Make lock(&foo) syntax nominally lock the subroutine

In 5.10, lock(&foo) was an error for non-lvalue subs.  For lvalue
subs, it passed &foo to the lockhook and return \&foo.

In 5.12, lock(&foo) was still an error for non-lvalue subs.  For
lvalue subs, it would pass &foo to the lockhook and then either
trip an assertion (-DDEBUGGING) or return &foo, resulting in inter-
esting bugs.

Commit f4df43b5e changed lock(&lvalue_sub) to call the sub and lock
its return value.

As Reini Urban pointed out in
<CAHiT=DE5cVZbuCR3kb=Q5oCa18vo3jr5jZKmURHYha2PwF4pEQ@mail.gmail.com>,
locking a subroutine does have its uses.

Since lock(&foo) has never really worked anyway, we can still
change this.

So, for lvalue subs, this reverts back to the 5.10 behaviour.  For
non-lvalue subs, it now behaves the same way, the lvalue flag making
no difference.  Note that it still causes an error at run-time, if
threads::shared is loaded, as its lockhook is conservative in what
it accepts.

But this change allows for future extensibility, unlike f4df43b5e.

A note about the implementation: There are two pieces of code (at
least) in op.c that convert an entersub op into an rv2cv, one in
S_doref and the other in Perl_op_lvalue_flags.  Originally (before
f4df43b5e) it was S_doref that took care of that for OP_LOCK.  But
Perl_op_lvalue_flags is called first, so it would assume it was an
assignment to a sub call and croak if there was no lvalue sub in the
symbol table.  This commit adds back the special case for OP_LOCK, but
in Perl_op_lvalue_flags, not S_doref.

1 files changed, 2 insertions, 1 deletions

diff --git a/op.c b/op.c
index 4f8693a4b9..b3c5f86f31 100644
--- a/op.c
+++ b/op.c
@@ -1705,7 +1705,7 @@ Perl_op_lvalue_flags(pTHX_ OP *o, I32 type, U32 flags)
 	    break;
 	goto nomod;
     case OP_ENTERSUB:
-	if ((type == OP_UNDEF || type == OP_REFGEN) &&
+	if ((type == OP_UNDEF || type == OP_REFGEN || type == OP_LOCK) &&
 	    !(o->op_flags & OPf_STACKED)) {
 	    o->op_type = OP_RV2CV;		/* entersub => rv2cv */
 	    /* Both ENTERSUB and RV2CV use this bit, but for different pur-
@@ -10415,6 +10415,7 @@ Perl_core_prototype(pTHX_ SV *sv, const char *name, const int code,
 	    str[n++] = '$';
 	    str[n++] = '@';
 	    str[n++] = '%';
+	    if (i == OP_LOCK) str[n++] = '&';
 	    str[n++] = '*';
 	    str[n++] = ']';
 	}