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[perl5.git] / mro.c

/*    mro.c
 *
 *    Copyright (c) 2007 Brandon L Black
 *    Copyright (c) 2007, 2008 Larry Wall and others
 *
 *    You may distribute under the terms of either the GNU General Public
 *    License or the Artistic License, as specified in the README file.
 *
 */

/*
 * 'Which order shall we go in?' said Frodo.  'Eldest first, or quickest first?
 *  You'll be last either way, Master Peregrin.'
 *
 *     [p.101 of _The Lord of the Rings_, I/iii: "A Conspiracy Unmasked"]
 */

/*
=head1 MRO Functions

These functions are related to the method resolution order of perl classes

=cut
*/

#include "EXTERN.h"
#define PERL_IN_MRO_C
#include "perl.h"

static const struct mro_alg dfs_alg =
    {S_mro_get_linear_isa_dfs, "dfs", 3, 0, 0};

SV *
Perl_mro_get_private_data(pTHX_ struct mro_meta *const smeta,
			  const struct mro_alg *const which)
{
    SV **data;
    PERL_ARGS_ASSERT_MRO_GET_PRIVATE_DATA;

    data = (SV **)Perl_hv_common(aTHX_ smeta->mro_linear_all, NULL,
				 which->name, which->length, which->kflags,
				 HV_FETCH_JUST_SV, NULL, which->hash);
    if (!data)
	return NULL;

    /* If we've been asked to look up the private data for the current MRO, then
       cache it.  */
    if (smeta->mro_which == which)
	smeta->mro_linear_current = *data;

    return *data;
}

SV *
Perl_mro_set_private_data(pTHX_ struct mro_meta *const smeta,
			  const struct mro_alg *const which, SV *const data)
{
    PERL_ARGS_ASSERT_MRO_SET_PRIVATE_DATA;

    if (!smeta->mro_linear_all) {
	if (smeta->mro_which == which) {
	    /* If all we need to store is the current MRO's data, then don't use
	       memory on a hash with 1 element - store it direct, and signal
	       this by leaving the would-be-hash NULL.  */
	    smeta->mro_linear_current = data;
	    return data;
	} else {
	    HV *const hv = newHV();
	    /* Start with 2 buckets. It's unlikely we'll need more. */
	    HvMAX(hv) = 1;	
	    smeta->mro_linear_all = hv;

	    if (smeta->mro_linear_current) {
		/* If we were storing something directly, put it in the hash
		   before we lose it. */
		Perl_mro_set_private_data(aTHX_ smeta, smeta->mro_which, 
					  smeta->mro_linear_current);
	    }
	}
    }

    /* We get here if we're storing more than one linearisation for this stash,
       or the linearisation we are storing is not that if its current MRO.  */

    if (smeta->mro_which == which) {
	/* If we've been asked to store the private data for the current MRO,
	   then cache it.  */
	smeta->mro_linear_current = data;
    }

    if (!Perl_hv_common(aTHX_ smeta->mro_linear_all, NULL,
			which->name, which->length, which->kflags,
			HV_FETCH_ISSTORE, data, which->hash)) {
	Perl_croak(aTHX_ "panic: hv_store() failed in set_mro_private_data() "
		   "for '%.*s' %d", (int) which->length, which->name,
		   which->kflags);
    }

    return data;
}

const struct mro_alg *
Perl_mro_get_from_name(pTHX_ SV *name) {
    SV **data;

    PERL_ARGS_ASSERT_MRO_GET_FROM_NAME;

    data = (SV **)Perl_hv_common(aTHX_ PL_registered_mros, name, NULL, 0, 0,
				 HV_FETCH_JUST_SV, NULL, 0);
    if (!data)
	return NULL;
    assert(SvTYPE(*data) == SVt_IV);
    assert(SvIOK(*data));
    return INT2PTR(const struct mro_alg *, SvUVX(*data));
}

void
Perl_mro_register(pTHX_ const struct mro_alg *mro) {
    SV *wrapper = newSVuv(PTR2UV(mro));

    PERL_ARGS_ASSERT_MRO_REGISTER;

    
    if (!Perl_hv_common(aTHX_ PL_registered_mros, NULL,
			mro->name, mro->length, mro->kflags,
			HV_FETCH_ISSTORE, wrapper, mro->hash)) {
	SvREFCNT_dec(wrapper);
	Perl_croak(aTHX_ "panic: hv_store() failed in mro_register() "
		   "for '%.*s' %d", (int) mro->length, mro->name, mro->kflags);
    }
}

struct mro_meta*
Perl_mro_meta_init(pTHX_ HV* stash)
{
    struct mro_meta* newmeta;

    PERL_ARGS_ASSERT_MRO_META_INIT;
    assert(HvAUX(stash));
    assert(!(HvAUX(stash)->xhv_mro_meta));
    Newxz(newmeta, 1, struct mro_meta);
    HvAUX(stash)->xhv_mro_meta = newmeta;
    newmeta->cache_gen = 1;
    newmeta->pkg_gen = 1;
    newmeta->mro_which = &dfs_alg;

    return newmeta;
}

#if defined(USE_ITHREADS)

/* for sv_dup on new threads */
struct mro_meta*
Perl_mro_meta_dup(pTHX_ struct mro_meta* smeta, CLONE_PARAMS* param)
{
    struct mro_meta* newmeta;

    PERL_ARGS_ASSERT_MRO_META_DUP;

    Newx(newmeta, 1, struct mro_meta);
    Copy(smeta, newmeta, 1, struct mro_meta);

    if (newmeta->mro_linear_all) {
	newmeta->mro_linear_all
	    = MUTABLE_HV(sv_dup_inc((const SV *)newmeta->mro_linear_all, param));
	/* This is just acting as a shortcut pointer, and will be automatically
	   updated on the first get.  */
	newmeta->mro_linear_current = NULL;
    } else if (newmeta->mro_linear_current) {
	/* Only the current MRO is stored, so this owns the data.  */
	newmeta->mro_linear_current
	    = sv_dup_inc((const SV *)newmeta->mro_linear_current, param);
    }

    if (newmeta->mro_nextmethod)
	newmeta->mro_nextmethod
	    = MUTABLE_HV(sv_dup_inc((const SV *)newmeta->mro_nextmethod, param));
    if (newmeta->isa)
	newmeta->isa
	    = MUTABLE_HV(sv_dup_inc((const SV *)newmeta->isa, param));

    return newmeta;
}

#endif /* USE_ITHREADS */

/*
=for apidoc mro_get_linear_isa_dfs

Returns the Depth-First Search linearization of @ISA
the given stash.  The return value is a read-only AV*.
C<level> should be 0 (it is used internally in this
function's recursion).

