You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
165 lines
8.3 KiB
165 lines
8.3 KiB
2014/04/16 - Pointer assignments during processing of the HTTP body
|
|
|
|
In HAProxy, a struct http_msg is a descriptor for an HTTP message, which stores
|
|
the state of an HTTP parser at any given instant, relative to a buffer which
|
|
contains part of the message being inspected.
|
|
|
|
Currently, an http_msg holds a few pointers and offsets to some important
|
|
locations in a message depending on the state the parser is in. Some of these
|
|
pointers and offsets may move when data are inserted into or removed from the
|
|
buffer, others won't move.
|
|
|
|
An important point is that the state of the parser only translates what the
|
|
parser is reading, and not at all what is being done on the message (eg:
|
|
forwarding).
|
|
|
|
For an HTTP message <msg> and a buffer <buf>, we have the following elements
|
|
to work with :
|
|
|
|
|
|
Buffer :
|
|
--------
|
|
|
|
buf.size : the allocated size of the buffer. A message cannot be larger than
|
|
this size. In general, a message will even be smaller because the
|
|
size is almost always reduced by global.maxrewrite bytes.
|
|
|
|
buf.data : memory area containing the part of the message being worked on. This
|
|
area is exactly <buf.size> bytes long. It should be seen as a sliding
|
|
window over the message, but in terms of implementation, it's closer
|
|
to a wrapping window. For ease of processing, new messages (requests
|
|
or responses) are aligned to the beginning of the buffer so that they
|
|
never wrap and common string processing functions can be used.
|
|
|
|
buf.p : memory pointer (char *) to the beginning of the buffer as the parser
|
|
understands it. It commonly refers to the first character of an HTTP
|
|
request or response, but during forwarding, it can point to other
|
|
locations. This pointer always points to a location in <buf.data>.
|
|
|
|
buf.i : number of bytes after <buf.p> that are available in the buffer. If
|
|
<buf.p + buf.i> exceeds <buf.data + buf.size>, then the pending data
|
|
wrap at the end of the buffer and continue at <buf.data>.
|
|
|
|
buf.o : number of bytes already processed before <buf.p> that are pending
|
|
for departure. These bytes may leave at any instant once a connection
|
|
is established. These ones may wrap before <buf.data> to start before
|
|
<buf.data + buf.size>.
|
|
|
|
It's common to call the part between buf.p and buf.p+buf.i the input buffer, and
|
|
the part between buf.p-buf.o and buf.p the output buffer. This design permits
|
|
efficient forwarding without copies. As a result, forwarding one byte from the
|
|
input buffer to the output buffer only consists in :
|
|
- incrementing buf.p
|
|
- incrementing buf.o
|
|
- decrementing buf.i
|
|
|
|
|
|
Message :
|
|
---------
|
|
Unless stated otherwise, all values are relative to <buf.p>, and are always
|
|
comprised between 0 and <buf.i>. These values are relative offsets and they do
|
|
not need to take wrapping into account, they are used as if the buffer was an
|
|
infinite length sliding window. The buffer management functions handle the
|
|
wrapping automatically.
|
|
|
|
msg.next : points to the next byte to inspect. This offset is automatically
|
|
adjusted when inserting/removing some headers. In data states, it is
|
|
automatically adjusted to the number of bytes already inspected.
|
|
|
|
msg.sov : start of value. First character of the header's value in the header
|
|
states, start of the body in the data states. Strictly positive
|
|
values indicate that headers were not forwarded yet (<buf.p> is
|
|
before the start of the body), and null or negative values are seen
|
|
after headers are forwarded (<buf.p> is at or past the start of the
|
|
body). The value stops changing when data start to leave the buffer
|
|
(in order to avoid integer overflows). So the maximum possible range
|
|
is -<buf.size> to +<buf.size>. This offset is automatically adjusted
|
|
when inserting or removing some headers. It is useful to rewind the
|
|
request buffer to the beginning of the body at any phase. The
|
|
response buffer does not really use it since it is immediately
|
|
forwarded to the client.
