fix runtime issue of IOBuffer backend (#704)

Author: JonasIsensee

src/io/bufferedio.jl

@@ -63,7 +63,8 @@ Base.show(io::IO, ::BufferedReader) = print(io, "BufferedReader")
 
 function readmore!(io::BufferedReader, n::Integer)
     f = io.f
-    amount = max(bytesavailable(f), n) #TODO: check if this reads way to much
+    sz = _getsize(f)
+    amount = min(max(n, 2^14), sz-position(f))
     amount < n && throw(EOFError())
     buffer = io.buffer
     pos = bufferpos(io)

src/io/io_wrappers.jl

@@ -172,17 +172,14 @@ Base.isreadable(::RWBuffer) = true
 Base.iswritable(::RWBuffer) = true
 isseekable(::RWBuffer) = true
 
+_getsize(io::RWBuffer) = io.size
+
 function _getsize(io::IO)
-    try
-        return filesize(io)
-    catch
-        # fallback
-        pos = position(io)
-        seekend(io)
-        sz = position(io)
-        seek(io, pos)
-        return sz
-    end
+    pos = position(io)
+    seekend(io)
+    sz = position(io)
+    seek(io, pos)
+    return sz
 end
 
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