/**    mdnblocks() -- Get the number of blocks stored in a relation.**        Important side effect: all active segments of the relation are opened*        and added to the mdfd_chain list.  If this routine has not been*        called, then only segments up to the last one actually touched*        are present in the chain.*/
BlockNumber
mdnblocks(SMgrRelation reln, ForkNumber forknum)
{MdfdVec    *v = mdopen(reln, forknum, EXTENSION_FAIL);BlockNumber nblocks;BlockNumber segno = 0;/** Skip through any segments that aren't the last one, to avoid redundant* seeks on them.  We have previously verified that these segments are* exactly RELSEG_SIZE long, and it's useless to recheck that each time.** NOTE: this assumption could only be wrong if another backend has* truncated the relation.    We rely on higher code levels to handle that* scenario by closing and re-opening the md fd, which is handled via* relcache flush.    (Since the checkpointer doesn't participate in* relcache flush, it could have segment chain entries for inactive* segments; that's OK because the checkpointer never needs to compute* relation size.)*/while (v->mdfd_chain != NULL){segno++;v = v->mdfd_chain;}for (;;){nblocks = _mdnblocks(reln, forknum, v);　　　　 if (nblocks > ((BlockNumber) RELSEG_SIZE))　　　　　　　　elog(FATAL, "segment too big");　　　　 if (nblocks < ((BlockNumber) RELSEG_SIZE))　　　　　　　　return (segno * ((BlockNumber) RELSEG_SIZE)) + nblocks;

        ...v = v->mdfd_chain;}
}