Fix random crashing of ClientContext::write(Stream) and write_P(PGM_P buf, size_t size) (#2504) (#4530)

* Fix random crashing of ClientContext::write(Stream) and write_P(PGM_P buf, size_t size) (#2504)

* - Allow partial buffer release

* - Refined comments

libraries/ESP8266WiFi/src/include/DataSource.h

@@ -66,20 +66,56 @@ public:
     const uint8_t* get_buffer(size_t size) override
     {
         assert(_pos + size <= _size);
-        if (_bufferSize < size) {
+
-            _buffer.reset(new uint8_t[size]);
+        //Data that was already read from the stream but not released (e.g. if tcp_write error occured). Otherwise this should be 0.
-            _bufferSize = size;
+        const size_t stream_read = _streamPos - _pos;
+
+        //Min required buffer size: max(requested size, previous stream data already in buffer)
+        const size_t min_buffer_size = size > stream_read ? size : stream_read;
+
+        //Buffer too small?
+        if (_bufferSize < min_buffer_size) {
+            uint8_t *new_buffer = new uint8_t[min_buffer_size];
+            //If stream reading is ahead, than some data is already in the old buffer and needs to be copied to new resized buffer
+            if (_buffer && stream_read > 0) {
+                memcpy(new_buffer, _buffer.get(), stream_read);
             }
-        size_t cb = _stream.readBytes(reinterpret_cast<char*>(_buffer.get()), size);
+            _buffer.reset(new_buffer);
-        assert(cb == size);
+            _bufferSize = min_buffer_size;
+        }
+
+        //Fetch remaining data from stream
+        //If error in tcp_write in ClientContext::_write_some() occured earlier and therefore release_buffer was not called last time, than the requested stream data is already in the buffer.
+        if (size > stream_read) {
+            //Remaining bytes to read from stream
+            const size_t stream_rem = size - stream_read;
+            const size_t cb = _stream.readBytes(reinterpret_cast<char*>(_buffer.get() + stream_read), stream_rem);
+            assert(cb == stream_rem);
             (void)cb;
+            _streamPos += stream_rem;
+        }
         return _buffer.get();
+
     }
 
     void release_buffer(const uint8_t* buffer, size_t size) override
     {
+        if (size == 0) {
+            return;
+        }
+
         (void)buffer;
         _pos += size;     
+
+        //Cannot release more than acquired through get_buffer
+        assert(_pos <= _streamPos);
+
+        //Release less than requested with get_buffer?
+        if (_pos < _streamPos) {
+            // Move unreleased stream data in buffer to front
+            assert(_buffer);
+            memmove(_buffer.get(), _buffer.get() + size, _streamPos - _pos);
+        }
     }
 
 protected:
@@ -88,6 +124,7 @@ protected:
     size_t _size;
     size_t _pos = 0;
     size_t _bufferSize = 0;
+    size_t _streamPos = 0;
 };
 
 class ProgmemStream