Any real-world object will exceed the pipe buffer in size. Unless the
filter program which we invoke implemented an infinite buffer (and
requiring something like this would miss the point of stream
processing), we can only write to the program as much data as it
accepts for its input buffering (plus the pipe buffer size), before we
need to read its output.

Thus, we need to ensure that write(fds[STDIN_FILENO].fd) does not
block.

Only the first filter has access to the objcore.

fixes:

Assert error in vdp_init(), vdfp_pipe.c line 189:
  Condition((objcore) != NULL) not true.
version = varnish-trunk revision
136736b092206c5bc764f12f80d9092efde6c9dc, vrt api = 13.0
ident = Linux,5.10.12-200.fc33.x86_64,x86_64,-junix,-sfile,-sdefault,-hcritbit,epoll
now = 3808647.356313 (mono), 1618849595.965169 (real)
Backtrace:
  0x559444057399: /usr/sbin/varnishd(+0x57399) [0x559444057399]
  0x5594440ce273: /usr/sbin/varnishd(VAS_Fail+0x43) [0x5594440ce273]
  0x7fc6745f820f: ./vmod_cache/_vmod_pipe.f336a3498bf9765ce992b39693196344e9c1ddda789775d4d0b0fd3b4939272e(+0x220f) [0x7fc6745f820f]
  0x5594440381a3: /usr/sbin/varnishd(VDP_Push+0x103) [0x5594440381a3]
  0x55944406df96: /usr/sbin/varnishd(VCL_StackVDP+0x76) [0x55944406df96]
  0x55944405ce02: /usr/sbin/varnishd(+0x5ce02) [0x55944405ce02]
  0x55944405ec48: /usr/sbin/varnishd(CNT_Request+0xd8) [0x55944405ec48]
  0x559444088bbb: /usr/sbin/varnishd(+0x88bbb) [0x559444088bbb]
  0x55944407cee7: /usr/sbin/varnishd(+0x7cee7) [0x55944407cee7]
  0x55944407d494: /usr/sbin/varnishd(+0x7d494) [0x55944407d494]

Ref:
https://github.com/varnishcache/varnish-cache/commit/681c11998ec56dc4460a30585210f95a7850ea3b

Support the task-scoped invocation in vcl_deliver.

Use a VSTAILQ for the setenv list. VSLIST pushes only to the head,
so setenv() calls were in reverse order compared to the VCL calls.
We want later invocations of .setenv() overwrite the value of previous
invocations for the same variable (if overwrite is true). So use
a tail queue and push to the tail.