You are responsible for C<SvREFCNT_inc()> on the
return value if you plan to store it anywhere
semi-permanently (otherwise it might be deleted
out from under you the next time the cache is
invalidated).

=cut
*/
static AV*
S_mro_get_linear_isa_dfs(pTHX_ HV *stash, U32 level)
{
    AV* retval;
    GV** gvp;
    GV* gv;
    AV* av;
    const HEK* stashhek;
    struct mro_meta* meta;
    SV *our_name;
    HV *stored = NULL;

    PERL_ARGS_ASSERT_MRO_GET_LINEAR_ISA_DFS;
    assert(HvAUX(stash));

    stashhek
     = HvAUX(stash)->xhv_name_u.xhvnameu_name && HvENAME_HEK_NN(stash)
        ? HvENAME_HEK_NN(stash)
        : HvNAME_HEK(stash);

    if (!stashhek)
      Perl_croak(aTHX_ "Can't linearize anonymous symbol table");

    if (level > 100)
        Perl_croak(aTHX_ "Recursive inheritance detected in package '%s'",
		   HEK_KEY(stashhek));

    meta = HvMROMETA(stash);

    /* return cache if valid */
    if((retval = MUTABLE_AV(MRO_GET_PRIVATE_DATA(meta, &dfs_alg)))) {
        return retval;
    }

    /* not in cache, make a new one */

    retval = MUTABLE_AV(sv_2mortal(MUTABLE_SV(newAV())));
    /* We use this later in this function, but don't need a reference to it
       beyond the end of this function, so reference count is fine.  */
    our_name = newSVhek(stashhek);
    av_push(retval, our_name); /* add ourselves at the top */

    /* fetch our @ISA */
    gvp = (GV**)hv_fetchs(stash, "ISA", FALSE);
    av = (gvp && (gv = *gvp) && isGV_with_GP(gv)) ? GvAV(gv) : NULL;

    /* "stored" is used to keep track of all of the classnames we have added to
       the MRO so far, so we can do a quick exists check and avoid adding
       duplicate classnames to the MRO as we go.
       It's then retained to be re-used as a fast lookup for ->isa(), by adding
       our own name and "UNIVERSAL" to it.  */

    if(av && AvFILLp(av) >= 0) {

        SV **svp = AvARRAY(av);
        I32 items = AvFILLp(av) + 1;

        /* foreach(@ISA) */
        while (items--) {
            SV* const sv = *svp++;
            HV* const basestash = gv_stashsv(sv, 0);
	    SV *const *subrv_p;
	    I32 subrv_items;

            if (!basestash) {
                /* if no stash exists for this @ISA member,
                   simply add it to the MRO and move on */
		subrv_p = &sv;
		subrv_items = 1;
            }
            else {
                /* otherwise, recurse into ourselves for the MRO
                   of this @ISA member, and append their MRO to ours.
		   The recursive call could throw an exception, which
		   has memory management implications here, hence the use of
		   the mortal.  */
		const AV *const subrv
		    = mro_get_linear_isa_dfs(basestash, level + 1);

		subrv_p = AvARRAY(subrv);
		subrv_items = AvFILLp(subrv) + 1;
	    }
	    if (stored) {
		while(subrv_items--) {
		    SV *const subsv = *subrv_p++;
		    /* LVALUE fetch will create a new undefined SV if necessary
		     */
		    HE *const he = hv_fetch_ent(stored, subsv, 1, 0);
		    assert(he);
		    if(HeVAL(he) != &PL_sv_undef) {
			/* It was newly created.  Steal it for our new SV, and
			   replace it in the hash with the "real" thing.  */
			SV *const val = HeVAL(he);
			HEK *const key = HeKEY_hek(he);

			HeVAL(he) = &PL_sv_undef;
			/* Save copying by making a shared hash key scalar. We
			   inline this here rather than calling
			   Perl_newSVpvn_share because we already have the
			   scalar, and we already have the hash key.  */
			assert(SvTYPE(val) == SVt_NULL);
			sv_upgrade(val, SVt_PV);
			SvPV_set(val, HEK_KEY(share_hek_hek(key)));
			SvCUR_set(val, HEK_LEN(key));
			SvREADONLY_on(val);
			SvFAKE_on(val);
			SvPOK_on(val);
			if (HEK_UTF8(key))
			    SvUTF8_on(val);

			av_push(retval, val);
		    }
		}
            } else {
		/* We are the first (or only) parent. We can short cut the
		   complexity above, because our @ISA is simply us prepended
		   to our parent's @ISA, and our ->isa cache is simply our
		   parent's, with our name added.  */
		/* newSVsv() is slow. This code is only faster if we can avoid
		   it by ensuring that SVs in the arrays are shared hash key
		   scalar SVs, because we can "copy" them very efficiently.
		   Although to be fair, we can't *ensure* this, as a reference
		   to the internal array is returned by mro::get_linear_isa(),
		   so we'll have to be defensive just in case someone faffed
		   with it.  */
		if (basestash) {
		    SV **svp;
		    stored = MUTABLE_HV(sv_2mortal((SV*)newHVhv(HvMROMETA(basestash)->isa)));
		    av_extend(retval, subrv_items);
		    AvFILLp(retval) = subrv_items;
		    svp = AvARRAY(retval);
		    while(subrv_items--) {
			SV *const val = *subrv_p++;
			*++svp = SvIsCOW_shared_hash(val)
			    ? newSVhek(SvSHARED_HEK_FROM_PV(SvPVX(val)))
			    : newSVsv(val);
		    }
		} else {
		    /* They have no stash.  So create ourselves an ->isa cache
		       as if we'd copied it from what theirs should be.  */
		    stored = MUTABLE_HV(sv_2mortal(MUTABLE_SV(newHV())));
		    (void) hv_store(stored, "UNIVERSAL", 9, &PL_sv_undef, 0);
		    av_push(retval,
			    newSVhek(HeKEY_hek(hv_store_ent(stored, sv,
							    &PL_sv_undef, 0))));
		}
	    }
        }
    } else {
	/* We have no parents.  */
	stored = MUTABLE_HV(sv_2mortal(MUTABLE_SV(newHV())));
	(void) hv_store(stored, "UNIVERSAL", 9, &PL_sv_undef, 0);
    }

