channel: check for existance before trying to set key

This avoids an error message in async context, where it is not safe.

1 files changed, 6 insertions, 2 deletions

diff --git a/runtime/doc/channel.txt b/runtime/doc/channel.txt
index c94c64eb84..c4f7eb1ff1 100644
--- a/runtime/doc/channel.txt
+++ b/runtime/doc/channel.txt
@@ -43,7 +43,7 @@ bytes. Additionally, for a job channel using rpc, bytes can still be
 read over its stderr. Similarily, only bytes can be written to nvim's own stderr.
 
 						    *channel-callback* *buffered*
-					*on_stdout* *on_stderr* *on_stdin* *on_data*
+				*E5210* *on_stdout* *on_stderr* *on_stdin* *on_data*
 A callback function `on_{stream}` will be invoked with data read from the
 channel.  By default, the callback will be invoked immediately when data is
 available, to facilitate interactive communication. The same callback will
@@ -52,7 +52,11 @@ Alternatively the `{stream}_buffered` option can be set to invoke the callback
 only when the underlying stream reaches EOF, and will then be passed in
 complete output.  This is helpful when only the complete output is useful, and
 not partial data. Futhermore if `{stream}_buffered` is set but not a callback,
-the data is saved in the options dict, with the stream name as key.
+the data is saved in the options dict, with the stream name as key. For this
+to work a new options dict must be used for each opened channel. If a script
+uses a global `s:job_opts` dict, it can be copied with |copy()| before supplying
+it to |jobstart()|. If a dict is reused, so that the dict key already is
+occupied, error `E5210` will be raised.
 
 - The arguments passed to the callback function are: