sendf.c: added ability to call recv() before send() as workaround

WinSock destroys recv() buffer if send() is failed. As result - server
response may be lost if server sent it while curl is still sending
request. This behavior noticeable on HTTP server short replies if
libcurl use several send() for request (usually for POST request).
To workaround this problem, libcurl use recv() before every send() and
keeps received data in intermediate buffer for further processing.

Fixes: #657
Closes: #668

1 files changed, 101 insertions, 1 deletions

diff --git a/lib/sendf.c b/lib/sendf.c
index a75c5c743..23bcfa2c9 100644
--- a/lib/sendf.c
+++ b/lib/sendf.c
@@ -33,6 +33,7 @@
 #include "non-ascii.h"
 #include "curl_printf.h"
 #include "strerror.h"
+#include "select.h"
 
 /* The last #include files should be: */
 #include "curl_memory.h"
@@ -120,6 +121,90 @@ static size_t convert_lineends(struct SessionHandle *data,
 }
 #endif /* CURL_DO_LINEEND_CONV */
 
+#ifdef USE_RECV_BEFORE_SEND_WORKAROUND
+static void pre_receive_plain(struct connectdata *conn, int num)
+{
+  const curl_socket_t sockfd = conn->sock[num];
+  struct postponed_data * const psnd = &(conn->postponed[num]);
+  size_t bytestorecv = psnd->allocated_size - psnd->recv_size;
+  /* WinSock will destroy unread received data if send() is
+     failed.
+     To avoid lossage of received data, recv() must be
+     performed before every send() if any incoming data is
+     available. However, skip this, if buffer is already full. */
+  if((conn->handler->protocol&PROTO_FAMILY_HTTP) != 0 &&
+     conn->recv[num] == Curl_recv_plain &&
+     (!psnd->buffer || bytestorecv)) {
+    const int readymask = Curl_socket_check(sockfd, CURL_SOCKET_BAD,
+                                            CURL_SOCKET_BAD, 0);
+    if(readymask != -1 && (readymask & CURL_CSELECT_IN) != 0) {
+      /* Have some incoming data */
+      if(!psnd->buffer) {
+        /* Use buffer double default size for intermediate buffer */
+        psnd->allocated_size = 2 * BUFSIZE;
+        psnd->buffer = malloc(psnd->allocated_size);
+        psnd->recv_size = 0;
+        psnd->recv_processed = 0;
+#ifdef DEBUGBUILD
+        psnd->bindsock = sockfd; /* Used only for DEBUGASSERT */
+#endif /* DEBUGBUILD */
+        bytestorecv = psnd->allocated_size;
+      }
+      if(psnd->buffer) {
+        ssize_t recvedbytes;
+        DEBUGASSERT(psnd->bindsock == sockfd);
+        recvedbytes = sread(sockfd, psnd->buffer + psnd->recv_size,
+                            bytestorecv);
+        if(recvedbytes > 0)
+          psnd->recv_size += recvedbytes;
+      }
+      else
+        psnd->allocated_size = 0;
+    }
+  }
+}
+
+static ssize_t get_pre_recved(struct connectdata *conn, int num, char *buf,
+                              size_t len)
+{
+  struct postponed_data * const psnd = &(conn->postponed[num]);
+  size_t copysize;
+  if(!psnd->buffer)
+    return 0;
+
+  DEBUGASSERT(psnd->allocated_size > 0);
+  DEBUGASSERT(psnd->recv_size <= psnd->allocated_size);
+  DEBUGASSERT(psnd->recv_processed <= psnd->recv_size);
+  /* Check and process data that already received and storied in internal
+     intermediate buffer */
+  if(psnd->recv_size > psnd->recv_processed) {
+    DEBUGASSERT(psnd->bindsock == conn->sock[num]);
+    copysize = CURLMIN(len, psnd->recv_size - psnd->recv_processed);
+    memcpy(buf, psnd->buffer + psnd->recv_processed, copysize);
+    psnd->recv_processed += copysize;
+  }
+  else
+    copysize = 0; /* buffer was allocated, but nothing was received */
+
+  /* Free intermediate buffer if it has no unprocessed data */
+  if(psnd->recv_processed == psnd->recv_size) {
+    free(psnd->buffer);
+    psnd->buffer = NULL;
+    psnd->allocated_size = 0;
+    psnd->recv_size = 0;
+    psnd->recv_processed = 0;
+#ifdef DEBUGBUILD
+    psnd->bindsock = CURL_SOCKET_BAD;
+#endif /* DEBUGBUILD */
+  }
+  return (ssize_t)copysize;
+}
+#else  /* ! USE_RECV_BEFORE_SEND_WORKAROUND */
+/* Use "do-nothing" macros instead of functions when workaround not used */
+#define pre_receive_plain(c,n) do {} WHILE_FALSE
+#define get_pre_recved(c,n,b,l) 0
+#endif /* ! USE_RECV_BEFORE_SEND_WORKAROUND */
+
 /* Curl_infof() is for info message along the way */
 
 void Curl_infof(struct SessionHandle *data, const char *fmt, ...)
@@ -255,6 +340,12 @@ ssize_t Curl_send_plain(struct connectdata *conn, int num,
 {
   curl_socket_t sockfd = conn->sock[num];
   ssize_t bytes_written;
+  /* WinSock will destroy unread received data if send() is
+     failed.
+     To avoid lossage of received data, recv() must be
+     performed before every send() if any incoming data is
+     available. */
+  pre_receive_plain(conn, num);
 
 #ifdef MSG_FASTOPEN /* Linux */
   if(conn->bits.tcp_fastopen) {
@@ -322,7 +413,16 @@ ssize_t Curl_recv_plain(struct connectdata *conn, int num, char *buf,
                         size_t len, CURLcode *code)
 {
   curl_socket_t sockfd = conn->sock[num];
-  ssize_t nread = sread(sockfd, buf, len);
+  ssize_t nread;
+  /* Check and return data that already received and storied in internal
+     intermediate buffer */
+  nread = get_pre_recved(conn, num, buf, len);
+  if(nread > 0) {
+    *code = CURLE_OK;
+    return nread;
+  }
+
+  nread = sread(sockfd, buf, len);
 
   *code = CURLE_OK;
   if(-1 == nread) {