Hold a VCL reference in the debug vmod

By default the reference is released as soon as the VCL goes cold, which
doesn't harm the existing tests yet allows to cover the new VRT_ref_vcl
and VRT_rel_vcl functions.

It is possible to ask the vmod to release the reference after a delay.

lib/libvmod_debug/vmod.vcc

@@ -138,3 +138,7 @@ Mark a workspace as overflowed.
 $Function INT workspace_free(ENUM { client, backend, session, thread })
 
 Find how much unallocated space there is left in a workspace.
+
+$Function VOID vcl_release_delay(DURATION)
+
+Hold a reference to the VCL when it goes cold for the given delay.

lib/libvmod_debug/vmod_debug.c

@@ -28,6 +28,7 @@
 
 #include "config.h"
 
+#include <pthread.h>
 #include <stdlib.h>
 #include <stdio.h>
 
@@ -46,6 +47,8 @@ struct priv_vcl {
 	uintptr_t		exp_cb;
 };
 
+VCL_DURATION vcl_release_delay = 0.0;
+
 VCL_VOID __match_proto__(td_debug_panic)
 vmod_panic(VRT_CTX, const char *str, ...)
 {
@@ -280,14 +283,38 @@ event_warm(VRT_CTX)
 		return (-1);
 	}
 
+	VRT_ref_vcl(ctx);
 	return (0);
 }
 
+static void*
+cooldown_thread(void *priv)
+{
+	struct vrt_ctx ctx;
+
+	AN(priv);
+	INIT_OBJ(&ctx, VRT_CTX_MAGIC);
+	ctx.vcl = (struct vcl*)priv;
+
+	VTIM_sleep(vcl_release_delay);
+	VRT_rel_vcl(&ctx);
+	return (NULL);
+}
+
 static int
 event_cold(VRT_CTX)
 {
+	pthread_t thread;
 
 	VSL(SLT_Debug, 0, "%s: VCL_EVENT_COLD", VCL_Name(ctx->vcl));
+
+	if (vcl_release_delay == 0.0) {
+		VRT_rel_vcl(ctx);
+		return (0);
+	}
+
+	AZ(pthread_create(&thread, NULL, cooldown_thread, ctx->vcl));
+	AZ(pthread_detach(thread));
 	return (0);
 }
 
@@ -379,3 +406,12 @@ vmod_workspace_overflow(VRT_CTX, VCL_ENUM which)
 
 	WS_MarkOverflow(ws);
 }
+
+void
+vmod_vcl_release_delay(VRT_CTX, VCL_DURATION delay)
+{
+
+	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
+	assert(delay > 0.0);
+	vcl_release_delay = delay;
+}