events: Refactor how event deferral is handled

- Remove all *_set_defer methods and the 'defer' flag from rstream/jobs
- Added {signal,rstream,job}_event_source functions. Each return a pointer that
  represent the event source for the object in question(For signals, a static
  pointer is returned)
- Added a 'source' field to the Event struct, which is set to the appropriate
  value by the code that created the event.
- Added a 'sources' parameter to `event_poll`. It should point to a
  NULL-terminated array of event sources that will be used to decide which
  events should be processed immediately
- Added a 'source_override' parameter to `rstream_new`. This was required to use
  jobs as event sources of RStream instances(When "focusing" on a job, for
  example).
- Extracted `process_from` static function from `event_process`.
- Remove 'defer' parameter from `event_process`, which now operates only on
  deferred events.
- Refactor `channel_send_call` to use the new lock mechanism

What changed in a single sentence: Code that calls `event_poll` have to specify
which event sources should NOT be deferred. This change was necessary for a
number of reasons:

- To fix a bug where due to race conditions, a client request
  could end in the deferred queue in the middle of a `channel_send_call`
  invocation, resulting in a deadlock since the client process would never
  receive a response, and channel_send_call would never return because
  the client would still be waiting for the response.
- To handle "event locking" correctly in recursive `channel_send_call`
  invocations when the frames are waiting for responses from different
  clients. Not much of an issue now since there's only a python client, but
  could break things later.
- To simplify the process of implementing synchronous functions that depend on
  asynchronous events.

12 files changed, 152 insertions, 86 deletions

diff --git a/src/nvim/edit.c b/src/nvim/edit.c
index 8cfaddde86..3dddaea39d 100644
--- a/src/nvim/edit.c
+++ b/src/nvim/edit.c
@@ -944,7 +944,7 @@ doESCkey:
       break;
 
     case K_EVENT:
-      event_process(true);
+      event_process();
       break;
 
     case K_HOME:        /* <Home> */
diff --git a/src/nvim/ex_getln.c b/src/nvim/ex_getln.c
index 67748fa164..bf5076bdc3 100644
--- a/src/nvim/ex_getln.c
+++ b/src/nvim/ex_getln.c
@@ -762,7 +762,7 @@ getcmdline (
      */
     switch (c) {
     case K_EVENT:
-      event_process(true);
+      event_process();
       // Force a redraw even though the command line didn't change
       shell_resized();
       goto cmdline_not_changed;
@@ -1878,7 +1878,7 @@ redraw:
 
       if (IS_SPECIAL(c1)) {
         // Process deferred events
-        event_process(true);
+        event_process();
         // Ignore other special key codes
         continue;
       }
diff --git a/src/nvim/getchar.c b/src/nvim/getchar.c
index 31fa6a702f..340b31d80a 100644
--- a/src/nvim/getchar.c
+++ b/src/nvim/getchar.c
@@ -2473,7 +2473,7 @@ inchar (
       char_u dum[DUM_LEN + 1];
 
       for (;; ) {
-        event_process(true);
+        event_process();
         len = ui_inchar(dum, DUM_LEN, 0L, 0);
         if (len == 0 || (len == 1 && dum[0] == 3))
           break;
diff --git a/src/nvim/message.c b/src/nvim/message.c
index dea02e21fa..ef0faa35ee 100644
--- a/src/nvim/message.c
+++ b/src/nvim/message.c
@@ -2074,7 +2074,7 @@ static int do_more_prompt(int typed_char)
     toscroll = 0;
     switch (c) {
     case K_EVENT:
-      event_process(true);
+      event_process();
       break;
     case BS:                    /* scroll one line back */
     case K_BS:
@@ -2734,8 +2734,11 @@ do_dialog (
       retval = 0;
       break;
     default:                  /* Could be a hotkey? */
-      if (c < 0)              /* special keys are ignored here */
+      if (c < 0) {            /* special keys are ignored here */
+        // drain event queue to prevent infinite loop
+        event_process();
         continue;
+      }
       if (c == ':' && ex_cmd) {
         retval = dfltbutton;
         ins_char_typebuf(':');
diff --git a/src/nvim/normal.c b/src/nvim/normal.c
index 5a4c3a326a..55b86f61dd 100644
--- a/src/nvim/normal.c
+++ b/src/nvim/normal.c
@@ -7375,5 +7375,5 @@ static void nv_cursorhold(cmdarg_T *cap)
 
