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, 1 insertions, 1 deletions

diff --git a/pod/perlfunc.pod b/pod/perlfunc.pod
index 264fbdcbcd..04c6a05bf9 100644
--- a/pod/perlfunc.pod
+++ b/pod/perlfunc.pod
@@ -3095,7 +3095,7 @@ This function places an advisory lock on a shared variable or referenced
 object contained in I<THING> until the lock goes out of scope.
 
 The value returned is the scalar itself, if the argument is a scalar, or a
-reference, if the argument is a hash or array.
+reference, if the argument is a hash, array or subroutine.
 
 lock() is a "weak keyword" : this means that if you've defined a function
 by this name (before any calls to it), that function will be called