    (void) hv_store_ent(stored, our_name, &PL_sv_undef, 0);

    SvREFCNT_inc_simple_void_NN(stored);
    SvTEMP_off(stored);
    SvREADONLY_on(stored);

    meta->isa = stored;

    /* now that we're past the exception dangers, grab our own reference to
       the AV we're about to use for the result. The reference owned by the
       mortals' stack will be released soon, so everything will balance.  */
    SvREFCNT_inc_simple_void_NN(retval);
    SvTEMP_off(retval);

    /* we don't want anyone modifying the cache entry but us,
       and we do so by replacing it completely */
    SvREADONLY_on(retval);

    return MUTABLE_AV(Perl_mro_set_private_data(aTHX_ meta, &dfs_alg,
						MUTABLE_SV(retval)));
}

/*
=for apidoc mro_get_linear_isa

Returns either C<mro_get_linear_isa_c3> or
C<mro_get_linear_isa_dfs> for the given stash,
dependant upon which MRO is in effect
for that stash.  The return value is a
read-only AV*.

You are responsible for C<SvREFCNT_inc()> on the
return value if you plan to store it anywhere
semi-permanently (otherwise it might be deleted
out from under you the next time the cache is
invalidated).

=cut
*/
AV*
Perl_mro_get_linear_isa(pTHX_ HV *stash)
{
    struct mro_meta* meta;
    AV *isa;

    PERL_ARGS_ASSERT_MRO_GET_LINEAR_ISA;
    if(!SvOOK(stash))
        Perl_croak(aTHX_ "Can't linearize anonymous symbol table");

    meta = HvMROMETA(stash);
    if (!meta->mro_which)
        Perl_croak(aTHX_ "panic: invalid MRO!");
    isa = meta->mro_which->resolve(aTHX_ stash, 0);

    if (!meta->isa) {
	    HV *const isa_hash = newHV();
	    /* Linearisation didn't build it for us, so do it here.  */
	    SV *const *svp = AvARRAY(isa);
	    SV *const *const svp_end = svp + AvFILLp(isa) + 1;
	    const HEK *canon_name = HvENAME_HEK(stash);
	    if (!canon_name) canon_name = HvNAME_HEK(stash);

	    while (svp < svp_end) {
		(void) hv_store_ent(isa_hash, *svp++, &PL_sv_undef, 0);
	    }

	    (void) hv_common(isa_hash, NULL, HEK_KEY(canon_name),
			     HEK_LEN(canon_name), HEK_FLAGS(canon_name),
			     HV_FETCH_ISSTORE, &PL_sv_undef,
			     HEK_HASH(canon_name));
	    (void) hv_store(isa_hash, "UNIVERSAL", 9, &PL_sv_undef, 0);

	    SvREADONLY_on(isa_hash);

	    meta->isa = isa_hash;
    }

    return isa;
}

/*
=for apidoc mro_isa_changed_in

Takes the necessary steps (cache invalidations, mostly)
when the @ISA of the given package has changed.  Invoked
by the C<setisa> magic, should not need to invoke directly.

=cut
*/

/* Macro to avoid repeating the code five times. */
#define CLEAR_LINEAR(mEta)                                     \
    if (mEta->mro_linear_all) {                                 \
	SvREFCNT_dec(MUTABLE_SV(mEta->mro_linear_all));          \
	mEta->mro_linear_all = NULL;                              \
	/* This is just acting as a shortcut pointer.  */          \
	mEta->mro_linear_current = NULL;                            \
    } else if (mEta->mro_linear_current) {                           \
	/* Only the current MRO is stored, so this owns the data.  */ \
	SvREFCNT_dec(mEta->mro_linear_current);                        \
	mEta->mro_linear_current = NULL;                                \
    }

void
Perl_mro_isa_changed_in(pTHX_ HV* stash)
{
    dVAR;
    HV* isarev;
    AV* linear_mro;
    HE* iter;
    SV** svp;
    I32 items;
    bool is_universal;
    struct mro_meta * meta;
    HV *isa = NULL;

    const char * const stashname = HvENAME_get(stash);
    const STRLEN stashname_len = HvENAMELEN_get(stash);

    PERL_ARGS_ASSERT_MRO_ISA_CHANGED_IN;

    if(!stashname)
        Perl_croak(aTHX_ "Can't call mro_isa_changed_in() on anonymous symbol table");


    /* wipe out the cached linearizations for this stash */
    meta = HvMROMETA(stash);
    CLEAR_LINEAR(meta);
    if (meta->isa) {
	/* Steal it for our own purposes. */
	isa = (HV *)sv_2mortal((SV *)meta->isa);
	meta->isa = NULL;
    }

    /* Inc the package generation, since our @ISA changed */
    meta->pkg_gen++;

    /* Wipe the global method cache if this package
       is UNIVERSAL or one of its parents */

    svp = hv_fetch(PL_isarev, stashname, stashname_len, 0);
    isarev = svp ? MUTABLE_HV(*svp) : NULL;

    if((stashname_len == 9 && strEQ(stashname, "UNIVERSAL"))
        || (isarev && hv_exists(isarev, "UNIVERSAL", 9))) {
        PL_sub_generation++;
        is_universal = TRUE;
    }
    else { /* Wipe the local method cache otherwise */
        meta->cache_gen++;
	is_universal = FALSE;
    }

