Fix broken PERL_MEM_LOG under threads

This fixes GH #18341

There are problems with getenv() on threaded perls wchich can lead to
incorrect results when compiled with PERL_MEM_LOG.

Commit 0b83dfe6dd9b0bda197566adec923f16b9a693cd fixed this for some
platforms, but as Tony Cook, pointed out there may be
standards-compliant platforms that that didn't fix.

The detailed comments outline the issues and (complicated) full solution.

1 files changed, 4 insertions, 0 deletions

diff --git a/sv.c b/sv.c
index 18d9e0436b..d4df78fb02 100644
--- a/sv.c
+++ b/sv.c
@@ -15407,6 +15407,10 @@ perl_clone_using(PerlInterpreter *proto_perl, UV flags,
     /* op_free() hook */
     PL_opfreehook	= proto_perl->Iopfreehook;
 
+#  ifdef PERL_MEM_LOG
+    Zero(PL_mem_log, sizeof(PL_mem_log), char);
+#  endif
+
 #ifdef USE_REENTRANT_API
     /* XXX: things like -Dm will segfault here in perlio, but doing
      *  PERL_SET_CONTEXT(proto_perl);