Update the multipart chapter

doc/src/guide/multipart.asciidoc

@@ -2,36 +2,21 @@
 == Multipart requests
 
 Multipart originates from MIME, an Internet standard that
-extends the format of emails. Multipart messages are a
-container for parts of any content-type.
+extends the format of emails.
 
-For example, a multipart message may have a part
-containing text and a second part containing an
-image. This is what allows you to attach files
-to emails.
+A multipart message is a list of parts. A part contains
+headers and a body. The body of the parts may be
+of any media type, and contain text or binary data.
+It is possible for parts to contain a multipart media
+type.
 
 In the context of HTTP, multipart is most often used
-with the `multipart/form-data` content-type. This is
-the content-type you have to use when you want browsers
-to be allowed to upload files through HTML forms.
+with the `multipart/form-data` media type. It is what
+browsers use to upload files through HTML forms.
 
-Multipart is of course not required for uploading
-files, it is only required when you want to do so
-through HTML forms.
-
-You can read and parse multipart messages using the
-Req object directly.
-
-Cowboy defines two functions that allows you to get
-information about each part and read their contents.
-
-=== Structure
-
-A multipart message is a list of parts. Parts may
-contain either a multipart message or a non-multipart
-content-type. This allows parts to be arranged in a
-tree structure, although this is a rare case as far
-as the Web is concerned.
+The `multipart/byteranges` is also common. It is the
+media type used to send arbitrary bytes from a resource,
+enabling clients to resume downloads.
 
 === Form-data
 
@@ -42,29 +27,24 @@ values and is therefore not fit for uploading files.
 
 That's where the `multipart/form-data` content-type
 comes in. When the form is configured to use this
-content-type, the browser will use one part of the
-message for each form field. This means that a file
-input field will be sent in its own part, but the
-same applies to all other kinds of fields.
+content-type, the browser will create a multipart
+message where each part corresponds to a field on
+the form. For files, it also adds some metadata in
+the part headers, like the file name.
 
 A form with a text input, a file input and a select
 choice box will result in a multipart message with
 three parts, one for each field.
 
-The browser does its best to determine the content-type
+The browser does its best to determine the media type
 of the files it sends this way, but you should not
 rely on it for determining the contents of the file.
 Proper investigation of the contents is recommended.
 
-=== Checking the content-type
+=== Checking for multipart messages
 
-While there is a variety of multipart messages, the
-most common on the Web is `multipart/form-data`. It's
-the type of message being sent when an HTML form
-allows uploading files.
-
-You can quickly figure out if a multipart message
-has been sent by parsing the `content-type` header.
+The content-type header indicates the presence of
+a multipart message:
 
 [source,erlang]
 ----
@@ -74,96 +54,116 @@ has been sent by parsing the `content-type` header.
 
 === Reading a multipart message
 
-To read a message you have to iterate over all its
-parts. Then, for each part, you can inspect its headers
-and read its body.
+Cowboy provides two sets of functions for reading
+request bodies as multipart messages.
+
+The `cowboy_req:read_part/1,2` functions return the
+next part's headers, if any.
+
+The `cowboy_req:read_part_body/1,2` functions return
+the current part's body. For large bodies you may
+need to call the function multiple times.
+
+To read a multipart message you need to iterate over
+all its parts:
 
 [source,erlang]
 ----
-multipart(Req) ->
-    case cowboy_req:part(Req) of
-        {ok, _Headers, Req2} ->
-            {ok, _Body, Req3} = cowboy_req:part_body(Req2),
-            multipart(Req3);
-        {done, Req2} ->
-            Req2
+multipart(Req0) ->
+    case cowboy_req:read_part(Req0) of
+        {ok, _Headers, Req1} ->
+            {ok, _Body, Req} = cowboy_req:read_part_body(Req1),
+            multipart(Req);
+        {done, Req} ->
+            Req
     end.
 ----
 
-Parts do not have a size limit. When a part body is
-too big, Cowboy will return what it read so far and
-allow you to continue if you wish to do so.
+When part bodies are too large, Cowboy will return
+a `more` tuple, and allow you to loop until the part
+body has been fully read.
 
