uart: fix wdt input overrun

[d-a-v]

WDT is triggerred when uart is flooded but not read (quickly enough) by user.
Here's a fix with a message on debug console (generally same uart though).

MCVE:

#define PRINT_INTV_MS 5000

unsigned long next;

void setup() {
  Serial.setRxBufferSize(8);
  Serial.begin(115200);
  Serial.setDebugOutput(true);
  Serial.println("please type in chars");
  next = millis() + PRINT_INTV_MS;
}

void loop() {
  static int last = 0;
  int avail = Serial.available();
  if (last != avail)
  {
    last = avail;
    Serial.printf("available: %i\n", avail);
  }

  if (millis() > next)
  {
    Serial.print("reading serial: ");
    while (Serial.available())
      Serial.print((char)Serial.read());
    Serial.println();
    next += PRINT_INTV_MS;
  }
}

A choice has to be made when receive buffer is full:
discard oldest data, or newest data ?
Current master tries to discard newest data (with the wdt bug)
This PR has the two (fixed) ways, I personally prefer discarding the oldest
(with this scheme, I may not be the last leaving among you :)

newest discarded (input is '1234567890'):

00:42:35.122 -> SDK:2.2.1(cfd48f3)/Core:2.4.1-32-g546c8b7/lwIP:2.0.3(STABLE-2_0_3_RELEASE/glue:arduino-2.4.1)
00:42:35.122 -> please type in chars
00:42:38.464 -> available: 1
00:42:38.465 -> available: 4
00:42:38.465 -> available: 5
00:42:38.497 -> available: 6
00:42:38.497 -> available: 7
00:42:38.497 -> available: 8
00:42:38.497 -> available: 9
00:42:38.497 -> available: 10
00:42:38.497 -> available: 11
00:42:38.497 -> uart input full!
00:42:38.497 -> available: 7
00:42:40.139 -> reading serial: 1234567
00:42:40.139 -> available: 0

oldest discarded (input is '1234567890'):

00:49:12.791 -> SDK:2.2.1(cfd48f3)/Core:2.4.1-32-g546c8b7/lwIP:2.0.3(STABLE-2_0_3_RELEASE/glue:arduino-2.4.1)
00:49:12.794 -> please type in chars
00:49:15.531 -> available: 1
00:49:15.532 -> available: 4
00:49:15.532 -> available: 5
00:49:15.532 -> available: 6
00:49:15.533 -> available: 7
00:49:15.533 -> available: 8
00:49:15.534 -> available: 9
00:49:15.534 -> available: 10
00:49:15.534 -> available: 11
00:49:15.535 -> uart input full!
00:49:15.535 -> available: 7
00:49:17.796 -> reading serial: 567890
00:49:17.796 -> 
00:49:17.796 -> available: 0
00:49:22.770 -> reading serial: 
00:49:27.779 -> reading serial: 
00:49:32.791 -> reading serial: 

[devyte]

Maybe it would be better to put this flag inside the uart struct. Given that the uart struct is opaque, also add an api to check/clear it.

[d-a-v]

Done

[s0170071]

the example I provided works now, still I get

 ets Jan  8 2013,rst cause:4, boot mode:(3,7)

wdt reset
load 0x4010f000, len 1384, room 16 
tail 8
chksum 0x2d
csum 0x2d
v54cc4634
~ld
⸮U

when I run ESPEasy with serial data incoming and core >2.3.0

[devyte]

I'm ok with either option here.

[devyte]

I'm ok with either action to take

[devyte]

I prefer discard newest data, because it keeps the data stream contiguous, but I'm ok with either option.

[devyte]

HardwareSerial needs a method to expose uart_has_overrun to the user

[d-a-v]

I will intentionally leave the old/new discard #if option choice for further reference.

[d-a-v]

@s0170071 thanks for your feedback!
Then further investigations will be needed on ESPeasy side

[s0170071]

@d-a-v The WD is triggered in WiFi.softAP().

[d-a-v]

@s0170071 please open a new issue with MCVE