BREAKING: Add Print::availableForWrite method (#7658)

* Add Print::availableForWrite method

Adds an availableForWrite() method to the Print class, matching current
ArduinoCore-API commit 398e70f188e2b861c10d9ffe5e2bfcb6a4a4f489 .

Hook availableForWrite into the SDFS filesystem (other FSes don't have
this capability built-in).

Fixes #7650

* WiFiClient::availableForWrite proto matching Print

* Fix Netdump signedness warning

* Clean up Serial availableForWrite

This is evidently a breaking change due to the type difference.
Arduino's `availableForWrite` returns an `int`, while the
(multiply-implemented, non-virtual) core `availableForWrite` returned
`size_t`.

cores/esp8266/FS.cpp

@@ -46,6 +46,14 @@ int File::available() {
     return _p->size() - _p->position();
 }
 
+int File::availableForWrite() {
+    if (!_p)
+        return false;
+
+    return _p->availableForWrite();
+}
+
+
 int File::read() {
     if (!_p)
         return -1;

cores/esp8266/FS.h

@@ -57,6 +57,7 @@ public:
     // Print methods:
     size_t write(uint8_t) override;
     size_t write(const uint8_t *buf, size_t size) override;
+    int availableForWrite() override;
 
     // Stream methods:
     int available() override;

cores/esp8266/FSImpl.h

@@ -35,6 +35,7 @@ public:
     virtual bool seek(uint32_t pos, SeekMode mode) = 0;
     virtual size_t position() const = 0;
     virtual size_t size() const = 0;
+    virtual int availableForWrite() { return 0; }
     virtual bool truncate(uint32_t size) = 0;
     virtual void close() = 0;
     virtual const char* name() const = 0;

cores/esp8266/HardwareSerial.h

@@ -150,7 +150,7 @@ public:
     {
         return readBytes((char*)buffer, size);
     }
-    int availableForWrite(void)
+    int availableForWrite(void) override
     {
         return static_cast<int>(uart_tx_free(_uart));
     }

cores/esp8266/Print.h

@@ -75,6 +75,10 @@ class Print {
         inline size_t write(char c) { return write((uint8_t) c); }
         inline size_t write(int8_t c) { return write((uint8_t) c); }
 
+        // default to zero, meaning "a single write may block"
+        // should be overriden by subclasses with buffering
+        virtual int availableForWrite() { return 0; }
+
         size_t printf(const char * format, ...)  __attribute__ ((format (printf, 2, 3)));
         size_t printf_P(PGM_P format, ...) __attribute__((format(printf, 2, 3)));
         size_t print(const __FlashStringHelper *);

libraries/ESP8266WiFi/examples/WiFiTelnetToSerial/WiFiTelnetToSerial.ino

@@ -173,10 +173,10 @@ void loop() {
 
   // determine maximum output size "fair TCP use"
   // client.availableForWrite() returns 0 when !client.connected()
-  size_t maxToTcp = 0;
+  int maxToTcp = 0;
   for (int i = 0; i < MAX_SRV_CLIENTS; i++)
     if (serverClients[i]) {
-      size_t afw = serverClients[i].availableForWrite();
+      int afw = serverClients[i].availableForWrite();
       if (afw) {
         if (!maxToTcp) {
           maxToTcp = afw;
@@ -190,11 +190,11 @@ void loop() {
     }
 
   //check UART for data
-  size_t len = std::min((size_t)Serial.available(), maxToTcp);
+  size_t len = std::min(Serial.available(), maxToTcp);
   len = std::min(len, (size_t)STACK_PROTECTOR);
   if (len) {
     uint8_t sbuf[len];
-    size_t serial_got = Serial.readBytes(sbuf, len);
+    int serial_got = Serial.readBytes(sbuf, len);
     // push UART data to all connected telnet clients
     for (int i = 0; i < MAX_SRV_CLIENTS; i++)
       // if client.availableForWrite() was 0 (congested)

libraries/ESP8266WiFi/src/WiFiClient.cpp

@@ -195,7 +195,7 @@ bool WiFiClient::getSync() const
     return _client->getSync();
 }
 
-size_t WiFiClient::availableForWrite ()
+int WiFiClient::availableForWrite ()
 {
     return _client? _client->availableForWrite(): 0;
 }

libraries/ESP8266WiFi/src/WiFiClient.h

@@ -86,7 +86,7 @@ public:
 
   static void setLocalPortStart(uint16_t port) { _localPort = port; }
 
-  size_t availableForWrite();
+  int availableForWrite() override;
 
   friend class WiFiServer;
 

libraries/Netdump/src/Netdump.cpp

@@ -176,7 +176,7 @@ void Netdump::tcpDumpProcess(const Packet& np)
         bufferIndex += incl_len;
     }
 
-    if (bufferIndex && tcpDumpClient && tcpDumpClient.availableForWrite() >= bufferIndex)
+    if (bufferIndex && tcpDumpClient && tcpDumpClient.availableForWrite() >= (int)bufferIndex)
     {
         tcpDumpClient.write(packetBuffer, bufferIndex);
         bufferIndex = 0;
@@ -202,7 +202,7 @@ void Netdump::tcpDumpLoop(WiFiServer &tcpDumpServer, const Filter nf)
     {
         setCallback(nullptr);
     }
-    if (bufferIndex && tcpDumpClient && tcpDumpClient.availableForWrite() >= bufferIndex)
+    if (bufferIndex && tcpDumpClient && tcpDumpClient.availableForWrite() >= (int)bufferIndex)
     {
         tcpDumpClient.write(packetBuffer, bufferIndex);
         bufferIndex = 0;

libraries/SDFS/src/SDFS.h

@@ -268,6 +268,11 @@ public:
         close();
     }
 
+    int availableForWrite() override
+    {
+        return _opened ? _fd->availableForWrite() : 0;
+    }
+
     size_t write(const uint8_t *buf, size_t size) override
     {
         return _opened ? _fd->write(buf, size) : -1;