Add readavailable(::FileBuffer)

This adds a method for `FileBuffer`s to `readavailable` that precisely
matches the behavior for `IOBuffer`s. This definition permits downstream
niceties such as `CSV.read(::S3Path, ...)`.

Author: ararslan

src/buffer.jl

@@ -35,6 +35,7 @@ end
 Base.isopen(buffer::FileBuffer) = isopen(buffer.io)
 Base.isreadable(buffer::FileBuffer) = buffer.read
 Base.iswritable(buffer::FileBuffer) = buffer.write
+Base.readavailable(buffer::FileBuffer) = read(buffer)
 Base.seek(buffer::FileBuffer, n::Integer) = (_read(buffer); seek(buffer.io, n))
 Base.skip(buffer::FileBuffer, n::Integer) = (_read(buffer); skip(buffer.io, n))
 Base.seekstart(buffer::FileBuffer) = seekstart(buffer.io)

test/buffer.jl

@@ -33,6 +33,8 @@ using FilePathsBase: FileBuffer
             close(io)
         end
 
+        @test readavailable(FileBuffer(p)) == readavailable(IOBuffer(read(p)))
+
         # issue #126: data on first read
         mktemp(SystemPath) do p, _
             write(p, "testing")