GiST splitting on empty pages

This is from Bug #11555, which is still in moderation as I type this
(analysis was done via IRC).

The GiST insertion code appears to have no length checks at all on the
inserted entry. index_form_tuple checks for length <= 8191, with the
default blocksize, but obviously a tuple less than 8191 bytes may
still not fit on the page due to page header info etc.

However, the gist insertion code assumes that if the tuple doesn't fit,
then it must have to split the page, with no check whether the page is
already empty.

So this crashes with infinite recursion in gistSplit (which also lacks
a check_stack_depth() call):

create extension pg_trgm;

create table t1 (a text);
create index on t1 using gist (a gist_trgm_ops);

insert into t1 values (
'vvsehbxxlezgwtyvbgdyburmhxmuzbwvwoaepxbbbaiyuguwnxvprmbzkotkqqfnegruds'
'wftedolykzvfonsqndehouxuibazwdrtjlynzjlkihqxvjnimrpbmnvupvtlzlejxdwwmh'
'hvpxtkggstyivlvqgkmawmlbvjerfrzmnokgyrnrllagwwxdgjddwofrxjidbiqowbvusi'
'mdumkrihuprxsnmyekhnojvexsftmybzcwlmntuijlfcyracciqqrmuoeairzkqbgcouvi'
'cfthhszhvbplshxmwcnetnokovmdpimrnfzuxzcsaseszcfvetaxgoivjuzzclqprqkopn'
'hqgmjgoocsicqpqylatkzvvqlmhwbwjjmpvvwkkyctatirstsldsgzismqonmxxzntvkdf'
'ifzizharbsdfkjetcrjqfwocvcvqmywuevddddvevyjgiozkfialfpnarjqdinymibqlem'
'qakzgtofeuoeftutulclpkynxgoostaitkizewfirunxnhqhttsiervbxkpqdqyxbhxfdc'
'nvwbskiirbckkgbfizqypuoorpvovzqiunjnxswpuyaefbkobbmrvbgmrbbmbsvwffjcxf'
'ssesxjtiyvjkmemsrdusqvklspqbsohkhlcevwmtrveeaqwrurjknuwfkngcbnnjzpnvma'
'odvsiwjfnewxpjslocyveajsjjhxeuxsxtlgqvldzhbortagvybazlsjuagyueqsycyoxj'
'swrtljnlpikrjjccswczuxelpdnorlyjhpdszqdozngjxilqfoqalumaxapplnzscclctp'
'rtdxdagorlchmocypayepjrpcusowldnfgkihrxzcagoojndjmizwzoyugmqsqeyxpgege'
'ejelytulxeyfdufsszzfqrvupskwxrbbafnyzhkwlicpchivhhaxywsopdlnumpusctrje'
'ovmqlpytlfamdziwnxzyltlaodciummihzzsoxmadmmgluczscxdwiekmgsgsfpaeostme'
'tprfwcazbtbzwyibiuhbbahqalftfryyhpeeseduxftudcvmwdoxdvodgtxllvktkoxdta'
'xrgqmjtiwqlknpfctmwqyhliawxyzrywienvogdkwovkeanxmjnkrztsvrqviprquflimp'
'tjeouiphfcrtnisgaoxrjfgbwahijuxddbsxkhfqjwjwfcdgrbxagdcdekmoekshmkfwsl'
'mbivynyctpdrqkutnzdaohkgpwqvsihfkpajczlwoonfziynibnwjxczttumcbrnrswtri'
'qgxelwmjjvlwruuutnoozqpregjbaajrhhvsdicndnkvhepbseprvfjzmsamtkearzsuiu'
'ilhsgpwwqoafgvkpuwhujbenbwnuqvoygwrnlnjccjhiesyyogtyhymiuzclvrkbobpapy'
'crhjalcykreepmdbvyaxkvpuxdwmdllfcspdesbpjguysyaowbmhwbcufyhiksonleqpws'
'ffyzerxefufrcctexydegnxvajzrywjebiegzckfxqxzsqdpohuvusrvbrmanwepeivelg'
'jiwhhoxlemszimraisrvterytwhpasvkarrhgptlclklyblhuccnhumbqtqrllcldutkkn'
'vmyfyxhkecmhqubcvsvmkgxmsbgllqyhdxmbuulzwygmtipoakakqywjadvltusxrfymzk'
'mwjsjcayqbirlzpiipmebfyucqabcampwvigxieoknfwnvfvlranxyiaoibringfjolgxq'
'uhdaeqwjmhamvxldxzlzqunxawmmdjcyrgzvxvfjcfwydhbbhmbxhovhlhtoqwnicmeahj'
'jkpgitojuvwvtekomvwfkncxvfkzfrjpcyvlskvmfrizwsoiokoyxqwsvhsazbpbalmsvh'
'fbznavgoeuystwjpoexhfwjvxkgkdcridrwdncsrxrkqntgbydjdzszwcfgghyolqlodnh'
'ukyfblyhnwkwajpzgsfnynlnybynfmuzxseyddfrapnaycafugsstdfsfefkqaknsplwsq'
'ntgbufdukybcrugxnmbsxrsielxqiqhwjnxdtbydzzgqunnhzoawgsflecbmtjjcxggqhe'
'tgeaxynkgmzgjgzordrtqkdznaftqnyktdkrcxcikbouiniarathkxgyxmsnzrytuikwfm'
'eqotkxgtxxtrfeomclyvzymxrggcdmpicebmbifyfzpldexgqvbptnnlutnxfdfihhuipa'
'hvaxgdbdkszliszvetpsrvvxddeymuytpyrvctzmlyytrxovreojzjhcnlazgzsvykrbdq'
'nopmhgjwcbaqlaasdneemkdfgcpxdtoqhddoknlmomdzmdrprvtegxkmzajctytacxpmka'
'zzncyzgqpxmjcsgmfgmojfndgpawckwbjjeijlzzjmilfpxkwkzfqmjxbjteuqfeaknjvm'
'iezrqegnodynjpasmbbffwvlavwnfraowzfmdmaspygyograisgopcaqxwednerkexwijw'
'azvhyjnpkwiqkxsloqhsuvwlfbjbjtykturrefhpcpfnnyybpftjaqvfsfhbygmraejekq'
'umfzztyxuocoydftixzqzxwlpxpyczowmuwlnuiiilxgocaxaaozxklnialkaagmucyixh'
'qgsnnhqnfqntpleaymbkxckdpfgnnduejnrwuikayytokyoilqtisdmhisvwwpafcscxan'
'xylrnvpcebsxjlbvtkoogkegqhzsfzgdyrulnknslgqusrqmbebhpfofnnysnewlvqxjal'
'cmrshkjxwkcxsrdhwquujhzftvwqexbgjtyqdioatqxliatfrnabvzhoueeybgflzecdmq'
'dghbsqclvuyvvtudiohm'
);

Suggested fixes (probably all of these are appropriate):

1. gistSplit should have check_stack_depth()

2. gistSplit should probably refuse to split anything if called with
only one item (which is the value being inserted).

3. somewhere before reaching gistSplit it might make sense to check
explicitly (e.g. in gistFormTuple) whether the tuple will actually
fit on a page.

4. pg_trgm probably should do something more sensible with large leaf
items, but this is a peripheral issue since ultimately the gist core
must enforce these limits rather than rely on the opclass.

--
Andrew (irc:RhodiumToad)

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