[http] Automatically retry request on a 503 Service Unavailable

A web server may return a 503 Service Unavailable response along with
a Retry-After header to direct the client to retry the request at a
later time.

The Retry-After header may be a number of seconds, or a full HTTP
timestamp (e.g. "Fri, 7 Mar 2014 17:22:14 GMT").  We have no
reasonable way of parsing a full HTTP timestamp; if the server chooses
to use this format then we simply retry after a fixed 5-second delay.

As per RFC 2616, in the absence of a Retry-After header we treat a
status code of 503 Service Unavailable as being equivalent to 500
Internal Server Error, and immediately fail the request.

Requested-by: Suresh Sundriyal <ssundriy@vmware.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/tcp/httpcore.c

 #include <ipxe/tcpip.h>
 #include <ipxe/process.h>
 #include <ipxe/retry.h>
+#include <ipxe/timer.h>
 #include <ipxe/linebuf.h>
 #include <ipxe/base64.h>
 #include <ipxe/base16.h>
 /** Block size used for HTTP block device request */
 #define HTTP_BLKSIZE 512
 
+/** Retry delay used when we cannot understand the Retry-After header */
+#define HTTP_RETRY_SECONDS 5
+
 /** HTTP flags */
 enum http_flags {
 	/** Request is waiting to be transmitted */
 
 	/** Request retry timer */
 	struct retry_timer timer;
+	/** Retry delay (in timer ticks) */
+	unsigned long retry_delay;
 };
 
 /**
 	}
 
 	/* Start request retry timer */
-	start_timer_nodelay ( &http->timer );
+	start_timer_fixed ( &http->timer, http->retry_delay );
+	http->retry_delay = 0;
 }
 
 /**
 	return 0;
 }
 
+/**
+ * Handle HTTP Retry-After header
+ *
+ * @v http		HTTP request
+ * @v value		HTTP header value
+ * @ret rc		Return status code
+ */
+static int http_rx_retry_after ( struct http_request *http, char *value ) {
+	unsigned long seconds;
+	char *endp;
+
+	DBGC ( http, "HTTP %p retry requested (%s)\n", http, value );
+
+	/* If we received a 503 Service Unavailable response, then
+	 * retry after the specified number of seconds.  If the value
+	 * is not a simple number of seconds (e.g. a full HTTP date),
+	 * then retry after a fixed delay, since we don't have code
+	 * able to parse full HTTP dates.
+	 */
+	if ( http->code == 503 ) {
+		seconds = strtoul ( value, &endp, 10 );
+		if ( *endp != '\0' ) {
+			seconds = HTTP_RETRY_SECONDS;
+			DBGC ( http, "HTTP %p cannot understand \"%s\"; "
+			       "using %ld seconds\n", http, value, seconds );
+		}
+		http->flags |= HTTP_TRY_AGAIN;
+		http->retry_delay = ( seconds * TICKS_PER_SEC );
+	}
+
+	return 0;
+}
+
 /** An HTTP header handler */
 struct http_header_handler {
 	/** Name (e.g. "Content-Length") */
 		.header = "WWW-Authenticate",
 		.rx = http_rx_www_authenticate,
 	},
+	{
+		.header = "Retry-After",
+		.rx = http_rx_retry_after,
+	},
 	{ NULL, NULL }
 };