    /* wipe next::method cache too */
    if(meta->mro_nextmethod) hv_clear(meta->mro_nextmethod);

    /* Iterate the isarev (classes that are our children),
       wiping out their linearization, method and isa caches
       and upating PL_isarev. */
    if(isarev) {
        HV *isa_hashes = NULL;

       /* We have to iterate through isarev twice to avoid a chicken and
        * egg problem: if A inherits from B and both are in isarev, A might
        * be processed before B and use B’s previous linearisation.
        */

       /* First iteration: Wipe everything, but stash away the isa hashes
        * since we still need them for updating PL_isarev.
        */

        if(hv_iterinit(isarev)) {
            /* Only create the hash if we need it; i.e., if isarev has
               any elements. */
            isa_hashes = (HV *)sv_2mortal((SV *)newHV());
        }
        while((iter = hv_iternext(isarev))) {
	    I32 len;
            const char* const revkey = hv_iterkey(iter, &len);
            HV* revstash = gv_stashpvn(revkey, len, 0);
            struct mro_meta* revmeta;

            if(!revstash) continue;
            revmeta = HvMROMETA(revstash);
	    CLEAR_LINEAR(revmeta);
            if(!is_universal)
                revmeta->cache_gen++;
            if(revmeta->mro_nextmethod)
                hv_clear(revmeta->mro_nextmethod);

	    (void)
	      hv_store(
	       isa_hashes, (const char*)&revstash, sizeof(HV *),
	       revmeta->isa ? (SV *)revmeta->isa : &PL_sv_undef, 0
	      );
	    revmeta->isa = NULL;
        }

       /* Second pass: Update PL_isarev. We can just use isa_hashes to
        * avoid another round of stash lookups. */

       /* isarev might be deleted from PL_isarev during this loop, so hang
        * on to it. */
        SvREFCNT_inc_simple_void_NN(sv_2mortal((SV *)isarev));

        if(isa_hashes) {
            hv_iterinit(isa_hashes);
            while((iter = hv_iternext(isa_hashes))) {
                HV* const revstash = *(HV **)HEK_KEY(HeKEY_hek(iter));
                HV * const isa = (HV *)HeVAL(iter);
                const HEK *namehek;

                /* We're starting at the 2nd element, skipping revstash */
                linear_mro = mro_get_linear_isa(revstash);
                svp = AvARRAY(linear_mro) + 1;
                items = AvFILLp(linear_mro);

                namehek = HvENAME_HEK(revstash);
                if (!namehek) namehek = HvNAME_HEK(revstash);

                while (items--) {
                    SV* const sv = *svp++;
                    HV* mroisarev;

                    HE *he = hv_fetch_ent(PL_isarev, sv, TRUE, 0);

                    /* That fetch should not fail.  But if it had to create
                       a new SV for us, then will need to upgrade it to an
                       HV (which sv_upgrade() can now do for us). */

                    mroisarev = MUTABLE_HV(HeVAL(he));

                    SvUPGRADE(MUTABLE_SV(mroisarev), SVt_PVHV);

                    /* This hash only ever contains PL_sv_yes. Storing it
                       over itself is almost as cheap as calling hv_exists,
                       so on aggregate we expect to save time by not making
                       two calls to the common HV code for the case where
                       it doesn't exist.  */
	   
                    (void)
                      hv_store(
                       mroisarev, HEK_KEY(namehek), HEK_LEN(namehek),
                       &PL_sv_yes, 0
                      );
                }

                if((SV *)isa != &PL_sv_undef)
                    mro_clean_isarev(
                     isa, HEK_KEY(namehek), HEK_LEN(namehek),
                     HvMROMETA(revstash)->isa
                    );
            }
        }
    }

    /* Now iterate our MRO (parents), adding ourselves and everything from
       our isarev to their isarev.
    */

    /* We're starting at the 2nd element, skipping ourselves here */
    linear_mro = mro_get_linear_isa(stash);
    svp = AvARRAY(linear_mro) + 1;
    items = AvFILLp(linear_mro);

    while (items--) {
        SV* const sv = *svp++;
        HV* mroisarev;

        HE *he = hv_fetch_ent(PL_isarev, sv, TRUE, 0);

	/* That fetch should not fail.  But if it had to create a new SV for
	   us, then will need to upgrade it to an HV (which sv_upgrade() can
	   now do for us. */

        mroisarev = MUTABLE_HV(HeVAL(he));

	SvUPGRADE(MUTABLE_SV(mroisarev), SVt_PVHV);

	/* This hash only ever contains PL_sv_yes. Storing it over itself is
	   almost as cheap as calling hv_exists, so on aggregate we expect to
	   save time by not making two calls to the common HV code for the
	   case where it doesn't exist.  */
	   
	(void)hv_store(mroisarev, stashname, stashname_len, &PL_sv_yes, 0);
    }

    /* Delete our name from our former parents’ isarevs. */
    if(isa && HvARRAY(isa))
        mro_clean_isarev(isa, stashname, stashname_len, meta->isa);
}

/* Deletes name from all the isarev entries listed in isa */
STATIC void
S_mro_clean_isarev(pTHX_ HV * const isa, const char * const name,
                         const STRLEN len, HV * const exceptions)
{
    HE* iter;

    PERL_ARGS_ASSERT_MRO_CLEAN_ISAREV;

    /* Delete our name from our former parents’ isarevs. */
    if(isa && HvARRAY(isa) && hv_iterinit(isa)) {
        SV **svp;
        while((iter = hv_iternext(isa))) {
            I32 klen;
            const char * const key = hv_iterkey(iter, &klen);
            if(exceptions && hv_exists(exceptions, key, klen)) continue;
            svp = hv_fetch(PL_isarev, key, klen, 0);
            if(svp) {
                HV * const isarev = (HV *)*svp;
                (void)hv_delete(isarev, name, len, G_DISCARD);
                if(!HvARRAY(isarev) || !HvUSEDKEYS(isarev))
                    (void)hv_delete(PL_isarev, key, klen, G_DISCARD);
            }
        }
    }
}

/*
=for apidoc mro_package_moved

Call this function to signal to a stash that it has been assigned to
another spot in the stash hierarchy. C<stash> is the stash that has been
assigned. C<oldstash> is the stash it replaces, if any. C<gv> is the glob
that is actually being assigned to.

