stream: fix reading LibuvStream into array

Adds a new abstraction `take!(::Array{T,N}, ::Array{T,N})` for doing an
efficient `copyto!` equivalent. Previously it was assumed that `compact`
did this automatically, which wasn't a great assumption.

Fixes #56078

base/array.jl

@@ -351,6 +351,17 @@ copy
     return $(Expr(:new, :(typeof(a)), :(memoryref(newmem)), :(a.size)))
 end
 
+# a mutating version of copyto! that results in dst aliasing src afterwards
+function take!(dst::Array{T,N}, src::Array{T,N}) where {T,N}
+    if getfield(dst, :ref) !== getfield(src, :ref)
+        setfield!(dst, :ref, getfield(src, :ref))
+    end
+    if getfield(dst, :size) !== getfield(src, :size)
+        setfield!(dst, :size, getfield(src, :size))
+    end
+    return dst
+end
+
 ## Constructors ##
 
 similar(a::Array{T,1}) where {T}                    = Vector{T}(undef, size(a,1))

base/io.jl

@@ -1541,3 +1541,7 @@ function countlines(io::IO; eol::AbstractChar='\n')
 end
 
 countlines(f::AbstractString; eol::AbstractChar = '\n') = open(io->countlines(io, eol = eol), f)::Int
+
+# double mutating version of take!
+take!(b::Vector{UInt8}, io::IO) = take!(b, take!(io))
+_unsafe_take!(b::Vector{UInt8}, io::IO) = take!(b, _unsafe_take!(io))

base/stream.jl

@@ -944,7 +944,7 @@ function readbytes!(s::LibuvStream, a::Vector{UInt8}, nb::Int)
         finally
             s.buffer = sbuf
         end
-        compact(newbuf)
+        _unsafe_take!(a, newbuf)
     end
     iolock_end()
     return nread