lib: pass in 'struct Curl_easy *' to most functions

... in most cases instead of 'struct connectdata *' but in some cases in
addition to.

- We mostly operate on transfers and not connections.

- We need the transfer handle to log, store data and more. Everything in
  libcurl is driven by a transfer (the CURL * in the public API).

- This work clarifies and separates the transfers from the connections
  better.

- We should avoid "conn->data". Since individual connections can be used
  by many transfers when multiplexing, making sure that conn->data
  points to the current and correct transfer at all times is difficult
  and has been notoriously error-prone over the years. The goal is to
  ultimately remove the conn->data pointer for this reason.

Closes #6425

lib/pingpong.h

@@ -7,7 +7,7 @@
  *                            | (__| |_| |  _ <| |___
  *                             \___|\___/|_| \_\_____|
  *
- * Copyright (C) 1998 - 2020, Daniel Stenberg, <daniel@haxx.se>, et al.
+ * Copyright (C) 1998 - 2021, Daniel Stenberg, <daniel@haxx.se>, et al.
  *
  * This software is licensed as described in the file COPYING, which
  * you should have received as part of this distribution. The terms
@@ -62,37 +62,44 @@ struct pingpong {
                                off, used to time-out response reading */
   timediff_t response_time; /* When no timeout is given, this is the amount of
                                milliseconds we await for a server response. */
-  struct connectdata *conn; /* points to the connectdata struct that this
-                               belongs to */
+  struct connectdata *conn; /* the connection */
   struct dynbuf sendbuf;
 
   /* Function pointers the protocols MUST implement and provide for the
      pingpong layer to function */
 
-  CURLcode (*statemach_act)(struct connectdata *conn);
-
-  bool (*endofresp)(struct connectdata *conn, char *ptr, size_t len,
-                    int *code);
+  CURLcode (*statemachine)(struct Curl_easy *data, struct connectdata *conn);
+  bool (*endofresp)(struct Curl_easy *data, struct connectdata *conn,
+                    char *ptr, size_t len, int *code);
 };
 
+#define PINGPONG_SETUP(pp,s,e)                   \
+  do {                                           \
+    pp->response_time = RESP_TIMEOUT;            \
+    pp->statemachine = s;                        \
+    pp->endofresp = e;                           \
+    pp->conn = conn;                             \
+  } while(0)
+
 /*
  * Curl_pp_statemach()
  *
  * called repeatedly until done. Set 'wait' to make it wait a while on the
  * socket if there's no traffic.
  */
-CURLcode Curl_pp_statemach(struct pingpong *pp, bool block,
-                           bool disconnecting);
+CURLcode Curl_pp_statemach(struct Curl_easy *data, struct pingpong *pp,
+                           bool block, bool disconnecting);
 
 /* initialize stuff to prepare for reading a fresh new response */
-void Curl_pp_init(struct pingpong *pp);
+void Curl_pp_init(struct Curl_easy *data, struct pingpong *pp);
 
 /* setup for the transfer */
 void Curl_pp_setup(struct pingpong *pp);
 
 /* Returns timeout in ms. 0 or negative number means the timeout has already
    triggered */
-timediff_t Curl_pp_state_timeout(struct pingpong *pp, bool disconnecting);
+timediff_t Curl_pp_state_timeout(struct Curl_easy *data,
+                                 struct pingpong *pp, bool disconnecting);
 
 
 /***********************************************************************
@@ -105,7 +112,8 @@ timediff_t Curl_pp_state_timeout(struct pingpong *pp, bool disconnecting);
  *
  * made to never block
  */
-CURLcode Curl_pp_sendf(struct pingpong *pp,
+CURLcode Curl_pp_sendf(struct Curl_easy *data,
+                       struct pingpong *pp,
                        const char *fmt, ...);
 
 /***********************************************************************
@@ -118,7 +126,8 @@ CURLcode Curl_pp_sendf(struct pingpong *pp,
  *
  * made to never block
  */
-CURLcode Curl_pp_vsendf(struct pingpong *pp,
+CURLcode Curl_pp_vsendf(struct Curl_easy *data,
+                        struct pingpong *pp,
                         const char *fmt,
                         va_list args);
 
@@ -127,7 +136,8 @@ CURLcode Curl_pp_vsendf(struct pingpong *pp,
  *
  * Reads a piece of a server response.
  */
-CURLcode Curl_pp_readresp(curl_socket_t sockfd,
+CURLcode Curl_pp_readresp(struct Curl_easy *data,
+                          curl_socket_t sockfd,
                           struct pingpong *pp,
                           int *code, /* return the server code if done */
                           size_t *size); /* size of the response */