Reapply "fetch: A beresp body cannot be BS_TAKEN"

[dridi]

This reverts commit 8c4ae4f.

I dispute the claim from the revert commit that bo->htc->body_status
refers to the bereq, since this is managed by the V1F (HTTP/1 Fetch)
code. What we fetch on the backend side is a beresp.

[nigoroll]

Sorry, I was wrong.
It very much looks like I was under the impression of surrounding commits and simply misread the diff.

To gain confidence that I am not wrong again, I ran the test suite with this patch. It made some logexps fail, but the result is what you say:

diff --git a/bin/varnishd/cache/cache_fetch.c b/bin/varnishd/cache/cache_fetch.c
index 196817512..2cf8b5216 100644
--- a/bin/varnishd/cache/cache_fetch.c
+++ b/bin/varnishd/cache/cache_fetch.c
@@ -1115,6 +1115,12 @@ vbf_fetch_thread(struct worker *wrk, void *priv)
                AN(stp->name);
                AN(stp->func);
                stp = stp->func(wrk, bo);
+               if (bo->htc == NULL)
+                       VSLb(bo->vsl, SLT_Debug, "htc == NULL");
+               else {
+                       VSLb(bo->vsl, SLT_Debug, "htc->body_status = %s",
+                           bo->htc->body_status->name);
+               }
        }
 
        assert(bo->director_state == DIR_S_NULL);

$ find . -name \*.log | xargs grep -hE 'Debug.*htc' | sed 's:^.*Debug:Debug:'| sort | uniq -c | sort -rn
   4432 Debug           b htc == NULL
   2626 Debug           b htc->body_status = length
   1185 Debug           b htc->body_status = none
    358 Debug           b htc->body_status = chunked
     66 Debug           b htc->body_status = eof
      6 Debug          b "htc == NULL%00"                                                                                                                                                            
      4 Debug          b "htc->body_status = none%00"                                                                                                                                                
      3 Debug          "htc == NULL%00"
      3 Debug          b "htc->body_status = length%00"
      1 Debug          b "htc == NULL%00"