This can also be called with a null first argument to
indicate that C<oldstash> has been deleted.

This function invalidates isa caches on the old stash, on all subpackages
nested inside it, and on the subclasses of all those, including
non-existent packages that have corresponding entries in C<stash>.

It also sets the effective names (C<HvENAME>) on all the stashes as
appropriate.

If the C<gv> is present and is not in the symbol table, then this function
simply returns. This checked will be skipped if C<flags & 1>.

=cut
*/
void
Perl_mro_package_moved(pTHX_ HV * const stash, HV * const oldstash,
                       const GV * const gv, U32 flags)
{
    SV *namesv;
    HEK **namep;
    I32 name_count;
    HV *stashes;
    HE* iter;

    PERL_ARGS_ASSERT_MRO_PACKAGE_MOVED;
    assert(stash || oldstash);

    /* Determine the name(s) of the location that stash was assigned to
     * or from which oldstash was removed.
     *
     * We cannot reliably use the name in oldstash, because it may have
     * been deleted from the location in the symbol table that its name
     * suggests, as in this case:
     *
     *   $globref = \*foo::bar::;
     *   Symbol::delete_package("foo");
     *   *$globref = \%baz::;
     *   *$globref = *frelp::;
     *      # calls mro_package_moved(%frelp::, %baz::, *$globref, NULL, 0)
     *
     * So we get it from the gv. But, since the gv may no longer be in the
     * symbol table, we check that first. The only reliable way to tell is
     * to see whether its stash has an effective name and whether the gv
     * resides in that stash under its name. That effective name may be
     * different from what gv_fullname4 would use.
     * If flags & 1, the caller has asked us to skip the check.
     */
    if(!(flags & 1)) {
	SV **svp;
	if(
	 !GvSTASH(gv) || !HvENAME(GvSTASH(gv)) ||
	 !(svp = hv_fetch(GvSTASH(gv), GvNAME(gv), GvNAMELEN(gv), 0)) ||
	 *svp != (SV *)gv
	) return;
    }
    assert(SvOOK(GvSTASH(gv)));
    assert(GvNAMELEN(gv));
    assert(GvNAME(gv)[GvNAMELEN(gv) - 1] == ':');
    assert(GvNAMELEN(gv) == 1 || GvNAME(gv)[GvNAMELEN(gv) - 2] == ':');
    name_count = HvAUX(GvSTASH(gv))->xhv_name_count;
    if (!name_count) {
	name_count = 1;
	namep = &HvAUX(GvSTASH(gv))->xhv_name_u.xhvnameu_name;
    }
    else {
	namep = HvAUX(GvSTASH(gv))->xhv_name_u.xhvnameu_names;
	if (name_count < 0) ++namep, name_count = -name_count - 1;
    }
    if (name_count == 1) {
	if (HEK_LEN(*namep) == 4 && strnEQ(HEK_KEY(*namep), "main", 4)) {
	    namesv = GvNAMELEN(gv) == 1
		? newSVpvs_flags(":", SVs_TEMP)
		: newSVpvs_flags("",  SVs_TEMP);
	}
	else {
	    namesv = sv_2mortal(newSVhek(*namep));
	    if (GvNAMELEN(gv) == 1) sv_catpvs(namesv, ":");
	    else                    sv_catpvs(namesv, "::");
	}
	if (GvNAMELEN(gv) != 1)
	    sv_catpvn(namesv, GvNAME(gv), GvNAMELEN(gv) - 2);
	                                  /* skip trailing :: */
    }
    else {
	SV *aname;
	namesv = sv_2mortal((SV *)newAV());
	while (name_count--) {
	    if(HEK_LEN(*namep) == 4 && strnEQ(HEK_KEY(*namep), "main", 4)){
		aname = GvNAMELEN(gv) == 1
		         ? newSVpvs(":")
		         : newSVpvs("");
		namep++;
	    }
	    else {
		aname = newSVhek(*namep++);
		if (GvNAMELEN(gv) == 1) sv_catpvs(aname, ":");
		else                    sv_catpvs(aname, "::");
	    }
	    if (GvNAMELEN(gv) != 1)
		sv_catpvn(aname, GvNAME(gv), GvNAMELEN(gv) - 2);
	                                  /* skip trailing :: */
	    av_push((AV *)namesv, aname);
	}
    }

    /* Get a list of all the affected classes. */
    /* We cannot simply pass them all to mro_isa_changed_in to avoid
       the list, as that function assumes that only one package has
       changed. It does not work with:

          @foo::ISA = qw( B B::B );
          *B:: = delete $::{"A::"};

       as neither B nor B::B can be updated before the other, since they
       will reset caches on foo, which will see either B or B::B with the
       wrong name. The names must be set on *all* affected stashes before
       we do anything else. (And linearisations must be cleared, too.)
     */
    stashes = (HV *) sv_2mortal((SV *)newHV());
    mro_gather_and_rename(
     stashes, (HV *) sv_2mortal((SV *)newHV()),
     stash, oldstash, namesv
    );

