vmod: Teach VMODs to use REGEX expressions

When they take a regular expression as an argument, they can benefit
from compile time checks and avoid PRIV_CALL or on-the-fly regex
compilation.

bin/varnishtest/tests/c00103.vtc

+varnishtest "REGEX expressions in VCL"
+
+varnish v1 -vcl {
+	import debug;
+	backend be none;
+	sub vcl_recv {
+		# NB: the REGEX expression below is needlessly complicated
+		# on purpose to ensure we don't treat REGEX expressions
+		# differently.
+		if (req.url ~ (debug.just_return_regex(
+		    debug.just_return_regex("hello")))) {
+			return (synth(200));
+		}
+		return (synth(500));
+	}
+	sub vcl_synth {
+		set resp.reason = regsub(resp.reason,
+		    debug.just_return_regex("OK"), "world");
+	}
+} -start
+
+client c1 {
+	txreq
+	rxresp
+	expect resp.status == 500
+	expect resp.reason == "Internal Server Error"
+
+	txreq -url "/hello"
+	rxresp
+	expect resp.status == 200
+	expect resp.reason == world
+} -run

doc/sphinx/reference/vmod.rst

@@ -381,6 +381,11 @@ REAL
 
 	A floating point value.
 
+REGEX
+	C-type: ``const struct vre *``
+
+	This is an opaque type for regular expressions with a VCL scope.
+
 STRING
 	C-type: ``const char *``
 

lib/libvcc/vmodtool.py

@@ -110,6 +110,7 @@ CTYPES = {
     'IP':          "VCL_IP",
     'PROBE':       "VCL_PROBE",
     'REAL':        "VCL_REAL",
+    'REGEX':       "VCL_REGEX",
     'STEVEDORE':   "VCL_STEVEDORE",
     'STRANDS':     "VCL_STRANDS",
     'STRING':      "VCL_STRING",

vmod/vmod_debug.c

@@ -1278,3 +1278,12 @@ xyzzy_validhdr(VRT_CTX, VCL_STRANDS s)
 	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
 	return (VRT_ValidHdr(ctx, s));
 }
+
+VCL_REGEX v_matchproto_(td_xyzzy_regex)
+xyzzy_just_return_regex(VRT_CTX, VCL_REGEX r)
+{
+
+	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
+	AN(r);
+	return (r);
+}

vmod/vmod_debug.vcc

@@ -319,3 +319,7 @@ A function mixing a named PRIV_TASK with optional parameters.
 $Function BOOL validhdr(STRANDS)
 
 Test if the argument is a valid header according to RFC7230 section 3.2.
+
+$Function REGEX just_return_regex(REGEX)
+
+Take a REGEX argument and return it.