|
|
|
|
msg.sol : start of line. Points to the beginning of the current header line
|
|
while parsing headers. It is cleared to zero in the BODY state,
|
|
and contains exactly the number of bytes comprising the preceding
|
|
chunk size in the DATA state (which can be zero), so that the sum of
|
|
msg.sov + msg.sol always points to the beginning of data for all
|
|
states starting with DATA. For chunked encoded messages, this sum
|
|
always corresponds to the beginning of the current chunk of data as
|
|
it appears in the buffer, or to be more precise, it corresponds to
|
|
the first of the remaining bytes of chunked data to be inspected. In
|
|
TRAILERS state, it contains the length of the last parsed part of
|
|
the trailer headers.
|
|
|
|
msg.eoh : end of headers. Points to the CRLF (or LF) preceding the body and
|
|
marking the end of headers. It is where new headers are appended.
|
|
This offset is automatically adjusted when inserting/removing some
|
|
headers. It always contains the size of the headers excluding the
|
|
trailing CRLF even after headers have been forwarded.
|
|
|
|
msg.eol : end of line. Points to the CRLF or LF of the current header line
|
|
being inspected during the various header states. In data states, it
|
|
holds the trailing CRLF length (1 or 2) so that msg.eoh + msg.eol
|
|
always equals the exact header length. It is not affected during data
|
|
states nor by forwarding.
|
|
|
|
The beginning of the message headers can always be found this way even after
|
|
headers or data have been forwarded, provided that everything is still present
|
|
in the buffer :
|
|
|
|
headers = buf.p + msg->sov - msg->eoh - msg->eol
|
|
|
|
|
|
Message length :
|
|
----------------
|
|
msg.chunk_len : amount of bytes of the current chunk or total message body
|
|
remaining to be inspected after msg.next. It is automatically
|
|
incremented when parsing a chunk size, and decremented as data
|
|
are forwarded.
|
|
|
|
msg.body_len : total message body length, for logging. Equals Content-Length
|
|
when used, otherwise is the sum of all correctly parsed chunks.
|
|
|
|
|
|
Message state :
|
|
---------------
|
|
msg.msg_state contains the current parser state, one of HTTP_MSG_*. The state
|
|
indicates what byte is expected at msg->next.
|
|
|
|
HTTP_MSG_BODY : all headers have been parsed, parsing of body has not
|
|
started yet.
|
|
|
|
HTTP_MSG_100_SENT : parsing of body has started. If a 100-Continue was needed
|
|
it has already been sent.
|
|
|
|
HTTP_MSG_DATA : some bytes are remaining for either the whole body when
|
|
the message size is determined by Content-Length, or for
|
|
the current chunk in chunked-encoded mode.
|
|
|
|
HTTP_MSG_CHUNK_CRLF : msg->next points to the CRLF after the current data chunk.
|
|
|
|
HTTP_MSG_TRAILERS : msg->next points to the beginning of a possibly empty
|
|
trailer line after the final empty chunk.
|
|
|
|
HTTP_MSG_DONE : all the Content-Length data has been inspected, or the
|
|
final CRLF after trailers has been met.
|
|
|
|
|
|
Message forwarding :
|
|
--------------------
|
|
Forwarding part of a message consists in advancing buf.p up to the point where
|
|
it points to the byte following the last one to be forwarded. This can be done
|
|
inline if enough bytes are present in the buffer, or in multiple steps if more
|
|
buffers need to be forwarded (possibly including splicing). Thus by definition,
|
|
after a block has been scheduled for being forwarded, msg->next and msg->sov
|
|
must be reset.
|
|
|
|
The communication channel between the producer and the consumer holds a counter
|
|
of extra bytes remaining to be forwarded directly without consulting analysers,
|
|
after buf.p. This counter is called to_forward. It commonly holds the advertised
|
|
chunk length or content-length that does not fit in the buffer. For example, if
|
|
2000 bytes are to be forwarded, and 10 bytes are present after buf.p as reported
|
|
by buf.i, then both buf.o and buf.p will advance by 10, buf.i will be reset, and
|
|
to_forward will be set to 1990 so that in total, 2000 bytes will be forwarded.
|
|
At the end of the forwarding, buf.p will point to the first byte to be inspected
|
|
after the 2000 forwarded bytes.
|
|
|