 static void nv_event(cmdarg_T *cap)
 {
-  event_process(true);
+  event_process();
 }
diff --git a/src/nvim/os/channel.c b/src/nvim/os/channel.c
index 11a58f246a..d5f29aa667 100644
--- a/src/nvim/os/channel.c
+++ b/src/nvim/os/channel.c
@@ -118,7 +118,7 @@ void channel_from_stream(uv_stream_t *stream)
   stream->data = NULL;
   channel->is_job = false;
   // read stream
-  channel->data.streams.read = rstream_new(parse_msgpack, 1024, channel, true);
+  channel->data.streams.read = rstream_new(parse_msgpack, 1024, channel, NULL);
   rstream_set_stream(channel->data.streams.read, stream);
   rstream_start(channel->data.streams.read);
   // write stream
@@ -189,16 +189,10 @@ bool channel_send_call(uint64_t id,
   // Send the msgpack-rpc request
   send_request(channel, request_id, name, arg);
 
-  if (!kv_size(channel->call_stack)) {
-    // This is the first frame, we must disable event deferral for this
-    // channel because we won't be returning until the client sends a
-    // response
-    if (channel->is_job) {
-      job_set_defer(channel->data.job, false);
-    } else {
-      rstream_set_defer(channel->data.streams.read, false);
-    }
-  }
+  EventSource channel_source = channel->is_job
+    ? job_event_source(channel->data.job)
+    : rstream_event_source(channel->data.streams.read);
+  EventSource sources[] = {channel_source, NULL};
 
   // Push the frame
   ChannelCallFrame frame = {request_id, false, NIL};
@@ -206,24 +200,18 @@ bool channel_send_call(uint64_t id,
   size_t size = kv_size(channel->call_stack);
 
   do {
-    event_poll(-1);
+    event_poll(-1, sources);
   } while (
       // Continue running if ...
       channel->enabled &&  // the channel is still enabled
       kv_size(channel->call_stack) >= size);  // the call didn't return
 
-  if (!kv_size(channel->call_stack)) {
-    // Popped last frame, restore event deferral
-    if (channel->is_job) {
-      job_set_defer(channel->data.job, true);
-    } else {
-      rstream_set_defer(channel->data.streams.read, true);
-    }
-    if (!channel->enabled && !channel->rpc_call_level) {
+  if (!(kv_size(channel->call_stack)
+        || channel->enabled
+        || channel->rpc_call_level)) {
       // Close the channel if it has been disabled and we have not been called
       // by `parse_msgpack`(It would be unsafe to close the channel otherwise)
       close_channel(channel);
-    }
   }
 
   *errored = frame.errored;
diff --git a/src/nvim/os/event.c b/src/nvim/os/event.c
index 6367a60af7..a460b2db96 100644
--- a/src/nvim/os/event.c
+++ b/src/nvim/os/event.c
@@ -33,6 +33,11 @@ typedef struct {
 # include "os/event.c.generated.h"
 #endif
 static klist_t(Event) *deferred_events, *immediate_events;
+// NULL-terminated array of event sources that we should process immediately.
+//
+// Events from sources that are not contained in this array are processed
+// later when `event_process` is called
+static EventSource *immediate_sources = NULL;
 
 void event_init(void)
 {
@@ -63,7 +68,8 @@ void event_teardown(void)
 }
 
 // Wait for some event
-bool event_poll(int32_t ms)
+bool event_poll(int32_t ms, EventSource sources[])
+  FUNC_ATTR_NONNULL_ARG(2)
 {
   uv_run_mode run_mode = UV_RUN_ONCE;
 
