Add VRT_VCL_Busy() and VRT_VCL_Unbusy() so VMODs can prevent

their VCL from going cold.

One half of #2471

bin/varnishd/cache/cache_vrt_vcl.c

@@ -144,6 +144,28 @@ VCL_Rel(struct vcl **vcc)
 
 /*--------------------------------------------------------------------*/
 
+void
+VRT_VCL_Busy(VRT_CTX)
+{
+	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
+
+	CHECK_OBJ_NOTNULL(ctx->vcl, VCL_MAGIC);
+	VCL_Ref(ctx->vcl);
+}
+
+void
+VRT_VCL_Unbusy(VRT_CTX)
+{
+	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
+	struct vcl *vcl;
+
+	CHECK_OBJ_NOTNULL(ctx->vcl, VCL_MAGIC);
+	vcl = ctx->vcl;
+	VCL_Rel(&vcl);
+}
+
+/*--------------------------------------------------------------------*/
+
 VCL_BACKEND
 VRT_AddDirector(VRT_CTX, const struct vdi_methods *m, void *priv,
     const char *fmt, ...)

bin/varnishtest/tests/r02471.vtc

+varnishtest "test VRT_VCL_(Un)Busy()"
+
+server s1 {
+	rxreq
+	txresp
+} -start
+
+varnish v1 -vcl+backend {
+	import debug;
+
+	sub vcl_recv {
+		if (req.url == "/hold") {
+			debug.hold_vcl_busy();
+		} else if (req.url == "/release") {
+			debug.release_vcl_busy();
+		}
+		return (synth(200));
+	}
+} -start
+
+varnish v1 -vcl+backend {}
+
+varnish v1 -cliok "vcl.state vcl1 cold"
+
+# Nothing holds vcl1, so it should go gold.
+varnish v1 -cliexpect "cold    cold         0    vcl1" "vcl.list"
+
+
+# Grab hold of vcl1
+varnish v1 -cliok "vcl.use vcl1"
+client c1 {
+	txreq -url "/hold"
+	rxresp
+} -run
+
+# Flush worker threads hold
+varnish v1 -cliok "vcl.use vcl2"
+client c1 {
+	txreq
+	rxresp
+} -run
+
+# There should still be a single busy hold on vcl1
+varnish v1 -cliok "vcl.state vcl1 cold"
+varnish v1 -cliexpect "cold    busy         1    vcl1" "vcl.list"
+
+# Release hold on vcl1
+varnish v1 -cliok "vcl.use vcl1"
+client c1 {
+	txreq -url "/release"
+	rxresp
+} -run
+
+# Flush worker threads hold
+varnish v1 -cliok "vcl.use vcl2"
+client c1 {
+	txreq
+	rxresp
+} -run
+
+# Nothing holds vcl1, so it should go gold.
+varnish v1 -cliok "vcl.state vcl1 cold"
+varnish v1 -cliexpect "cold    cold         0    vcl1" "vcl.list"

include/vrt.h

@@ -52,6 +52,7 @@
  * binary/load-time compatible, increment MAJOR version
  *
  * unreleased (planned for 2019-09-15)
+ *	VRT_VCL_Busy() and VRT_VCL_Unbusy() added.
  *	VRT_vcl_get moved to vcc_interface.h
  *	VRT_vcl_rel emoved to vcc_interface.h
  *	VRT_vcl_select emoved to vcc_interface.h
@@ -575,3 +576,10 @@ void VRT_VSC_Destroy(const char *, struct vsc_seg *);
 void VRT_VSC_Hide(const struct vsc_seg *);
 void VRT_VSC_Reveal(const struct vsc_seg *);
 size_t VRT_VSC_Overhead(size_t);
+
+/*
+ * API to prevent VCL from going cold
+ */
+
+void VRT_VCL_Busy(VRT_CTX);
+void VRT_VCL_Unbusy(VRT_CTX);

lib/libvmod_debug/vmod.vcc

@@ -243,3 +243,11 @@ This function is by no means guaranteed to work cross platform and
 should now only be used for diagnostic purposes.
 
 0B is returned if no sensible value can be determined.
+
+$Function VOID hold_vcl_busy()
+
+Prevent VCL from going cold
+
+$Function VOID release_vcl_busy()
+
+Allow VCL to go cold

lib/libvmod_debug/vmod_debug.c

@@ -793,3 +793,17 @@ xyzzy_stk(VRT_CTX)
 
 	return (0);
 }
+
+VCL_VOID
+xyzzy_hold_vcl_busy(VRT_CTX)
+{
+
+	VRT_VCL_Busy(ctx);
+}
+
+VCL_VOID
+xyzzy_release_vcl_busy(VRT_CTX)
+{
+
+	VRT_VCL_Unbusy(ctx);
+}