    /* Once the caches have been wiped on all the classes, call
       mro_isa_changed_in on each. */
    hv_iterinit(stashes);
    while((iter = hv_iternext(stashes))) {
	HV * const stash = *(HV **)HEK_KEY(HeKEY_hek(iter));
	if(HvENAME(stash)) {
	    /* We have to restore the original meta->isa (that
	       mro_gather_and_rename set aside for us) this way, in case
	       one class in this list is a superclass of a another class
	       that we have already encountered. In such a case, meta->isa
	       will have been overwritten without old entries being deleted 
	       from PL_isarev. */
	    struct mro_meta * const meta = HvMROMETA(stash);
	    if(meta->isa != (HV *)HeVAL(iter)){
		SvREFCNT_dec(meta->isa);
		meta->isa
		 = HeVAL(iter) == &PL_sv_yes
		    ? NULL
		    : (HV *)HeVAL(iter);
		HeVAL(iter) = NULL; /* We donated our reference count. */
	    }
	    mro_isa_changed_in(stash);
	}
    }
}

void
S_mro_gather_and_rename(pTHX_ HV * const stashes, HV * const seen_stashes,
                              HV *stash, HV *oldstash, SV *namesv)
{
    register XPVHV* xhv;
    register HE *entry;
    I32 riter = -1;
    I32 items = 0;
    const bool stash_had_name = stash && HvENAME(stash);
    bool fetched_isarev = FALSE;
    HV *seen = NULL;
    HV *isarev = NULL;
    SV **svp = NULL;

    PERL_ARGS_ASSERT_MRO_GATHER_AND_RENAME;

    /* We use the seen_stashes hash to keep track of which packages have
       been encountered so far. This must be separate from the main list of
       stashes, as we need to distinguish between stashes being assigned
       and stashes being replaced/deleted. (A nested stash can be on both
       sides of an assignment. We cannot simply skip iterating through a
       stash on the right if we have seen it on the left, as it will not
       get its ename assigned to it.)

       To avoid allocating extra SVs, instead of a bitfield we can make
       bizarre use of immortals:

        &PL_sv_undef:  seen on the left  (oldstash)
        &PL_sv_no   :  seen on the right (stash)
        &PL_sv_yes  :  seen on both sides

     */

    if(oldstash) {
	/* Add to the big list. */
	struct mro_meta * meta;
	HE * const entry
	 = (HE *)
	     hv_common(
	      seen_stashes, NULL, (const char *)&oldstash, sizeof(HV *), 0,
	      HV_FETCH_LVALUE|HV_FETCH_EMPTY_HE, NULL, 0
	     );
	if(HeVAL(entry) == &PL_sv_undef || HeVAL(entry) == &PL_sv_yes) {
	    oldstash = NULL;
	    goto check_stash;
	}
	HeVAL(entry)
	 = HeVAL(entry) == &PL_sv_no ? &PL_sv_yes : &PL_sv_undef;
	meta = HvMROMETA(oldstash);
	(void)
	  hv_store(
	   stashes, (const char *)&oldstash, sizeof(HV *),
	   meta->isa
	    ? SvREFCNT_inc_simple_NN((SV *)meta->isa)
	    : &PL_sv_yes,
	   0
	  );
	CLEAR_LINEAR(meta);

	/* Update the effective name. */
	if(HvENAME_get(oldstash)) {
	    const HEK * const enamehek = HvENAME_HEK(oldstash);
	    if(SvTYPE(namesv) == SVt_PVAV) {
		items = AvFILLp((AV *)namesv) + 1;
		svp = AvARRAY((AV *)namesv);
	    }
	    else {
		items = 1;
		svp = &namesv;
	    }
	    while (items--) {
		STRLEN len;
		const char *name = SvPVx_const(*svp++, len);
		if(PL_stashcache)
		    (void)hv_delete(PL_stashcache, name, len, G_DISCARD);
	        hv_ename_delete(oldstash, name, len, 0);

		if (!fetched_isarev) {
		    /* If the name deletion caused a name change, then we
		     * are not going to call mro_isa_changed_in with this
		     * name (and not at all if it has become anonymous) so
		     * we need to delete old isarev entries here, both
		     * those in the superclasses and this class’s own list
		     * of subclasses. We simply delete the latter from
		     * PL_isarev, since we still need it. hv_delete morti-
		     * fies it for us, so sv_2mortal is not necessary. */
		    if(HvENAME_HEK(oldstash) != enamehek) {
			if(meta->isa && HvARRAY(meta->isa))
			    mro_clean_isarev(meta->isa, name, len, NULL);
			isarev = (HV *)hv_delete(PL_isarev, name, len, 0);
			fetched_isarev=TRUE;
		    }
		}
	    }
	}
    }
   check_stash:
    if(stash) {
	if(SvTYPE(namesv) == SVt_PVAV) {
	    items = AvFILLp((AV *)namesv) + 1;
	    svp = AvARRAY((AV *)namesv);
	}
	else {
	    items = 1;
	    svp = &namesv;
	}
	while (items--) {
	    STRLEN len;
	    const char *name = SvPVx_const(*svp++, len);
	    hv_ename_add(stash, name, len, 0);
	}

       /* Add it to the big list if it needs
	* mro_isa_changed_in called on it. That happens if it was
	* detached from the symbol table (so it had no HvENAME) before
	* being assigned to the spot named by the ‘name’ variable, because
	* its cached isa linearisation is now stale (the effective name
	* having changed), and subclasses will then use that cache when
	* mro_package_moved calls mro_isa_changed_in. (See
	* [perl #77358].)
	*
	* If it did have a name, then its previous name is still
	* used in isa caches, and there is no need for
	* mro_package_moved to call mro_isa_changed_in.
	*/

	entry
	 = (HE *)
	     hv_common(
	      seen_stashes, NULL, (const char *)&stash, sizeof(HV *), 0,
	      HV_FETCH_LVALUE|HV_FETCH_EMPTY_HE, NULL, 0
	     );
	if(HeVAL(entry) == &PL_sv_yes || HeVAL(entry) == &PL_sv_no)
	    stash = NULL;
	else {
	    HeVAL(entry)
	     = HeVAL(entry) == &PL_sv_undef ? &PL_sv_yes : &PL_sv_no;
	    if(!stash_had_name)
	    {
		struct mro_meta * const meta = HvMROMETA(stash);
		(void)
		  hv_store(
		   stashes, (const char *)&stash, sizeof(HV *),
		   meta->isa
		    ? SvREFCNT_inc_simple_NN((SV *)meta->isa)
		    : &PL_sv_yes,
		   0
		  );
		CLEAR_LINEAR(meta);
	    }
	}
    }

    if(!stash && !oldstash)
	/* Both stashes have been encountered already. */
	return;

    /* Add all the subclasses to the big list. */
    if(!fetched_isarev) {
	/* If oldstash is not null, then we can use its HvENAME to look up
	   the isarev hash, since all its subclasses will be listed there.
	   It will always have an HvENAME. It the HvENAME was removed
	   above, then fetch_isarev will be true, and this code will not be
	   reached.