@@ -99,10 +105,7 @@ bool event_poll(int32_t ms)
   do {
     // Run one event loop iteration, blocking for events if run_mode is
     // UV_RUN_ONCE
-    DLOG("Entering event loop");
-    uv_run(uv_default_loop(), run_mode);
-    processed_events = event_process(false);
-    DLOG("Exited event loop, processed %u events", processed_events);
+    processed_events = loop(run_mode, sources);
   } while (
       // Continue running if ...
       !processed_events &&   // we didn't process any immediate events
@@ -120,8 +123,7 @@ bool event_poll(int32_t ms)
     // once more to let libuv perform it's cleanup
     uv_close((uv_handle_t *)&timer, NULL);
     uv_close((uv_handle_t *)&timer_prepare, NULL);
-    uv_run(uv_default_loop(), UV_RUN_NOWAIT);
-    event_process(false);
+    processed_events += loop(UV_RUN_NOWAIT, sources);
   }
 
   return !timer_data.timed_out && (processed_events || event_has_deferred());
@@ -129,22 +131,41 @@ bool event_poll(int32_t ms)
 
 bool event_has_deferred(void)
 {
-  return !kl_empty(get_queue(true));
+  return !kl_empty(deferred_events);
 }
 
-// Push an event to the queue
-void event_push(Event event, bool deferred)
+// Queue an event
+void event_push(Event event)
 {
-  *kl_pushp(Event, get_queue(deferred)) = event;
+  bool defer = true;
+
+  if (immediate_sources) {
+    size_t i;
+    EventSource src;
+
+    for (src = immediate_sources[i = 0]; src; src = immediate_sources[++i]) {
+      if (src == event.source) {
+        defer = false;
+        break;
+      }
+    }
+  }
+
+  *kl_pushp(Event, defer ? deferred_events : immediate_events) = event;
+}
+
+void event_process(void)
+{
+  process_from(deferred_events);
 }
 
 // Runs the appropriate action for each queued event
-size_t event_process(bool deferred)
+static size_t process_from(klist_t(Event) *queue)
 {
   size_t count = 0;
   Event event;
 
-  while (kl_shift(Event, get_queue(deferred), &event) == 0) {
+  while (kl_shift(Event, queue, &event) == 0) {
     switch (event.type) {
       case kEventSignal:
         signal_handle(event);
@@ -161,6 +182,8 @@ size_t event_process(bool deferred)
     count++;
   }
 
+  DLOG("Processed %u events", count);
+
   return count;
 }
 
@@ -179,7 +202,42 @@ static void timer_prepare_cb(uv_prepare_t *handle)
   uv_prepare_stop(handle);
 }
 
-static klist_t(Event) *get_queue(bool deferred)
+static void requeue_deferred_events(void)
 {
-  return deferred ? deferred_events : immediate_events;
+  size_t remaining = deferred_events->size;
+
+  DLOG("Number of deferred events: %u", remaining);
+
+  while (remaining--) {
+    // Re-push each deferred event to ensure it will be in the right queue
+    Event event;
+    kl_shift(Event, deferred_events, &event);
+    event_push(event);
+    DLOG("Re-queueing event");
+  }
+
+  DLOG("Number of deferred events: %u", deferred_events->size);
+}
+
+static size_t loop(uv_run_mode run_mode, EventSource *sources)
+{
+  size_t count;
+  immediate_sources = sources;
+  // It's possible that some events from the immediate sources are waiting
+  // in the deferred queue. If so, move them to the immediate queue so they
+  // will be processed in order of arrival by the next `process_from` call.
+  requeue_deferred_events();
+  count = process_from(immediate_events);
+
+  if (count) {
+    // No need to enter libuv, events were already processed
+    return count;
+  }
+
+  DLOG("Enter event loop");
+  uv_run(uv_default_loop(), run_mode);
+  DLOG("Exit event loop");
+  immediate_sources = NULL;
+  count = process_from(immediate_events);
+  return count;
 }
diff --git a/src/nvim/os/event_defs.h b/src/nvim/os/event_defs.h
index ca2cabd75a..dbee3e2ba7 100644
--- a/src/nvim/os/event_defs.h
+++ b/src/nvim/os/event_defs.h
@@ -6,6 +6,8 @@
 #include "nvim/os/job_defs.h"
 #include "nvim/os/rstream_defs.h"
 
+typedef void * EventSource;
+
 typedef enum {
   kEventSignal,
   kEventRStreamData,
@@ -13,6 +15,7 @@ typedef enum {
 } EventType;
 
 typedef struct {
+  EventSource source;
   EventType type;
   union {
     int signum;
diff --git a/src/nvim/os/input.c b/src/nvim/os/input.c
index 58bdf0cf52..15aebdbf3d 100644
--- a/src/nvim/os/input.c
+++ b/src/nvim/os/input.c
@@ -34,7 +34,7 @@ static bool eof = false, started_reading = false;
 
 void input_init(void)
 {
-  read_stream = rstream_new(read_cb, READ_BUFFER_SIZE, NULL, false);
+  read_stream = rstream_new(read_cb, READ_BUFFER_SIZE, NULL, NULL);
   rstream_set_file(read_stream, read_cmd_fd);
 }
 
