Fix pipelining issue with multipart upload

Do not clear buffer at the end of multipart request to allow following request to proceed.

Closes https://github.com/cesanta/mongoose/pull/940

CL: mg: Fix pipelining issue with multipart upload

PUBLISHED_FROM=cc7d38b126eaa863553ee4e124cfafcd72030fcf

mongoose.c

@@ -6725,7 +6725,6 @@ static void mg_http_multipart_continue(struct mg_connection *c) {
         break;
       }
       case MPS_FINISHED: {
-        mbuf_remove(&c->recv_mbuf, c->recv_mbuf.len);
         return;
       }
     }

src/mg_http.c

@@ -1159,7 +1159,6 @@ static void mg_http_multipart_continue(struct mg_connection *c) {
         break;
       }
       case MPS_FINISHED: {
-        mbuf_remove(&c->recv_mbuf, c->recv_mbuf.len);
         return;
       }
     }

test/unit_test.c

@@ -4321,6 +4321,95 @@ static const char *test_http_multipart2(void) {
   return NULL;
 }
 
+static const char *test_http_multipart_pipeline(void) {
+  struct mg_mgr mgr;
+  struct mg_connection *nc_listen;
+
+  const char multi_part_req_fmt[] =
+      "%s"
+      "Content-Disposition: form-data; name=\"a\"; filename=\"foo\"\r\n"
+      "\r\n"
+      "%s"
+      "\r\n--Asrf456BGe4h\r\n"
+      "Content-Disposition: form-data; name=\"b\"\r\n"
+      "\r\n"
+      "%s"
+      "\r\n--Asrf456BGe4h\r\n"
+      "Content-Disposition: form-data; name=\"c\"; filename=\"bar\"\r\n"
+      "\r\n"
+      "%s"
+      "\r\n--Asrf456BGe4h--\r\n"
+      "\r\n";
+
+  char multi_part_req[1024 * 5];
+  struct mg_connection *c;
+  const char *ptr;
+  int i;
+
+  struct cb_mp_srv_data mpd;
+  memset(&mpd, 0, sizeof(mpd));
+  mbuf_init(&mpd.res, 0);
+
+  mg_mgr_init(&mgr, NULL);
+  nc_listen = mg_bind(&mgr, "8765", cb_mp_srv);
+  nc_listen->user_data = &mpd;
+
+  mg_set_protocol_http_websocket(nc_listen);
+
+  snprintf(multi_part_req, sizeof(multi_part_req), multi_part_req_fmt,
+           "\r\n--Asrf456BGe4h\r\n", b1, b2, b4);
+
+  ASSERT((c = mg_connect_http(&mgr, cb_mp_empty, "http://127.0.0.1:8765/test",
+                              "Content-Type: "
+                              "multipart/form-data;boundary=Asrf456BGe4h\r\n"
+                              "Connection: keep-alive",
+                              multi_part_req)) != NULL);
+
+  c->user_data = multi_part_req;
+
+  mg_printf(c,
+            "POST /test HTTP/1.1\r\n"
+            "Connection: keep-alive\r\n"
+            "Content-Type: multipart/form-data;boundary=Asrf456BGe4h\r\n"
+            "Content-Length: %d\r\n%s",
+            (int) strlen(multi_part_req), multi_part_req);
+
+  poll_until(&mgr, 10, c_int_eq, &mpd.request_end, (void *) 1);
+
+  ptr = mpd.res.buf;
+  for (i = 0; i < 2; i++) {
+    ASSERT_STREQ_NZ(ptr, "<MPRQ/test");
+    ptr += 10;
+    ASSERT_STREQ_NZ(ptr, "afoo");
+    ptr += 4;
+    ASSERT_STREQ_NZ(ptr, b1);
+    ptr += sizeof(b1) - 1;
+    ASSERT_STREQ_NZ(ptr, "afooFIN");
+    ptr += 7;
+    /* No file_name for second part */
+    ASSERT_STREQ_NZ(ptr, "b");
+    ptr++;
+    ASSERT_STREQ_NZ(ptr, b2);
+    ptr += sizeof(b2) - 1;
+    ASSERT_STREQ_NZ(ptr, "bFIN");
+    ptr += 4;
+    ASSERT_STREQ_NZ(ptr, "cbar");
+    ptr += 4;
+    ASSERT_STREQ_NZ(ptr, b4);
+    ptr += sizeof(b4) - 1;
+    ASSERT_STREQ_NZ(ptr, "cbarFIN");
+    ptr += 7;
+    ASSERT_STREQ_NZ(ptr, "+11MPRQ>");
+    ptr += 8;
+  }
+  ASSERT_EQ((size_t)(ptr - mpd.res.buf), mpd.res.len);
+
+  mbuf_free(&mpd.res);
+  mg_mgr_free(&mgr);
+
+  return NULL;
+}
+
 static struct mg_str upload_lfn_same(struct mg_connection *nc,
                                      struct mg_str fn) {
   if (fn.len == 0) {
@@ -5632,6 +5721,7 @@ const char *tests_run(const char *filter) {
   RUN_TEST(test_http_multipart);
 #if MG_ENABLE_HTTP_STREAMING_MULTIPART
   RUN_TEST(test_http_multipart2);
+  RUN_TEST(test_http_multipart_pipeline);
   RUN_TEST(test_http_multipart_upload);
 #endif
   RUN_TEST(test_parse_date_string);