varnishncsa: Add %{Varnish:default_format}x

This is an extended variable that has the same format as the default one
used when no format is specified.  The variable is useful for appending
or prepending other formatters to the default format.

bin/varnishncsa/varnishncsa.c

@@ -162,6 +162,8 @@ static struct ctx {
 	int64_t			vxid;
 } CTX;
 
+static void parse_format(const char *format);
+
 static void
 openout(int append)
 {
@@ -681,6 +683,10 @@ parse_x_format(char *buf)
 		addf_vsl((enum VSL_tag_e)slt, lval, r);
 		return;
 	}
+	if (!strcmp(buf, "Varnish:default_format")) {
+		parse_format(FORMAT);
+		return;
+	}
 	VUT_Error(vut, 1, "Unknown formatting extension: %s", buf);
 }
 

bin/varnishtest/tests/u00003.vtc

@@ -182,4 +182,14 @@ shell -expect ${localhost} {
 	varnishncsa -n ${v1_name} -d -q 'RespStatus == 400'
 }
 
+shell {
+	(
+		varnishncsa -n ${v1_name} -d -k 1
+		varnishncsa -n ${v1_name} -d -k 1 -F "%{User-Agent}i"
+	) >def_then_ua.txt
+	varnishncsa -n ${v1_name} -d -k 1 >def_with_ua.txt \
+		-F "%{Varnish:default_format}x\n%{User-Agent}i"
+	diff -u def_then_ua.txt def_with_ua.txt
+}
+
 # ESI coverage in e00003.vtc

doc/sphinx/reference/varnishncsa.rst

@@ -173,6 +173,10 @@ Supported formatters are:
 %{X}x
   Extended variables.  Supported variables are:
 
+  Varnish:default_format
+    The log format used when neither -f nor -F options are specified.
+    Useful for appending/prepending with other formatters.
+
   Varnish:time_firstbyte
     Time from when the request processing starts until the first
     byte is sent to the client, in seconds.  For backend mode: Time