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
Closes #4833

4 files changed, 58 insertions, 10 deletions

diff --git a/lib/easy.c b/lib/easy.c
index 1a6912748..85a882a97 100644
--- a/lib/easy.c
+++ b/lib/easy.c
@@ -983,6 +983,14 @@ CURLcode curl_easy_pause(struct Curl_easy *data, int action)
   newstate |= ((action & CURLPAUSE_RECV)?KEEP_RECV_PAUSE:0) |
     ((action & CURLPAUSE_SEND)?KEEP_SEND_PAUSE:0);
 
+  /* Unpause parts in active mime tree. */
+  if((k->keepon & ~newstate & KEEP_SEND_PAUSE) &&
+     (data->mstate == CURLM_STATE_PERFORM ||
+      data->mstate == CURLM_STATE_TOOFAST) &&
+     data->state.fread_func == (curl_read_callback) Curl_mime_read) {
+    Curl_mime_unpause(data->state.in);
+  }
+
   /* put it back in the keepon */
   k->keepon = newstate;
 
diff --git a/lib/formdata.c b/lib/formdata.c
index 429d479da..6103959cd 100644
--- a/lib/formdata.c
+++ b/lib/formdata.c
@@ -5,7 +5,7 @@
  *                            | (__| |_| |  _ <| |___
  *                             \___|\___/|_| \_\_____|
  *
- * Copyright (C) 1998 - 2019, 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
@@ -730,11 +730,15 @@ int curl_formget(struct curl_httppost *form, void *arg,
 
     switch(nread) {
     default:
-      if(append(arg, buffer, nread) != nread)
-        result = CURLE_READ_ERROR;
-      break;
+      if(append(arg, buffer, nread) == nread)
+        break;
+      /* FALLTHROUGH */
     case CURL_READFUNC_ABORT:
-    case CURL_READFUNC_PAUSE:
+      result = CURLE_ABORTED_BY_CALLBACK;
+      break;
+    case (size_t) -1:   /* Read error. */
+    case CURL_READFUNC_PAUSE:    /* Should not be paused. */
+      result = CURLE_READ_ERROR;
       break;
     }
   }
diff --git a/lib/mime.c b/lib/mime.c
index 081e51e50..5f928a171 100644
--- a/lib/mime.c
+++ b/lib/mime.c
@@ -5,7 +5,7 @@
  *                            | (__| |_| |  _ <| |___
  *                             \___|\___/|_| \_\_____|
  *
- * Copyright (C) 1998 - 2019, 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
@@ -740,9 +740,19 @@ static size_t read_part_content(curl_mimepart *part,
 {
   size_t sz = 0;
 
-  if(part->readfunc)
-    sz = part->readfunc(buffer, 1, bufsize, part->arg);
-  return sz;
+  switch(part->lastreadstatus) {
+  case 0:
+  case CURL_READFUNC_ABORT:
+  case CURL_READFUNC_PAUSE:
+  case READ_ERROR:
+    break;
+  default:
+    if(part->readfunc)
+      sz = part->readfunc(buffer, 1, bufsize, part->arg);
+    part->lastreadstatus = sz;
+    break;
+  }
+  return part->lastreadstatus;
 }
 
 /* Read and encode part content. */
@@ -1031,6 +1041,7 @@ static int mime_part_rewind(curl_mimepart *part)
   if(res == CURL_SEEKFUNC_OK)
     mimesetstate(&part->state, targetstate, NULL);
 
+  part->lastreadstatus = 1; /* Successful read status. */
   return res;
 }
 
@@ -1073,6 +1084,7 @@ static void cleanup_part_content(curl_mimepart *part)
   part->datasize = (curl_off_t) 0;    /* No size yet. */
   cleanup_encoder_state(&part->encstate);
   part->kind = MIMEKIND_NONE;
+  part->lastreadstatus = 1; /* Successful read status. */
 }
 
 static void mime_subparts_free(void *ptr)
@@ -1238,6 +1250,7 @@ void Curl_mime_initpart(curl_mimepart *part, struct Curl_easy *easy)
 {
   memset((char *) part, 0, sizeof(*part));
   part->easy = easy;
+  part->lastreadstatus = 1; /* Successful read status. */
   mimesetstate(&part->state, MIMESTATE_BEGIN, NULL);
 }
 
@@ -1805,6 +1818,26 @@ CURLcode Curl_mime_prepare_headers(curl_mimepart *part,
   return ret;
 }
 
+/* Recursively reset paused status in the given part. */
+void Curl_mime_unpause(curl_mimepart *part)
+{
+  if(part) {
+    if(part->lastreadstatus == CURL_READFUNC_PAUSE)
+      part->lastreadstatus = 1; /* Successful read status. */
+    if(part->kind == MIMEKIND_MULTIPART) {
+      curl_mime *mime = (curl_mime *) part->arg;
+
+      if(mime) {
+        curl_mimepart *subpart;
+
+        for(subpart = mime->firstpart; subpart; subpart = subpart->nextpart)
+          Curl_mime_unpause(subpart);
+      }
+    }
+  }
+}
+
+
 #else /* !CURL_DISABLE_HTTP || !CURL_DISABLE_SMTP || !CURL_DISABLE_IMAP */
 
 /* Mime not compiled in: define stubs for externally-referenced functions. */
diff --git a/lib/mime.h b/lib/mime.h
index 3241fdc1f..c6d374ec1 100644
--- a/lib/mime.h
+++ b/lib/mime.h
@@ -7,7 +7,7 @@
  *                            | (__| |_| |  _ <| |___
  *                             \___|\___/|_| \_\_____|
  *
- * Copyright (C) 1998 - 2019, 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
@@ -125,6 +125,7 @@ struct curl_mimepart_s {
   mime_state state;                /* Current readback state. */
   const mime_encoder *encoder;     /* Content data encoder. */
   mime_encoder_state encstate;     /* Data encoder state. */
+  size_t lastreadstatus;           /* Last read callback returned status. */
 };
 
 CURLcode Curl_mime_add_header(struct curl_slist **slp, const char *fmt, ...);
@@ -147,6 +148,7 @@ size_t Curl_mime_read(char *buffer, size_t size, size_t nitems,
                       void *instream);
 CURLcode Curl_mime_rewind(curl_mimepart *part);
 const char *Curl_mime_contenttype(const char *filename);
+void Curl_mime_unpause(curl_mimepart *part);
 
 #else
 /* if disabled */
@@ -158,6 +160,7 @@ const char *Curl_mime_contenttype(const char *filename);
 #define Curl_mime_size(x) (curl_off_t) -1
 #define Curl_mime_read NULL
 #define Curl_mime_rewind(x) ((void)x, CURLE_NOT_BUILT_IN)
+#define Curl_mime_unpause(x)
 #endif