Avoid deadlock with PERL_MEM_LOG

This fixes GH #18341

getenv() call allocates memory to squirrel safely away the result of
that getenv() call.  It does this while in a critical section so as to
make sure another thread can't interrupt it and destroy it.

The problem when using PERL_MEM_LOG is that the allocation of that
memory causes a recursive call to getenv() to see how to log that
allocation.  And it deadlocks trying to enter the critical section.

There are various solutions.  One is to use or emulate a general semaphore
instead of a binary one.  This is effectively what
PL_lc_numeric_mutex_depth does for another mutex, and the code for that
could be used as a template.

But given that this is an extreme edge case which requires Perl to be
specially compiled to enable this feature which is used only for
debugging, I thought it would be sufficient to just make a special case
to not try to lock during that recursive call.

inline.h

@@ -2601,7 +2601,17 @@ Perl_mortal_getenv(const char * str)
     /* Can't mortalize without stacks.  khw believes that no other threads
      * should be running, so no need to lock things, and this may be during a
      * phase when locking isn't even available */
-    if (UNLIKELY(PL_scopestack_ix == 0)) {
+    if (   UNLIKELY(PL_scopestack_ix == 0)
+
+#ifdef PERL_MEM_LOG
+            /* This is a kludge to prevent deadlock.  The process of creating a
+             * copy likely will call ourselves recursively to get the value of
+             * PERL_MEM_LOG to see about logging the memory used by that copy.
+             * Instead just return an unsafe version of it. */
+        || UNLIKELY(strEQ(str, "PERL_MEM_LOG"))
+#endif
+
+    ) {
         return getenv(str);
     }
 

util.c

@@ -5008,6 +5008,9 @@ S_mem_log_common(enum mem_log_type mlt, const UV n,
 
     PERL_ARGS_ASSERT_MEM_LOG_COMMON;
 
+    /* Note that 'pmlenv' is not protected from other threads overwriting it by
+     * calling getenv() themselves on platforms where getenv() returns an
+     * internal static pointer. */
     pmlenv = PerlEnv_getenv("PERL_MEM_LOG");
     if (!pmlenv)
 	return;