cli tool: reimplement stdin buffering in -F option.

If stdin is not a regular file, its content is memory-buffered to enable
a possible data "rewind".
In all cases, stdin data size is determined before real use to avoid
having an unknown part's size.

--libcurl generated code is left as an unbuffered stdin fread/fseek callback
part with unknown data size.

Buffering is not supported in deprecated curl_formadd() API.

1 files changed, 6 insertions, 3 deletions

diff --git a/docs/cmdline-opts/form.d b/docs/cmdline-opts/form.d
index 4cc5f5a39..b3251bdb8 100644
--- a/docs/cmdline-opts/form.d
+++ b/docs/cmdline-opts/form.d
@@ -25,9 +25,12 @@ the form-field to which portrait.jpg will be the input:
  curl -F profile=@portrait.jpg https://example.com/upload.cgi
 
 To read content from stdin instead of a file, use - as the filename. This goes
-for both @ and < constructs. For this case, as well as for others in which the
-full data size cannot be determined before the transfer starts (as named pipes
-or similar), data is transferred as chunks by HTTP and rejected by IMAP.
+for both @ and < constructs. If stdin is not attached to a regular file, it is
+buffered first to determine its size and allow a possible resend. Defining a
+part's data from a named non-regular file (such as a named pipe or similar) is
+unfortunately not subject to buffering and will be effectively read at
+transmission time; since the full size is unknown before the transfer starts,
+data is sent as chunks by HTTP and rejected by IMAP.
 
 You can also tell curl what Content-Type to use by using 'type=', in a manner
 similar to: