[peerdist] Limit number of concurrent raw block downloads

Raw block downloads are expensive if the origin server uses HTTPS,
since each concurrent download will require local TLS resources
(including potentially large received encrypted data buffers).

Raw block downloads may also be prohibitively slow to initiate when
the origin server is using HTTPS and client certificates.  Origin
servers for PeerDist downloads are likely to be running IIS, which has
a bug that breaks session resumption and requires each connection to
go through the full client certificate verification.

Limit the total number of concurrent raw block downloads to ameliorate
these problems.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/peerblk.h

 	struct peerdisc_client discovery;
 	/** Current position in discovered peer list */
 	struct peerdisc_peer *peer;
+	/** Block download queue */
+	struct peerdist_block_queue *queue;
+	/** List of queued block downloads */
+	struct list_head queued;
 	/** Retry timer */
 	struct retry_timer timer;
 	/** Number of full attempt cycles completed */
 	unsigned long attempted;
 };
 
+/** PeerDist block download queue */
+struct peerdist_block_queue {
+	/** Download opening process */
+	struct process process;
+	/** List of queued downloads */
+	struct list_head list;
+
+	/** Number of open downloads */
+	unsigned int count;
+	/** Maximum number of open downloads */
+	unsigned int max;
+
+	/** Open block download
+	 *
+	 * @v peerblk		PeerDist block download
+	 * @ret rc		Return status code
+	 */
+	int ( * open ) ( struct peerdist_block *peerblk );
+};
+
 /** Retrieval protocol block fetch response (including transport header)
  *
  * @v digestsize	Digest size

src/net/peerblk.c

  */
 #define PEERBLK_DECRYPT_CHUNKSIZE 2048
 
+/** PeerDist maximum number of concurrent raw block downloads
+ *
+ * Raw block downloads are expensive if the origin server uses HTTPS,
+ * since each concurrent download will require local TLS resources
+ * (including potentially large received encrypted data buffers).
+ *
+ * Raw block downloads may also be prohibitively slow to initiate when
+ * the origin server is using HTTPS and client certificates.  Origin
+ * servers for PeerDist downloads are likely to be running IIS, which
+ * has a bug that breaks session resumption and requires each
+ * connection to go through the full client certificate verification.
+ *
+ * Limit the total number of concurrent raw block downloads to
+ * ameliorate these problems.
+ *
+ * This is a policy decision.
+ */
+#define PEERBLK_RAW_MAX 2
+
 /** PeerDist raw block download attempt initial progress timeout
  *
  * This is a policy decision.
 static struct profiler peerblk_discovery_timeout_profiler __profiler =
 	{ .name = "peerblk.discovery.timeout" };
 
+static void peerblk_dequeue ( struct peerdist_block *peerblk );
+
 /**
  * Get profiling timestamp
  *
 	intf_restart ( &peerblk->raw, rc );
 	intf_restart ( &peerblk->retrieval, rc );
 
+	/* Remove from download queue, if applicable */
+	if ( peerblk->queue )
+		peerblk_dequeue ( peerblk );
+
 	/* Empty received data buffer */
 	xferbuf_free ( &peerblk->buffer );
 	peerblk->pos = 0;
 	peerblk_done ( peerblk, rc );
 }
 
+/******************************************************************************
+ *
+ * Block download queue
+ *
+ ******************************************************************************
+ */
+
+/**
+ * PeerDist block download queue process
+ *
+ * @v queue		Block download queue
+ */
+static void peerblk_step ( struct peerdist_block_queue *queue ) {
+	struct peerdist_block *peerblk;
+	int rc;
+
+	/* Do nothing yet if we have too many open block downloads */
+	if ( queue->count >= queue->max )
+		return;
+
+	/* Do nothing unless there are queued block downloads */
+	peerblk = list_first_entry ( &queue->list, struct peerdist_block,
+				     queued );
+	if ( ! peerblk )
+		return;
+
+	/* Reschedule queue process */
+	process_add ( &queue->process );
+
+	/* Remove block from queue */
+	list_del ( &peerblk->queued );
+	INIT_LIST_HEAD ( &peerblk->queued );
+
+	/* Attempt download */
+	if ( ( rc = queue->open ( peerblk ) ) != 0 ) {
+		peerblk_close ( peerblk, rc );
+		return;
+	}
+
+	/* Increment open block download count */
+	queue->count++;
+}
+
+/**
+ * Add block to download queue
+ *
+ * @v peerblk		PeerDist block download
+ * @v queue		Block download queue
+ */
+static void peerblk_enqueue ( struct peerdist_block *peerblk,
+			      struct peerdist_block_queue *queue ) {
+
+	/* Sanity checks */
+	assert ( peerblk->queue == NULL );
+	assert ( list_empty ( &peerblk->queued ) );
+
+	/* Add block to queue */
+	peerblk->queue = queue;
+	list_add_tail ( &peerblk->queued, &queue->list );
+
+	/* Schedule queue process */
+	process_add ( &queue->process );
+}
+
+/**
+ * Remove block from download queue
+ *
+ * @v peerblk		PeerDist block download
+ */
+static void peerblk_dequeue ( struct peerdist_block *peerblk ) {
+	struct peerdist_block_queue *queue = peerblk->queue;
+
+	/* Sanity checks */
+	assert ( queue != NULL );
+
+	/* Remove block from queue */
+	peerblk->queue = NULL;
+	if ( list_empty ( &peerblk->queued ) ) {
+
+		/* Open download: decrement count and reschedule queue */
+		queue->count--;
+		process_add ( &queue->process );
+
+	} else {
+
+		/* Queued download: remove from queue */
+		list_del ( &peerblk->queued );
+		INIT_LIST_HEAD ( &peerblk->queued );
+	}
+}
+
+/** PeerDist block download queue process descriptor */
+static struct process_descriptor peerblk_queue_desc =
+	PROC_DESC_ONCE ( struct peerdist_block_queue, process, peerblk_step );
+
+/** Raw block download queue */
+static struct peerdist_block_queue peerblk_raw_queue = {
+	.process = PROC_INIT ( peerblk_raw_queue.process, &peerblk_queue_desc ),
+	.list = LIST_HEAD_INIT ( peerblk_raw_queue.list ),
+	.max = PEERBLK_RAW_MAX,
+	.open = peerblk_raw_open,
+};
+
 /******************************************************************************
  *
  * Retrieval protocol block download attempts (using HTTP POST)
 		return;
 	}
 
-	/* Attempt raw download */
-	if ( ( rc = peerblk_raw_open ( peerblk ) ) != 0 )
-		goto err;
+	/* Add to raw download queue */
+	peerblk_enqueue ( peerblk, &peerblk_raw_queue );
 
 	return;
 
 	process_init_stopped ( &peerblk->process, &peerblk_process_desc,
 			       &peerblk->refcnt );
 	peerdisc_init ( &peerblk->discovery, &peerblk_discovery_operations );
+	INIT_LIST_HEAD ( &peerblk->queued );
 	timer_init ( &peerblk->timer, peerblk_expired, &peerblk->refcnt );
 	DBGC2 ( peerblk, "PEERBLK %p %d.%d id %02x%02x%02x%02x%02x..."
 		"%02x%02x%02x [%08zx,%08zx)", peerblk, peerblk->segment,