Multiple pipelines and limiting the number of connections.

Introducing a number of options to the multi interface that
allows for multiple pipelines to the same host, in order to
optimize the balance between the penalty for opening new
connections and the potential pipelining latency.

Two new options for limiting the number of connections:

CURLMOPT_MAX_HOST_CONNECTIONS - Limits the number of running connections
to the same host. When adding a handle that exceeds this limit,
that handle will be put in a pending state until another handle is
finished, so we can reuse the connection.

CURLMOPT_MAX_TOTAL_CONNECTIONS - Limits the number of connections in total.
When adding a handle that exceeds this limit,
that handle will be put in a pending state until another handle is
finished. The free connection will then be reused, if possible, or
closed if the pending handle can't reuse it.

Several new options for pipelining:

CURLMOPT_MAX_PIPELINE_LENGTH - Limits the pipeling length. If a
pipeline is "full" when a connection is to be reused, a new connection
will be opened if the CURLMOPT_MAX_xxx_CONNECTIONS limits allow it.
If not, the handle will be put in a pending state until a connection is
ready (either free or a pipe got shorter).

CURLMOPT_CONTENT_LENGTH_PENALTY_SIZE - A pipelined connection will not
be reused if it is currently processing a transfer with a content
length that is larger than this.

CURLMOPT_CHUNK_LENGTH_PENALTY_SIZE - A pipelined connection will not
be reused if it is currently processing a chunk larger than this.

CURLMOPT_PIPELINING_SITE_BL - A blacklist of hosts that don't allow
pipelining.

CURLMOPT_PIPELINING_SERVER_BL - A blacklist of server types that don't allow
pipelining.

See the curl_multi_setopt() man page for details.

2 files changed, 27 insertions, 0 deletions

diff --git a/include/curl/curl.h b/include/curl/curl.h
index 0b2b7ea44..c7a052d1e 100644
--- a/include/curl/curl.h
+++ b/include/curl/curl.h
@@ -507,6 +507,8 @@ typedef enum {
   CURLE_RTSP_SESSION_ERROR,      /* 86 - mismatch of RTSP Session Ids */
   CURLE_FTP_BAD_FILE_LIST,       /* 87 - unable to parse FTP file list */
   CURLE_CHUNK_FAILED,            /* 88 - chunk callback reported error */
+  CURLE_NO_CONNECTION_AVAILABLE, /* 89 - No connection available, the
+                                    session will be queued */
   CURL_LAST /* never use! */
 } CURLcode;
 
diff --git a/include/curl/multi.h b/include/curl/multi.h
index 6dcd2bac4..a5eb3c643 100644
--- a/include/curl/multi.h
+++ b/include/curl/multi.h
@@ -338,6 +338,31 @@ typedef enum {
   /* maximum number of entries in the connection cache */
   CINIT(MAXCONNECTS, LONG, 6),
 
+  /* maximum number of (pipelining) connections to one host */
+  CINIT(MAX_HOST_CONNECTIONS, LONG, 7),
+
+  /* maximum number of requests in a pipeline */
+  CINIT(MAX_PIPELINE_LENGTH, LONG, 8),
+
+  /* a connection with a content-length longer than this
+     will not be considered for pipelining */
+  CINIT(CONTENT_LENGTH_PENALTY_SIZE, OFF_T, 9),
+
+  /* a connection with a chunk length longer than this
+     will not be considered for pipelining */
+  CINIT(CHUNK_LENGTH_PENALTY_SIZE, OFF_T, 10),
+
+  /* a list of site names(+port) that are blacklisted from
+     pipelining */
+  CINIT(PIPELINING_SITE_BL, OBJECTPOINT, 11),
+
+  /* a list of server types that are blacklisted from
+     pipelining */
+  CINIT(PIPELINING_SERVER_BL, OBJECTPOINT, 12),
+
+  /* maximum number of open connections in total */
+  CINIT(MAX_TOTAL_CONNECTIONS, LONG, 13),
+
   CURLMOPT_LASTENTRY /* the last unused */
 } CURLMoption;