	   If oldstash is null, then this is an empty spot with no stash in
	   it, so subclasses could be listed in isarev hashes belonging to
	   any of the names, so we have to check all of them.
	 */
	assert(!oldstash || HvENAME(oldstash));
	if (oldstash) {
	    /* Extra variable to avoid a compiler warning */
	    char * const hvename = HvENAME(oldstash);
	    fetched_isarev = TRUE;
	    svp = hv_fetch(PL_isarev, hvename, HvENAMELEN_get(oldstash), 0);
	    if (svp) isarev = MUTABLE_HV(*svp);
	}
	else if(SvTYPE(namesv) == SVt_PVAV) {
	    items = AvFILLp((AV *)namesv) + 1;
	    svp = AvARRAY((AV *)namesv);
	}
	else {
	    items = 1;
	    svp = &namesv;
	}
    }
    if(
        isarev || !fetched_isarev
    ) {
      while (fetched_isarev || items--) {
	HE *iter;

	if (!fetched_isarev) {
	    HE * const he = hv_fetch_ent(PL_isarev, *svp++, 0, 0);
	    if (!he || !(isarev = MUTABLE_HV(HeVAL(he)))) continue;
	}

	hv_iterinit(isarev);
	while((iter = hv_iternext(isarev))) {
	    I32 len;
	    const char* const revkey = hv_iterkey(iter, &len);
	    HV* revstash = gv_stashpvn(revkey, len, 0);
	    struct mro_meta * meta;

	    if(!revstash) continue;
	    meta = HvMROMETA(revstash);
	    (void)
	      hv_store(
	       stashes, (const char *)&revstash, sizeof(HV *),
	       meta->isa
	        ? SvREFCNT_inc_simple_NN((SV *)meta->isa)
	        : &PL_sv_yes,
	       0
	      );
	    CLEAR_LINEAR(meta);
        }

	if (fetched_isarev) break;
      }
    }

    /* This is partly based on code in hv_iternext_flags. We are not call-
       ing that here, as we want to avoid resetting the hash iterator. */

    /* Skip the entire loop if the hash is empty.   */
    if(oldstash && HvUSEDKEYS(oldstash)) { 
	xhv = (XPVHV*)SvANY(oldstash);
	seen = (HV *) sv_2mortal((SV *)newHV());

	/* Iterate through entries in the oldstash, adding them to the
	   list, meanwhile doing the equivalent of $seen{$key} = 1.
	 */

	while (++riter <= (I32)xhv->xhv_max) {
	    entry = (HvARRAY(oldstash))[riter];

	    /* Iterate through the entries in this list */
	    for(; entry; entry = HeNEXT(entry)) {
		const char* key;
		I32 len;

		/* If this entry is not a glob, ignore it.
		   Try the next.  */
		if (!isGV(HeVAL(entry))) continue;

		key = hv_iterkey(entry, &len);
		if ((len > 1 && key[len-2] == ':' && key[len-1] == ':')
		 || (len == 1 && key[0] == ':')) {
		    HV * const oldsubstash = GvHV(HeVAL(entry));
		    SV ** const stashentry
		     = stash ? hv_fetch(stash, key, len, 0) : NULL;
		    HV *substash = NULL;

		    /* Avoid main::main::main::... */
		    if(oldsubstash == oldstash) continue;

		    if(
		        (
		            stashentry && *stashentry
		         && (substash = GvHV(*stashentry))
		        )
		     || (oldsubstash && HvENAME_get(oldsubstash))
		    )
		    {
			/* Add :: and the key (minus the trailing ::)
			   to each name. */
			SV *subname;
			if(SvTYPE(namesv) == SVt_PVAV) {
			    SV *aname;
			    items = AvFILLp((AV *)namesv) + 1;
			    svp = AvARRAY((AV *)namesv);
			    subname = sv_2mortal((SV *)newAV());
			    while (items--) {
				aname = newSVsv(*svp++);
				if (len == 1)
				    sv_catpvs(aname, ":");
				else {
				    sv_catpvs(aname, "::");
				    sv_catpvn(aname, key, len-2);
				}
				av_push((AV *)subname, aname);
			    }
			}
			else {
			    subname = sv_2mortal(newSVsv(namesv));
			    if (len == 1) sv_catpvs(subname, ":");
			    else {
				sv_catpvs(subname, "::");
				sv_catpvn(subname, key, len-2);
			    }
			}
			mro_gather_and_rename(
			     stashes, seen_stashes,
			     substash, oldsubstash, subname
			);
		    }

		    (void)hv_store(seen, key, len, &PL_sv_yes, 0);
		}
	    }
	}
    }

    /* Skip the entire loop if the hash is empty.   */
    if (stash && HvUSEDKEYS(stash)) {
	xhv = (XPVHV*)SvANY(stash);
	riter = -1;

	/* Iterate through the new stash, skipping $seen{$key} items,
	   calling mro_gather_and_rename(stashes,seen,entry,NULL, ...). */
	while (++riter <= (I32)xhv->xhv_max) {
	    entry = (HvARRAY(stash))[riter];

	    /* Iterate through the entries in this list */
	    for(; entry; entry = HeNEXT(entry)) {
		const char* key;
		I32 len;

