[mod_ssi] config ssi.conditional-requests

Summary:
A new SSI directive, "ssi.conditional-requests", allows to inform
lighttpd which SSI pages should be considered as cacheable and which
should not. In particular, the "ETag" & "Last-Modified" headers will
only be sent for those SSI pages for which the directive is enabled.

Long description:
"ETag" and "Last-Modified" headers were being sent for all SSI pages,
regardless of whether they were cacheable or not. And yet, there was
no cache validation at all for any SSI page.
This commit fixes these two minor issues by adding a new directive,
"ssi.conditional-requests", which allows to specify which SSI pages
are cacheable and which are not, and by adding cache validation to
those SSI pages which are cacheable. And since sending ETags for
non-cacheable documents is not appropriate, they are no longuer
computed nor sent for those SSI pages which are not cacheable.
Regarding the "Last-Modified" header for non-cacheable documents,
the standards allow to either send the current date and time for
that header or to simply skip it. The approach chosen is to not send
it for non-cacheable SSI pages. "ETag" and "Last-Modified" headers
are therefore only sent for an SSI page if ssi.conditional-requests
is enabled for that page.

The ssi.conditional-requests directive can be enabled or disabled
globally and/or in any context. It is disabled by default.

An index.shtml which only includes deterministic SSI commands such as:
<!--#echo var="LAST_MODIFIED"-->
is a trivial example of a dynamic SSI page that is cacheable.

NEWS

@@ -74,6 +74,7 @@ NEWS
   * [mod_dirlisting] class for dir <tr> (fixes #2304)
   * [core] define __STDC_WANT_LIB_EXT1__ (fixes #2722)
   * [core] setrlimit max-fds <= rlim_max for non-root (fixes #2723)
+  * [mod_ssi] config ssi.conditional-requests
 
 - 1.4.39 - 2016-01-02
   * [core] fix memset_s call (fixes #2698)

doc/config/conf.d/ssi.conf

@@ -1,6 +1,6 @@
 #######################################################################
 ##
-##  Server Side Includes 
+##  Server Side Includes
 ## -----------------------
 ##
 ## See http://redmine.lighttpd.net/projects/lighttpd/wiki/Docs_ModSSI
@@ -12,5 +12,34 @@ server.modules += ( "mod_ssi" )
 ##
 ssi.extension              = ( ".shtml" )
 
+##
+## The  ssi.conditional-requests  directive only affects requests
+## handled by the SSI module and allows to declare which SSI pages
+## are cacheable and which are not. This directive can be enabled
+## or disabled globally and/or in any context.
+##
+## As the name of this directive suggests, conditional requests will
+## be handled appropriately for any SSI page for which the directive
+## is enabled. In particular, the "ETag" and "Last-Modified" headers
+## will both be sent. Conversely, these headers will NOT be sent for
+## pages for which the directive is disabled.
+##
+## The directive should be set to "enable" ONLY for requests that are
+## known to generate cacheable documents. An SSI page which only
+## includes contents from other static files and/or which uses SSI
+## commands that produce predictable output (e.g. the echo command
+## for the LAST_MODIFIED variable) is likely to be cacheable.
+##
+## The directive should be set to "disable" for ALL other documents,
+## that is, for SSI pages which depend on non-predictable input such
+## as (but not limited to) output from ssi exec commands, data from
+## the client's request headers (other than the request URI), or any
+## other non constant input such as the current date or time, the
+## client's user-agent, etc...
+##
+## Disabled by default.
+##
+#ssi.conditional-requests = "enable"
+
 ##
 #######################################################################

src/mod_ssi.c

@@ -107,6 +107,7 @@ SETDEFAULTS_FUNC(mod_ssi_set_defaults) {
 	config_values_t cv[] = {
 		{ "ssi.extension",              NULL, T_CONFIG_ARRAY, T_CONFIG_SCOPE_CONNECTION },       /* 0 */
 		{ "ssi.content-type",           NULL, T_CONFIG_STRING, T_CONFIG_SCOPE_CONNECTION },      /* 1 */
+		{ "ssi.conditional-requests",   NULL, T_CONFIG_BOOLEAN, T_CONFIG_SCOPE_CONNECTION },     /* 2 */
 		{ NULL,                         NULL, T_CONFIG_UNSET, T_CONFIG_SCOPE_UNSET }
 	};
 
@@ -121,9 +122,11 @@ SETDEFAULTS_FUNC(mod_ssi_set_defaults) {
 		s = calloc(1, sizeof(plugin_config));
 		s->ssi_extension  = array_init();
 		s->content_type = buffer_init();
+		s->conditional_requests = 0;
 
 		cv[0].destination = s->ssi_extension;
 		cv[1].destination = s->content_type;
+		cv[2].destination = &(s->conditional_requests);
 
 		p->config_storage[i] = s;
 
@@ -1078,7 +1081,7 @@ static int mod_ssi_handle_request(server *srv, connection *con, plugin_data *p)
 		response_header_overwrite(srv, con, CONST_STR_LEN("Content-Type"), CONST_BUF_LEN(p->conf.content_type));
 	}
 
-	{
+	if (p->conf.conditional_requests) {
 		/* Generate "ETag" & "Last-Modified" headers */
 		time_t lm_time = 0;
 		buffer *mtime = NULL;
@@ -1093,6 +1096,13 @@ static int mod_ssi_handle_request(server *srv, connection *con, plugin_data *p)
 
 		mtime = strftime_cache_get(srv, lm_time);
 		response_header_overwrite(srv, con, CONST_STR_LEN("Last-Modified"), CONST_BUF_LEN(mtime));
+
+		if (HANDLER_FINISHED == http_response_handle_cachable(srv, con, mtime)) {
+			/* ok, the client already has our content,
+			 * no need to send it again */
+
+			chunkqueue_reset(con->write_queue);
+		}
 	}
 
 	/* Reset the modified time of included files */
@@ -1112,6 +1122,7 @@ static int mod_ssi_patch_connection(server *srv, connection *con, plugin_data *p
 
 	PATCH(ssi_extension);
 	PATCH(content_type);
+	PATCH(conditional_requests);
 
 	/* skip the first, the global context */
 	for (i = 1; i < srv->config_context->used; i++) {
@@ -1129,6 +1140,8 @@ static int mod_ssi_patch_connection(server *srv, connection *con, plugin_data *p
 				PATCH(ssi_extension);
 			} else if (buffer_is_equal_string(du->key, CONST_STR_LEN("ssi.content-type"))) {
 				PATCH(content_type);
+			} else if (buffer_is_equal_string(du->key, CONST_STR_LEN("ssi.conditional-requests"))) {
+				PATCH(conditional_requests);
 			}
 		}
 	}

src/mod_ssi.h

@@ -17,6 +17,7 @@
 typedef struct {
 	array *ssi_extension;
 	buffer *content_type;
+	unsigned short conditional_requests;
 } plugin_config;
 
 typedef struct {