string path, filename
 function util.splitFilePathName(file)
diff --git a/plugins/calibrecompanion.koplugin/main.lua b/plugins/calibrecompanion.koplugin/main.lua
index 6349f73f5..4598b0828 100644
--- a/plugins/calibrecompanion.koplugin/main.lua
+++ b/plugins/calibrecompanion.koplugin/main.lua
@@ -5,6 +5,7 @@ local JSON = require("json")
 local DEBUG = require("dbg")
 local _ = require("gettext")
 local NetworkMgr = require("ui/network/manager")
+local util = require("frontend/util")
 
 require("ffi/zeromq_h")
 
@@ -320,6 +321,7 @@ function CalibreCompanion:sendBook(arg)
     local inbox_dir = G_reader_settings:readSetting("inbox_dir")
     local filename = inbox_dir .. "/" .. arg.lpath
     DEBUG("write to file", filename)
+    util.makePath((util.splitFilePathName(filename)))
     local outfile = io.open(filename, "wb")
     local to_write_bytes = arg.length
     local calibre_device = self