 The function `cow_multipart:form_data/1` can be used
 to quickly obtain information about a part from a
-`multipart/form-data` message. This function will
-tell you if the part is for a normal field or if it
-is a file being uploaded.
+`multipart/form-data` message. The function returns
+a `data` or a `file` tuple depending on whether this
+is a normal field or a file being uploaded.
 
-This can be used for example to allow large part bodies
-for files but crash when a normal field is too large.
+The following snippet will use this function and
+use different strategies depending on whether the
+part is a file:
 
 [source,erlang]
 ----
-multipart(Req) ->
-    case cowboy_req:part(Req) of
-        {ok, Headers, Req2} ->
-            Req4 = case cow_multipart:form_data(Headers) of
+multipart(Req0) ->
+    case cowboy_req:read_part(Req0) of
+        {ok, Headers, Req1} ->
+            Req = case cow_multipart:form_data(Headers) of
                 {data, _FieldName} ->
-                    {ok, _Body, Req3} = cowboy_req:part_body(Req2),
-                    Req3;
+                    {ok, _Body, Req2} = cowboy_req:read_part_body(Req1),
+                    Req2;
                 {file, _FieldName, _Filename, _CType, _CTransferEncoding} ->
-                    stream_file(Req2)
+                    stream_file(Req1)
             end,
-            multipart(Req4);
-        {done, Req2} ->
-            Req2
+            multipart(Req);
+        {done, Req} ->
+            Req
     end.
 
-stream_file(Req) ->
-    case cowboy_req:part_body(Req) of
-        {ok, _Body, Req2} ->
-            Req2;
-        {more, _Body, Req2} ->
-            stream_file(Req2)
+stream_file(Req0) ->
+    case cowboy_req:read_part_body(Req0) of
+        {ok, _Body, Req} ->
+            Req;
+        {more, _Body, Req} ->
+            stream_file(Req)
     end.
 ----
 
-By default the body chunk Cowboy will return is limited
-to 8MB. This can of course be overriden. Both functions
-can take a second argument, the same list of options that
-will be passed to `cowboy_req:body/2` function.
+Both the part header and body reading functions can take
+options that will be given to the request body reading
+functions. By default, `cowboy_req:read_part/1` reads
+up to 64KB for up to 5 seconds. `cowboy_req:read_part_body/1`
+has the same defaults as `cowboy_req:read_body/1`.
+
+To change the defaults for part headers:
+
+[source,erlang]
+cowboy_req:read_part(Req, #{length => 128000}).
+
+And for part bodies:
+
+[source,erlang]
+cowboy_req:read_part_body(Req, #{length => 1000000, period => 7000}).
 
 === Skipping unwanted parts
 
-If you do not want to read a part's body, you can skip it.
-Skipping is easy. If you do not call the function to read
-the part's body, Cowboy will automatically skip it when
-you request the next part.
+Part bodies do not have to be read. Cowboy will automatically
+skip it when you request the next part's body.
 
 The following snippet reads all part headers and skips
 all bodies:
 
 [source,erlang]
 ----
-multipart(Req) ->
-    case cowboy_req:part(Req) of
-        {ok, _Headers, Req2} ->
-            multipart(Req2);
-        {done, Req2} ->
-            Req2
+multipart(Req0) ->
+    case cowboy_req:part(Req0) of
+        {ok, _Headers, Req} ->
+            multipart(Req);
+        {done, Req} ->
+            Req
     end.
 ----
 
 Similarly, if you start reading the body and it ends up
-being too big, you can simply continue with the next part,
+being too big, you can simply continue with the next part.
 Cowboy will automatically skip what remains.
 
-Note that the skipping rate may not be adequate for your
-application. If you observe poor performance when skipping,
-you might want to consider manually skipping by calling
-the `cowboy_req:part_body/1` function directly.
+While Cowboy can skip part bodies automatically, the read
+rate is not configurable. Depending on your application
+you may want to skip manually, in particular if you observe
+poor performance while skipping.
+
+You do not have to read all parts either. You can stop
+reading as soon as you find the data you need.
 
-And if you started reading the message but decide that you
-do not need the remaining parts, you can simply stop reading
-entirely and Cowboy will automatically figure out what to do.
+// @todo Cover the building of multipart messages.