Trying to parsefile an empty file triggers mmap exception

[IainNZ]

shell> touch empty.json

julia> import JSON

julia> JSON.parsefile("empty.json")
ERROR: memory mapping failed: Invalid argument
 in systemerror at /usr/local/Cellar/julia/0.3.6_1/lib/julia/sys.dylib
 in mmap at mmap.jl:35
 in mmap_array at mmap.jl:110
 in mmap_array at mmap.jl:4
 in anonymous at /Users/idunning/.julia/v0.3/JSON/src/JSON.jl:280
 in open at /usr/local/Cellar/julia/0.3.6_1/lib/julia/sys.dylib
 in parsefile at /Users/idunning/.julia/v0.3/JSON/src/JSON.jl:279

[stevengj]

It looks like this may be a bug (or unavoidable limitation? but it seems avoidable) in mmap_array. After doing touch foo to create an empty file, mmap_array(Int, (0,), open("foo", "r")) gives SystemError: memory mapping failed: Invalid argument.

[simonster]

An empty file is not valid JSON (see e.g. https://bugs.php.net/bug.php?id=68938), but the error could obviously be improved, and we should also throw an error for an empty string.

[IainNZ]

It was bad design/laziness on my part that led to even triggering this, but yeah, probably not the right error.

[IainNZ]

The error message is now

julia> JSON.parsefile("empty.json")
ERROR: ArgumentError: requested size must be > 0, got 0
 in mmap at mmap.jl:102
 in mmap at mmap.jl:153
 in anonymous at /Users/idunning/.julia/v0.4/JSON/src/JSON.jl:285
 in open at ./iostream.jl:114
 in parsefile at /Users/idunning/.julia/v0.4/JSON/src/JSON.jl:284

Which is easier to guess about, at least

[quinnj]

The way we handle this in Base is open(io->mmap(io, T, dims, offset; grow=grow, shared=shared), file, isfile(file) ? "r+" : "w+")::Array{eltype(T),N}. Basically, if it's already a file, we read it, if it's not a file, we open a new one to write. Not sure if that same logic works here, but thought I'd mention it.

[TotalVerb]

With JuliaLang/julia@a6f4615 and #140, the error message would become:

julia> JSON.parsefile("empty.json")
ERROR: Unexpected end of input
Line: 0
Around: ......
           ^

 in _error(::String, ::JSON.Parser.MemoryParserState) at /home/fengyang/.julia/v0.5/JSON/src/Parser/Parser.jl:135
 in byteat at /home/fengyang/.julia/v0.5/JSON/src/Parser/Parser.jl:49 [inlined]
 in parse_value(::JSON.Parser.MemoryParserState, ::Type{T}) at /home/fengyang/.julia/v0.5/JSON/src/Parser/Parser.jl:155
 in #parse#1(::Type{Dict{String,Any}}, ::Function, ::String) at /home/fengyang/.julia/v0.5/JSON/src/Parser/Parser.jl:383
 in (::JSON.Parser.#kw##parse)(::Array{Any,1}, ::JSON.Parser.#parse, ::String) at ./null:0
 in (::JSON.##6#7{DataType,Bool,Int64})(::IOStream) at /home/fengyang/.julia/v0.5/JSON/src/JSON.jl:223
 in open(::JSON.##6#7{DataType,Bool,Int64}, ::String) at ./iostream.jl:113
 in parsefile(::String) at /home/fengyang/.julia/v0.5/JSON/src/JSON.jl:220
 in eval(::Module, ::Any) at ./boot.jl:225
 in macro expansion at ./REPL.jl:92 [inlined]
 in (::Base.REPL.##1#2{Base.REPL.REPLBackend})() at ./event.jl:46

which I think is quite reasonable.