Assert return type of `eof`

[galenlynch]

Currently, the Julia compiler cannot infer the return type of eof. I am not 100% sure why it exhibits type instability, but we can at least assert that the return type will be a Bool, and limit the spread of that type instability.

This type assertion will hopefully be removed at some point when the code is made type stable.

[codecov[bot]]

Codecov Report

Merging #259 into master will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #259   +/-   ##
=======================================
  Coverage   76.83%   76.83%           
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  Files          14       14           
  Lines         600      600           
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  Hits          461      461           
  Misses        139      139           

Impacted Files	Coverage Δ
src/avio.jl	79.15% <100.00%> (ø)

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[IanButterworth]

Thanks again. Very much appreciate your careful review of all of this 🙏🏻

[galenlynch]

Thanks for the wonderful package, the only reason I'm finding so many of these things is because I've been using your package so much!

[timholy]

To explain: on Julia 1.5,

julia> using MethodAnalysis
[ Info: Precompiling MethodAnalysis [85b6ec6f-f7df-4429-9514-a64bcd9ee824]

julia> for m in methods(eof)
           _, argtypes = call_type(m.sig)
           println(m, " returns ", Base.return_types(eof, argtypes))
       end
eof(pipe::Base64.Base64DecodePipe) in Base64 at /home/tim/src/julia-1/usr/share/julia/stdlib/v1.5/Base64/src/decode.jl:126 returns Any[Any]
eof(io::Base.SecretBuffer) in Base at secretbuffer.jl:155 returns Any[Bool]
eof(f::Base.Filesystem.File) in Base.Filesystem at filesystem.jl:200 returns Any[Bool]
eof(s::Base.BufferStream) in Base at stream.jl:1287 returns Any[Bool]
eof(s::IOStream) in Base at iostream.jl:223 returns Any[Bool]
eof(::Base.DevNull) in Base at coreio.jl:22 returns Any[Bool]
eof(io::Base.AbstractPipe) in Base at io.jl:390 returns Any[Any]
eof(io::Base.GenericIOBuffer) in Base at iobuffer.jl:335 returns Any[Bool]
eof(s::Base.LibuvStream) in Base at stream.jl:46 returns Any[Bool, Union{Missing, Bool, Base.var"#64#65"{_A} where _A}]
eof(p::Pkg.TOML.Parser) in Pkg.TOML at /home/tim/src/julia-1/usr/share/julia/stdlib/v1.5/Pkg/ext/TOML/src/parser.jl:54 returns Any[Any]

so whether its inferrable depends on how specific the I type is in AVInput. Note also:

julia> subtypes(IO)
12-element Array{Any,1}:
 Base.AbstractPipe
 Base.DevNull
 Base.Filesystem.AbstractFile
 Base.GenericIOBuffer
 Base.LibuvStream
 Base.SecretBuffer
 Base64.Base64DecodePipe
 Base64.Base64EncodePipe
 Core.CoreSTDERR
 Core.CoreSTDOUT
 IOStream
 Mmap.Anonymous

julia> for T in subtypes(IO)
           println("eof(::$T) returns ", Base.return_types(eof, Tuple{T}))
       end
eof(::Base.AbstractPipe) returns Any[Any]
eof(::Base.DevNull) returns Any[Bool]
eof(::Base.Filesystem.AbstractFile) returns Any[Bool]
eof(::Base.GenericIOBuffer) returns Any[Bool]
eof(::Base.LibuvStream) returns Any[Bool, Union{Missing, Bool, Base.var"#64#65"{_A} where _A}]
eof(::Base.SecretBuffer) returns Any[Bool]
eof(::Base64.Base64DecodePipe) returns Any[Any]
eof(::Base64.Base64EncodePipe) returns Any[]
eof(::Core.CoreSTDERR) returns Any[]
eof(::Core.CoreSTDOUT) returns Any[]
eof(::IOStream) returns Any[Bool]
eof(::Mmap.Anonymous) returns Any[]

julia> Base.return_types(eof, Tuple{IO})
9-element Array{Any,1}:
 Any
 Bool
 Bool
 Bool
 Bool
 Bool
 Any
 Bool
 Union{Missing, Bool, Base.var"#64#65"{_A} where _A}

Julia 1.6 has been vastly improved as part of the invalidation work:

julia> Base.return_types(eof, Tuple{IO})
9-element Vector{Any}:
 Bool
 Bool
 Bool
 Bool
 Bool
 Bool
 Bool
 Bool
 Bool

Nevertheless if I is ever abstract, you still might need this simply because the number of applicable methods is higher than 3, the limit for which Julia will infer all methods and see if they all return the same type.

Consequently, when testing things like this it would be good to specify what specifc AVInput type inference was failing on. For many options it shouldn't have failed.

[galenlynch]

Very interesting, thanks for the explanation @timholy, as well as for your many packages that I use daily. I didn't know about MethodAnalysis, I will have to check it out.

Do you think we should not use the type assertion, then? I assumed that in practice VideoIO objects would wrap an IO type that would have a Bool return type, though perhaps that is not a justified assumption.

[timholy]

I think the type assertion is basically harmless, and very helpful for when inference fails. I'd recommend keeping it.

I was basically saying that when you make a fix like this it might be useful to report the specific circumstances in which you found it necessary, as there may be something deeper going on that would suggest an alternative fix.