@@ -129,7 +129,12 @@ bool os_isatty(int fd)
 
 static bool input_poll(int32_t ms)
 {
-  return input_ready() || event_poll(ms) || input_ready();
+  EventSource input_sources[] = {
+    rstream_event_source(read_stream),
+    NULL
+  };
+
+  return input_ready() || event_poll(ms, input_sources) || input_ready();
 }
 
 // This is a replacement for the old `WaitForChar` function in os_unix.c
diff --git a/src/nvim/os/job.c b/src/nvim/os/job.c
index 2f5b257b91..203aa2c990 100644
--- a/src/nvim/os/job.c
+++ b/src/nvim/os/job.c
@@ -214,8 +214,8 @@ Job *job_start(char **argv,
   job->in = wstream_new(maxmem);
   wstream_set_stream(job->in, (uv_stream_t *)&job->proc_stdin);
   // Start the readable streams
-  job->out = rstream_new(read_cb, JOB_BUFFER_SIZE, job, defer);
-  job->err = rstream_new(read_cb, JOB_BUFFER_SIZE, job, defer);
+  job->out = rstream_new(read_cb, JOB_BUFFER_SIZE, job, job_event_source(job));
+  job->err = rstream_new(read_cb, JOB_BUFFER_SIZE, job, job_event_source(job));
   rstream_set_stream(job->out, (uv_stream_t *)&job->proc_stdout);
   rstream_set_stream(job->err, (uv_stream_t *)&job->proc_stderr);
   rstream_start(job->out);
@@ -269,18 +269,6 @@ bool job_write(Job *job, WBuffer *buffer)
   return wstream_write(job->in, buffer);
 }
 
-/// Sets the `defer` flag for a Job instance
-///
-/// @param rstream The Job id
-/// @param defer The new value for the flag
-void job_set_defer(Job *job, bool defer)
-{
-  job->defer = defer;
-  rstream_set_defer(job->out, defer);
-  rstream_set_defer(job->err, defer);
-}
-
-
 /// Runs the read callback associated with the job exit event
 ///
 /// @param event Object containing data necessary to invoke the callback
@@ -307,6 +295,11 @@ void *job_data(Job *job)
   return job->data;
 }
 
+EventSource job_event_source(Job *job)
+{
+  return job;
+}
+
 static void job_exit_callback(Job *job)
 {
   // Free the slot now, 'exit_cb' may want to start another job to replace
@@ -391,10 +384,12 @@ static void exit_cb(uv_process_t *proc, int64_t status, int term_signal)
 
 static void emit_exit_event(Job *job)
 {
-  Event event;
-  event.type = kEventJobExit;
-  event.data.job = job;
-  event_push(event, true);
+  Event event = {
+    .source = job_event_source(job),
+    .type = kEventJobExit,
+    .data.job = job
+  };
+  event_push(event);
 }
 
 static void close_cb(uv_handle_t *handle)
diff --git a/src/nvim/os/rstream.c b/src/nvim/os/rstream.c
index 96dd26407a..d7ab5b8a64 100644
--- a/src/nvim/os/rstream.c
+++ b/src/nvim/os/rstream.c
@@ -26,7 +26,8 @@ struct rstream {
   uv_file fd;
   rstream_cb cb;
   size_t buffer_size, rpos, wpos, fpos;
-  bool reading, free_handle, defer;
+  bool reading, free_handle;
+  EventSource source_override;
 };
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
@@ -40,25 +41,25 @@ struct rstream {
 ///        for reading with `rstream_read`
 /// @param buffer_size Size in bytes of the internal buffer.
 /// @param data Some state to associate with the `RStream` instance
-/// @param defer Flag that specifies if callback invocation should be deferred
-///        to vim main loop(as a KE_EVENT special key)
+/// @param source_override Replacement for the default source used in events
+///        emitted by this RStream. If NULL, the default is used.
 /// @return The newly-allocated `RStream` instance
 RStream * rstream_new(rstream_cb cb,
                       size_t buffer_size,
                       void *data,
-                      bool defer)
+                      EventSource source_override)
 {
   RStream *rv = xmalloc(sizeof(RStream));
   rv->buffer = xmalloc(buffer_size);
   rv->buffer_size = buffer_size;
   rv->data = data;
-  rv->defer = defer;
   rv->cb = cb;
   rv->rpos = rv->wpos = rv->fpos = 0;
   rv->stream = NULL;
   rv->fread_idle = NULL;
   rv->free_handle = false;
   rv->file_type = UV_UNKNOWN_HANDLE;
+  rv->source_override = source_override ? source_override : rv;
 