		/* If this entry is not a glob, ignore it.
		   Try the next.  */
		if (!isGV(HeVAL(entry))) continue;

		key = hv_iterkey(entry, &len);
		if ((len > 1 && key[len-2] == ':' && key[len-1] == ':')
		 || (len == 1 && key[0] == ':')) {
		    HV *substash;

		    /* If this entry was seen when we iterated through the
		       oldstash, skip it. */
		    if(seen && hv_exists(seen, key, len)) continue;

		    /* We get here only if this stash has no corresponding
		       entry in the stash being replaced. */

		    substash = GvHV(HeVAL(entry));
		    if(substash) {
			SV *subname;

			/* Avoid checking main::main::main::... */
			if(substash == stash) continue;

			/* Add :: and the key (minus the trailing ::)
			   to each name. */
			if(SvTYPE(namesv) == SVt_PVAV) {
			    SV *aname;
			    items = AvFILLp((AV *)namesv) + 1;
			    svp = AvARRAY((AV *)namesv);
			    subname = sv_2mortal((SV *)newAV());
			    while (items--) {
				aname = newSVsv(*svp++);
				if (len == 1)
				    sv_catpvs(aname, ":");
				else {
				    sv_catpvs(aname, "::");
				    sv_catpvn(aname, key, len-2);
				}
				av_push((AV *)subname, aname);
			    }
			}
			else {
			    subname = sv_2mortal(newSVsv(namesv));
			    if (len == 1) sv_catpvs(subname, ":");
			    else {
				sv_catpvs(subname, "::");
				sv_catpvn(subname, key, len-2);
			    }
			}
			mro_gather_and_rename(
			  stashes, seen_stashes,
			  substash, NULL, subname
			);
		    }
		}
	    }
	}
    }
}

/*
=for apidoc mro_method_changed_in

Invalidates method caching on any child classes
of the given stash, so that they might notice
the changes in this one.

Ideally, all instances of C<PL_sub_generation++> in
perl source outside of C<mro.c> should be
replaced by calls to this.

Perl automatically handles most of the common
ways a method might be redefined.  However, there
are a few ways you could change a method in a stash
without the cache code noticing, in which case you
need to call this method afterwards:

1) Directly manipulating the stash HV entries from
XS code.

2) Assigning a reference to a readonly scalar
constant into a stash entry in order to create
a constant subroutine (like constant.pm
does).

This same method is available from pure perl
via, C<mro::method_changed_in(classname)>.

=cut
*/
void
Perl_mro_method_changed_in(pTHX_ HV *stash)
{
    const char * const stashname = HvENAME_get(stash);
    const STRLEN stashname_len = HvENAMELEN_get(stash);

    SV ** const svp = hv_fetch(PL_isarev, stashname, stashname_len, 0);
    HV * const isarev = svp ? MUTABLE_HV(*svp) : NULL;

    PERL_ARGS_ASSERT_MRO_METHOD_CHANGED_IN;

    if(!stashname)
        Perl_croak(aTHX_ "Can't call mro_method_changed_in() on anonymous symbol table");

    /* Inc the package generation, since a local method changed */
    HvMROMETA(stash)->pkg_gen++;

    /* If stash is UNIVERSAL, or one of UNIVERSAL's parents,
       invalidate all method caches globally */
    if((stashname_len == 9 && strEQ(stashname, "UNIVERSAL"))
        || (isarev && hv_exists(isarev, "UNIVERSAL", 9))) {
        PL_sub_generation++;
        return;
    }

    /* else, invalidate the method caches of all child classes,
       but not itself */
    if(isarev) {
	HE* iter;

        hv_iterinit(isarev);
        while((iter = hv_iternext(isarev))) {
	    I32 len;
            const char* const revkey = hv_iterkey(iter, &len);
            HV* const revstash = gv_stashpvn(revkey, len, 0);
            struct mro_meta* mrometa;

            if(!revstash) continue;
            mrometa = HvMROMETA(revstash);
            mrometa->cache_gen++;
            if(mrometa->mro_nextmethod)
                hv_clear(mrometa->mro_nextmethod);
        }
    }
}

void
Perl_mro_set_mro(pTHX_ struct mro_meta *const meta, SV *const name)
{
    const struct mro_alg *const which = Perl_mro_get_from_name(aTHX_ name);
 
    PERL_ARGS_ASSERT_MRO_SET_MRO;

    if (!which)
        Perl_croak(aTHX_ "Invalid mro name: '%"SVf"'", name);

    if(meta->mro_which != which) {
	if (meta->mro_linear_current && !meta->mro_linear_all) {
	    /* If we were storing something directly, put it in the hash before
	       we lose it. */
	    Perl_mro_set_private_data(aTHX_ meta, meta->mro_which, 
				      MUTABLE_SV(meta->mro_linear_current));
	}
	meta->mro_which = which;
	/* Scrub our cached pointer to the private data.  */
	meta->mro_linear_current = NULL;
        /* Only affects local method cache, not
           even child classes */
        meta->cache_gen++;
        if(meta->mro_nextmethod)
            hv_clear(meta->mro_nextmethod);
    }
}

#include "XSUB.h"

XS(XS_mro_method_changed_in);

void
Perl_boot_core_mro(pTHX)
{
    dVAR;
    static const char file[] = __FILE__;

    Perl_mro_register(aTHX_ &dfs_alg);

    newXSproto("mro::method_changed_in", XS_mro_method_changed_in, file, "$");
}

XS(XS_mro_method_changed_in)
{
    dVAR;
    dXSARGS;
    SV* classname;
    HV* class_stash;

    if(items != 1)
	croak_xs_usage(cv, "classname");
    
    classname = ST(0);

    class_stash = gv_stashsv(classname, 0);
    if(!class_stash) Perl_croak(aTHX_ "No such class: '%"SVf"'!", SVfARG(classname));

    mro_method_changed_in(class_stash);

    XSRETURN_EMPTY;
}

/*
 * Local variables:
 * c-indentation-style: bsd
 * c-basic-offset: 4
 * indent-tabs-mode: t
 * End:
 *
 * ex: set ts=8 sts=4 sw=4 noet:
 */