ESP8266: Trim TCP segment pbuf before sending

Because LWIP_NETIF_TX_SINGLE_PBUF is enabled (and must be enabled,
since lower leel stack can't do scatter/gather), all segments are
preallocated to full MSS, even if the resulting segment is much
smalller. When we are done with the segment and about to send it out,
trim the pbuf memory allocation to exactly the size required.
This is signioficant because pbufs can spend significant time in unacked
queue, and trimming them ASAP reduces peak memory usage when multiple
connections are active.

Signed-off-by: Deomid "rojer" Ryabkov <rojer@rojer.me>

src/esp8266/sdk_lwip/src/core/tcp_out.c

@@ -54,6 +54,7 @@
 #include "lwip/snmp.h"
 #include "netif/etharp.h"
 
+#include <stdlib.h>
 #include <string.h>
 
 #ifdef MEMLEAK_DEBUG
@@ -1170,6 +1171,24 @@ tcp_output_segment(struct tcp_seg *seg, struct tcp_pcb *pcb)
 #endif /* CHECKSUM_GEN_TCP */
   TCP_STATS_INC(tcp.xmit);
 
+  /*
+   * If the segment is a single RAM-backed pbuf, trim it to reduce memory usage.
+   * Note that what we want here is pbuf_realloc but it does not handle the case
+   * of buffer being relocated (unlikely in practice).
+   */
+  do {
+    struct pbuf *p = seg->p;
+    if (p->type != PBUF_RAM || p->next != NULL) break;
+    uint16_t payload_offset = (((char *) p->payload) - ((char *) p));
+    uint16_t trimmed_len = LWIP_MEM_ALIGN_SIZE(payload_offset + p->len);
+    struct pbuf *p2 = (struct pbuf *) realloc(p, trimmed_len);
+    if (p2 != NULL && p2 != p) {
+      p2->payload = ((char *) p2) + payload_offset;
+      seg->p = p2;
+      seg->tcphdr = p2->payload;
+    }
+  } while (0);
+
 #if LWIP_NETIF_HWADDRHINT
   ip_output_hinted(seg->p, &(pcb->local_ip), &(pcb->remote_ip), pcb->ttl, pcb->tos,
       IP_PROTO_TCP, &(pcb->addr_hint));