[master] 82eb76934 integrate the VCL_Rel() step into vcl_get()

[Nils Goroll]

commit 82eb76934ad782e7b6e2e752dafc22cbb95ff5f8
Author: Nils Goroll <nils.goroll at uplex.de>
Date:   Wed Jan 22 10:01:53 2020 +0100

    integrate the VCL_Rel() step into vcl_get()
    
    ... potentially saving a lock/unlock on the critical vcl_mtx
    
    Both callers of vcl_get(), VPI_vcl_select() and VCL_Refresh(),
    potentially have to unref another vcl first.
    
    For VCL_Refresh(), this happens when the active vcl changes and the
    previously active vcl is cached in (struct worker).vcl, which can be
    assumed to always be the case on a busy system.
    
    For VPI_vcl_select(), this happens when switching vcls at
    esi_level > 0.
    
    To summarize, this patch will primarily reduce contention on the
    vcl_mtx after a 'vcl.use' CLI command on a busy system and for
    'return(vcl(...))' with ESI.

diff --git a/bin/varnishd/cache/cache_vcl.c b/bin/varnishd/cache/cache_vcl.c
index 4dc504b03..fa653c636 100644
--- a/bin/varnishd/cache/cache_vcl.c
+++ b/bin/varnishd/cache/cache_vcl.c
@@ -259,10 +259,21 @@ VCL_Panic(struct vsb *vsb, const char *nm, const struct vcl *vcl)
 void
 vcl_get(struct vcl **vcc, struct vcl *vcl)
 {
+	struct vcl *old;
+
 	AN(vcc);
-	AZ(*vcc);
+
+	old = *vcc;
+	*vcc = NULL;
+
+	CHECK_OBJ_ORNULL(old, VCL_MAGIC);
 
 	Lck_Lock(&vcl_mtx);
+	if (old != NULL) {
+		assert(old->busy > 0);
+		old->busy--;
+	}
+
 	if (vcl == NULL)
 		vcl = vcl_active; /* Sample vcl_active under lock to avoid
 				   * race */
diff --git a/bin/varnishd/cache/cache_vpi.c b/bin/varnishd/cache/cache_vpi.c
index 298cac7a9..fd168fc18 100644
--- a/bin/varnishd/cache/cache_vpi.c
+++ b/bin/varnishd/cache/cache_vpi.c
@@ -108,8 +108,6 @@ VPI_vcl_select(VRT_CTX, VCL_VCL vcl)
 		AZ(req->top->vcl0);
 		req->top->vcl0 = req->vcl;
 		req->vcl = NULL;
-	} else {
-		VCL_Rel(&req->vcl);
 	}
 	vcl_get(&req->vcl, vcl);
 	VSLb(ctx->req->vsl, SLT_VCL_use, "%s via %s",
diff --git a/bin/varnishd/cache/cache_vrt_vcl.c b/bin/varnishd/cache/cache_vrt_vcl.c
index 2da13502f..d02f9bddc 100644
--- a/bin/varnishd/cache/cache_vrt_vcl.c
+++ b/bin/varnishd/cache/cache_vrt_vcl.c
@@ -84,8 +84,6 @@ VCL_Refresh(struct vcl **vcc)
 
 	if (*vcc == vcl_active)
 		return;
-	if (*vcc != NULL)
-		VCL_Rel(vcc);	/* XXX: optimize locking */
 
 	vcl_get(vcc, NULL);
 }