MEDIUM: h1: support partial message parsing

While it was not needed in the H2 mux which was reading full H1 messages
from the channel, it is mandatory for the H1 mux reading contents from
outside to be able to restart on a message. The problem is that the
headers are indexed on the fly, and it's not fun to have to store
everything between calls.

The solution here is to complete the first pass doing a partial restart,
and only once the end of message was found, to start over it again at
once, filling entries. This way there is a bounded number of passes on
the contents and no need to store an intermediary result anymore. Later
this principle could even be used to decide to completely drop an output
buffer to save memory.

src/h1.c

@@ -672,7 +672,15 @@ void http_msg_analyzer(struct http_msg *msg, struct hdr_idx *idx)
  * For now it's limited to the response. If the header block is incomplete,
  * 0 is returned, waiting to be called again with more data to try it again.
  * The caller is responsible for initializing h1m->state to H1_MSG_RPBEFORE,
- * and h1m->next to zero.
+ * and h1m->next to zero on the first call, the parser will do the rest. If
+ * an incomplete message is seen, the caller only needs to present h1m->state
+ * and h1m->next again, with an empty header list so that the parser can start
+ * again. In this case, it will detect that it interrupted a previous session
+ * and will first look for the end of the message before reparsing it again and
+ * indexing it at the same time. This ensures that incomplete messages fed 1
+ * character at a time are never processed entirely more than exactly twice,
+ * and that there is no need to store all the internal state and pre-parsed
+ * headers or start line between calls.
  *
  * A pointer to a start line descriptor may be passed in <slp>, in which case
  * the parser will fill it with whatever it found.
@@ -702,25 +710,39 @@ int h1_headers_to_hdr_list(char *start, const char *stop,
                            struct http_hdr *hdr, unsigned int hdr_num,
                            struct h1m *h1m, union h1_sl *slp)
 {
-	enum h1m_state state = h1m->state;
-	register char *ptr  = start + h1m->next;
-	register const char *end  = stop;
-	unsigned int hdr_count = 0;
-	unsigned int sol = 0;  /* start of line */
-	unsigned int col = 0;  /* position of the colon */
-	unsigned int eol = 0;  /* end of line */
-	unsigned int sov = 0;  /* start of value */
-	unsigned int skip = 0; /* number of bytes skipped at the beginning */
+	enum h1m_state state;
+	register char *ptr;
+	register const char *end;
+	unsigned int hdr_count;
+	unsigned int skip; /* number of bytes skipped at the beginning */
+	unsigned int sol;  /* start of line */
+	unsigned int col;  /* position of the colon */
+	unsigned int eol;  /* end of line */
+	unsigned int sov;  /* start of value */
 	union h1_sl sl;
-	int skip_update = 0;
+	int skip_update;
+	int restarting;
 	struct ist n, v;       /* header name and value during parsing */
 
+	skip = 0; // do it only once to keep track of the leading CRLF.
+
+ try_again:
+	hdr_count = sol = col = eol = sov = 0;
 	sl.st.status = 0;
+	skip_update = restarting = 0;
+
+	ptr   = start + h1m->next;
+	end   = stop;
+	state = h1m->state;
+
+	if (state != H1_MSG_RPBEFORE)
+		restarting = 1;
+
 	if (unlikely(ptr >= end))
 		goto http_msg_ood;
 
-	/* don't update output if hdr is NULL */
-	if (!hdr)
+	/* don't update output if hdr is NULL or if we're restarting */
+	if (!hdr || restarting)
 		skip_update = 1;
 
 	switch (state)	{
@@ -1060,8 +1082,14 @@ int h1_headers_to_hdr_list(char *start, const char *stop,
 	}
 
 	/* reaching here, we've parsed the whole message and the state is
-	 * H1_MSG_BODY.
+	 * H1_MSG_BODY. We may discover that we were already continuing an
+	 * interrupted parsing session, thus we were silently looking for
+	 * the end of message before deciding to parse it fully at once.
+	 * We won't come there again since restarting will turn zero.
 	 */
+	if (restarting)
+		goto restart;
+
 	if (slp && !skip_update)
 		*slp = sl;
 
@@ -1095,6 +1123,11 @@ int h1_headers_to_hdr_list(char *start, const char *stop,
 	h1m->err_state = h1m->state = state;
 	h1m->err_pos   = h1m->next  = ptr - start + skip;
 	return -2;
+
+ restart:
+	h1m->next  = 0;
+	h1m->state = H1_MSG_RPBEFORE;
+	goto try_again;
 }
 
 /* This function performs a very minimal parsing of the trailers block present