   return rv;
 }
@@ -213,15 +214,6 @@ size_t rstream_available(RStream *rstream)
   return rstream->wpos - rstream->rpos;
 }
 
-/// Sets the `defer` flag for a a RStream instance
-///
-/// @param rstream The RStream instance
-/// @param defer The new value for the flag
-void rstream_set_defer(RStream *rstream, bool defer)
-{
-  rstream->defer = defer;
-}
-
 /// Runs the read callback associated with the rstream
 ///
 /// @param event Object containing data necessary to invoke the callback
@@ -232,6 +224,11 @@ void rstream_read_event(Event event)
   rstream->cb(rstream, rstream->data, event.data.rstream.eof);
 }
 
+EventSource rstream_event_source(RStream *rstream)
+{
+  return rstream->source_override;
+}
+
 // Callbacks used by libuv
 
 // Called by libuv to allocate memory for reading.
@@ -260,7 +257,9 @@ static void read_cb(uv_stream_t *stream, ssize_t cnt, const uv_buf_t *buf)
 
   if (cnt <= 0) {
     if (cnt != UV_ENOBUFS) {
-      DLOG("Closing RStream(%p)", rstream);
+      DLOG("Closing RStream(address: %p, source: %p)",
+           rstream,
+           rstream_event_source(rstream));
       // Read error or EOF, either way stop the stream and invoke the callback
       // with eof == true
       uv_read_stop(stream);
@@ -275,12 +274,17 @@ static void read_cb(uv_stream_t *stream, ssize_t cnt, const uv_buf_t *buf)
   // Data was already written, so all we need is to update 'wpos' to reflect
   // the space actually used in the buffer.
   rstream->wpos += nread;
-  DLOG("Received %u bytes from RStream(%p)", (size_t)cnt, rstream);
+  DLOG("Received %u bytes from RStream(address: %p, source: %p)",
+       (size_t)cnt,
+       rstream,
+       rstream_event_source(rstream));
 
   if (rstream->wpos == rstream->buffer_size) {
     // The last read filled the buffer, stop reading for now
     rstream_stop(rstream);
-    DLOG("Buffer for RStream(%p) is full, stopping it", rstream);
+    DLOG("Buffer for RStream(address: %p, source: %p) is full, stopping it",
+         rstream,
+         rstream_event_source(rstream));
   }
 
   rstream->reading = false;
@@ -345,9 +349,13 @@ static void close_cb(uv_handle_t *handle)
 
 static void emit_read_event(RStream *rstream, bool eof)
 {
-  Event event;
-  event.type = kEventRStreamData;
-  event.data.rstream.ptr = rstream;
-  event.data.rstream.eof = eof;
-  event_push(event, rstream->defer);
+  Event event = {
+    .source = rstream_event_source(rstream),
+    .type = kEventRStreamData,
+    .data.rstream = {
+      .ptr = rstream,
+      .eof = eof
+    }
+  };
+  event_push(event);
 }
diff --git a/src/nvim/os/signal.c b/src/nvim/os/signal.c
index 65657fda9c..17f270a5cc 100644
--- a/src/nvim/os/signal.c
+++ b/src/nvim/os/signal.c
@@ -103,6 +103,11 @@ void signal_handle(Event event)
   }
 }
 
+EventSource signal_event_source(void)
+{
+  return &sint;
+}
+
 static char * signal_name(int signum)
 {
   switch (signum) {
@@ -155,10 +160,11 @@ static void signal_cb(uv_signal_t *handle, int signum)
   }
 
   Event event = {
+    .source = signal_event_source(),
     .type = kEventSignal,
     .data = {
       .signum = signum
     }
   };
-  event_push(event, true);
+  event_push(event);
 }