mime: latch last read callback status.

In case a read callback returns a status (pause, abort, eof,
error) instead of a byte count, drain the bytes read so far but
remember this status for further processing.
Takes care of not losing data when pausing, and properly resume a
paused mime structure when requested.
New tests 670-673 check unpausing cases, with easy or multi
interface and mime or form api.

Fixes #4813
Reported-by: MrdUkk on github

1 files changed, 11 insertions, 2 deletions

diff --git a/tests/libtest/lib643.c b/tests/libtest/lib643.c
index 7432dfce8..08c0f2e80 100644
--- a/tests/libtest/lib643.c
+++ b/tests/libtest/lib643.c
@@ -5,7 +5,7 @@
  *                            | (__| |_| |  _ <| |___
  *                             \___|\___/|_| \_\_____|
  *
- * Copyright (C) 1998 - 2017, Daniel Stenberg, <daniel@haxx.se>, et al.
+ * Copyright (C) 1998 - 2020, 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
@@ -41,11 +41,20 @@ struct WriteThis {
 static size_t read_callback(char *ptr, size_t size, size_t nmemb, void *userp)
 {
 #ifdef LIB644
+  static int count = 0;
   (void)ptr;
   (void)size;
   (void)nmemb;
   (void)userp;
-  return CURL_READFUNC_ABORT;
+  switch(count++) {
+  case 0: /* Return a single byte. */
+    *ptr = '\n';
+    return 1;
+  case 1: /* Request abort. */
+    return CURL_READFUNC_ABORT;
+  }
+  printf("Wrongly called >2 times\n");
+  exit(1); /* trigger major failure */
 #else
 
   struct WriteThis *pooh = (struct WriteThis *)userp;