[master] b825f55f3 vrt_vcl: Avoid retirement of assigned director

[Dridi Boukelmoune]

commit b825f55f348595cfc24f5eee92b86ecf2e880924
Author: Dridi Boukelmoune <dridi.boukelmoune at gmail.com>
Date:   Mon Dec 9 18:43:01 2024 +0100

    vrt_vcl: Avoid retirement of assigned director
    
    Consider the following VCL statement:
    
        set bereq.backend = bereq.backend;
    
    The director's refcount will be decremented before it is incremented. If
    that bereq.backend was holding the last reference of a dynamic backend,
    the backend is retired before acquiring a new reference.
    
    While this scenario is highly unlikely, we cannot discard the risk of
    assigning a director to a destination that already held a reference to
    the same director.

diff --git a/bin/varnishd/cache/cache_vrt_vcl.c b/bin/varnishd/cache/cache_vrt_vcl.c
index 82e6e10a5..33ba68241 100644
--- a/bin/varnishd/cache/cache_vrt_vcl.c
+++ b/bin/varnishd/cache/cache_vrt_vcl.c
@@ -336,6 +336,8 @@ VRT_Assign_Backend(VCL_BACKEND *dst, VCL_BACKEND src)
 	AN(dst);
 	CHECK_OBJ_ORNULL((*dst), DIRECTOR_MAGIC);
 	CHECK_OBJ_ORNULL(src, DIRECTOR_MAGIC);
+	if (*dst == src)
+		return;
 	if (*dst != NULL) {
 		vdir = (*dst)->vdir;
 		CHECK_OBJ_NOTNULL(vdir, VCLDIR_MAGIC);