[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>

diff --git a/src/net/tcp/httpcore.c b/src/net/tcp/httpcore.c
index 3b2aacdea5c8e105a9ff16ce6bee71b34aa8e809..aa15b11231da1a169a0d6c065a755ddca78ecec2 100644 (file)
--- a/src/net/tcp/httpcore.c
+++ b/src/net/tcp/httpcore.c
@@ -43,6 +43,7 @@ FILE_LICENCE ( GPL2_OR_LATER );
 #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>
@@ -88,6 +89,9 @@ FILE_LICENCE ( GPL2_OR_LATER );
 /** 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 */
@@ -185,6 +189,8 @@ struct http_request {
 
        /** Request retry timer */
        struct retry_timer timer;
+       /** Retry delay (in timer ticks) */
+       unsigned long retry_delay;
 };
 
 /**
@@ -342,7 +348,8 @@ static void http_done ( struct http_request *http ) {
        }
 
        /* Start request retry timer */
-       start_timer_nodelay ( &http->timer );
+       start_timer_fixed ( &http->timer, http->retry_delay );
+       http->retry_delay = 0;
 }
 
 /**
@@ -664,6 +671,39 @@ static int http_rx_www_authenticate ( struct http_request *http, char *value ) {
        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") */
@@ -701,6 +741,10 @@ static struct http_header_handler http_header_handlers[] = {
                .header = "WWW-Authenticate",
                .rx = http_rx_www_authenticate,
        },
+       {
+               .header = "Retry-After",
+               .rx = http_rx_retry_after,
+       },
        { NULL, NULL }
 };