WiFiClient write timeouts introduced in #3257 applied to the whole write
operation, which could take long time if data size was large. This
change makes the timeout happen per chunk. Timeout now happens if no
data has been delivered within a given interval.
Previously, _write_some function would be called each time TCP stack
notifies the application that some data was delivered (via the `sent`
callback). In turn, _write_some would obtain more data to be sent from
the DataSource. In case of a DataSource backed by a Stream, this would
read from a stream. Some libraries (such as SD) may call `yield` and
other blocking operations from Stream read function, which can not be
used in TCP stack callbacks.
This change moves the data sending loop back into the Arduino task, with
a negligible loss of performance. TCP callback now wakes the main task
via `esp_schedule`, which performs stream read and provides more data
to the TCP stack.
Possible future optimization would be to buffer Stream data ahead of
time. That way, buffered data could be sent immediately from the TCP
callback. On the other hand, this optimization would need extra TCP_MSS
of temporary storage, per connection.
Fixes#2399.
* minimum changes for libraries to compile with lwip2
* add WiFiClient::availableForWrite() (similar to Serial::) which indicates how much data can be sent without buffering
* Suppressed -Wunused-parameter and -Wunused-function by casting to void unused identifiers.
* Explicit initialization of all fields to suppress -Wmissing-field-initializers.
* Fixed signed/unsigned integer comparison.
* memset initialization of structs.
* More -Wunused-parameter fixes.
* WiFiClient: use DataSource for writes
* ESP8266WebServer: delegate writing to WiFiClient
* ESP8266WebServer: set write timeout before sending content
