Adjust tcp_write size when memory is tight (#729)

Increases the MEM_SIZE outstanding write buffer to 8K

Allows the ClientContext to attempt to send smaller buffer chunks in the
case where MEM_SIZE won't allow the full tcp_sndbuf() transfer.

Fixes #725

include/lwipopts.h

@@ -21,7 +21,7 @@ extern void interrupts();
 #define MEM_LIBC_MALLOC             0
 
 #define MEM_ALIGNMENT               4
-#define MEM_SIZE                    4000
+#define MEM_SIZE                    8192
 #define MEMP_NUM_TCP_SEG            32
 #define MEMP_NUM_ARP_QUEUE          10
 #define PBUF_POOL_SIZE              24

libraries/WiFi/src/include/ClientContext.h

@@ -491,6 +491,7 @@ protected:
         DEBUGV(":wr %d %d\r\n", _datalen - _written, _written);
 
         bool has_written = false;
+        int scale = 0;
 
         while (_written < _datalen) {
             if (state() == CLOSED) {
@@ -501,6 +502,10 @@ protected:
             {
                 LWIPMutex m;  // Block the timer sys_check_timeouts call, just for this call
                 next_chunk_size = std::min((size_t)tcp_sndbuf(_pcb), remaining);
+                // Potentially reduce transmit size if we are tight on memory, but only if it doesn't return a 0 chunk size
+                if (next_chunk_size > (size_t)(1 << scale)) {
+                    next_chunk_size >>= scale;
+                }
             }
             if (!next_chunk_size) {
                 break;
@@ -531,6 +536,13 @@ protected:
             if (err == ERR_OK) {
                 _written += next_chunk_size;
                 has_written = true;
+            } else if (err == ERR_MEM) {
+                if (scale < 4) {
+                    // Retry sending at 1/2 the chunk size
+                    scale ++;
+                } else {
+                    break;
+                }
             } else {
                 // ERR_MEM(-1) is a valid error meaning
                 // "come back later". It leaves state() opened

tools/libpico/lwipopts.h

@@ -21,7 +21,7 @@ extern void interrupts();
 #define MEM_LIBC_MALLOC             0
 
 #define MEM_ALIGNMENT               4
-#define MEM_SIZE                    4000
+#define MEM_SIZE                    8192
 #define MEMP_NUM_TCP_SEG            32
 #define MEMP_NUM_ARP_QUEUE          10
 #define PBUF_POOL_SIZE              24