[master] 60d3263b8 document vmod_vtc snapshot limitation

[Nils Goroll]

commit 60d3263b87d1587a8285e72a0b3a45127d3baaa0
Author: Nils Goroll <nils.goroll at uplex.de>
Date:   Wed Apr 24 16:08:55 2019 +0200

    document vmod_vtc snapshot limitation

diff --git a/lib/libvmod_vtc/vmod.vcc b/lib/libvmod_vtc/vmod.vcc
index e1644fffc..b6a8e4354 100644
--- a/lib/libvmod_vtc/vmod.vcc
+++ b/lib/libvmod_vtc/vmod.vcc
@@ -102,7 +102,9 @@ Find how much unallocated space there is left in a workspace.
 
 $Function VOID workspace_snapshot(ENUM { client, backend, session, thread})
 
-Snapshot a workspace. Only one snapshot may be active at a time.
+Snapshot a workspace. Only one snapshot may be active at a time and
+each VCL can save only one snapshot, so concurrent tasks requiring
+snapshots are not supported.
 
 $Function VOID workspace_reset(ENUM { client, backend, session, thread })
 
diff --git a/lib/libvmod_vtc/vmod_vtc.c b/lib/libvmod_vtc/vmod_vtc.c
index ee9d23fa0..b4022e869 100644
--- a/lib/libvmod_vtc/vmod_vtc.c
+++ b/lib/libvmod_vtc/vmod_vtc.c
@@ -124,6 +124,7 @@ vmod_sleep(VRT_CTX, VCL_DURATION t)
 
 /*--------------------------------------------------------------------*/
 
+// XXX this really should be PRIV_TASK state
 static uintptr_t vtc_ws_snapshot;
 
 static struct ws *