multi: add xfer_buf to multi handle

- can be borrowed by transfer during recv-write operation
- needs to be released before borrowing again
- adjustis size to `data->set.buffer_size`
- used in transfer.c readwrite_data()

Closes #12805

lib/multiif.h

@@ -94,4 +94,28 @@ CURLMcode Curl_multi_add_perform(struct Curl_multi *multi,
 /* Return the value of the CURLMOPT_MAX_CONCURRENT_STREAMS option */
 unsigned int Curl_multi_max_concurrent_streams(struct Curl_multi *multi);
 
+/**
+ * Borrow the transfer buffer from the multi, suitable
+ * for the given transfer `data`. The buffer may only be used in one
+ * multi processing of the easy handle. It MUST be returned to the
+ * multi before it can be borrowed again.
+ * Pointers into the buffer remain only valid as long as it is borrowed.
+ *
+ * @param data    the easy handle
+ * @param pbuf    on return, the buffer to use or NULL on error
+ * @param pbuflen on return, the size of *pbuf or 0 on error
+ * @return CURLE_OK when buffer is available and is returned.
+ *         CURLE_OUT_OF_MEMORy on failure to allocate the buffer,
+ *         CURLE_FAILED_INIT if the easy handle is without multi.
+ *         CURLE_AGAIN if the buffer is borrowed already.
+ */
+CURLcode Curl_multi_xfer_buf_borrow(struct Curl_easy *data,
+                                   char **pbuf, size_t *pbuflen);
+/**
+ * Release the borrowed buffer. All references into the buffer become
+ * invalid after this.
+ * @param buf the buffer pointer borrowed for coding error checks.
+ */
+void Curl_multi_xfer_buf_release(struct Curl_easy *data, char *buf);
+
 #endif /* HEADER_CURL_MULTIIF_H */