A follow-up issue has been reported in #13:

Assert error in Lck_Delete(), cache/cache_lck.c line 309:
  Condition((pthread_mutex_destroy(&ilck->mtx)) == 0) not true.

triggered from Lck_Delete(&bytes_tree->nodes_lock) at the bottom of
bytes_tree_fini().

Assuming everything else working correctly, the only scenario I
can see the moment is that we see the node->subreq.done == 1
earlier than Lck_Unlock() returned in vped_task(). In this
case, we could advance to destroying the lock while the other
thread still holds it.

The other use case of the shared lock is in fini_final(), where
we already go through an explicit lock/unlock.

Hopefully fixes #13 for real

(backport of 067c16e0 to 6.6)

Fixes https://gitlab.com/uplex/varnish/libvdp-pesi/-/issues/10

This is a custom patch version of Varnish 6.6.2 with a bugfix hypothesis
for issue 3775.

This 6.6 branch was forked from master after 6.6 release

This reverts commit 1e284a17.

I had removed these before the public open source release based on Flexelint output,
but Geoff is correct that, contrary to the C version macros like __STDC_VERSION__,
_POSIX_C_SOURCE is to be set by the programmer.

Ref: https://www.gnu.org/software/libc/manual/html_node/Feature-Test-Macros.html

On the downside, we do now see some noise from flexelint and it is unclear to me
how to suppress Note 9071.

--- Module:   pesi.c (C)
File pesi.c, Line 33
        _
    Note 9071: defined macro '_POSIX_C_SOURCE' is reserved to the compiler
File /usr/include/x86_64-linux-gnu/bits/types/struct_tm.h, Line 23
                      _
  long int __tm_gmtoff;		/* Seconds east of UTC.  */
    Error 114: Inconsistent structure declaration for tag 'tm'
File /usr/include/x86_64-linux-gnu/bits/types/struct_tm.h, Line 24
                       _
  const char *__tm_zone;	/* Timezone abbreviation.  */
    Error 114: Inconsistent structure declaration for tag 'tm'

--- Module:   vdp_pesi.c (C)
File vdp_pesi.c, Line 33
        _
    Note 9071: defined macro '_POSIX_C_SOURCE' is reserved to the compiler
File /usr/include/x86_64-linux-gnu/bits/types/struct_tm.h, Line 23
                      _
  long int __tm_gmtoff;		/* Seconds east of UTC.  */
    Error 114: Inconsistent structure declaration for tag 'tm'
File /usr/include/x86_64-linux-gnu/bits/types/struct_tm.h, Line 24
                       _
  const char *__tm_zone;	/* Timezone abbreviation.  */
    Error 114: Inconsistent structure declaration for tag 'tm'

Thank you, OTTO

Ref varnish-cache fd72157cd52e974d42dff2260cb608a79f1b9403