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

@@ -355,6 +355,17 @@ 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/stream.jl

@@ -941,6 +941,7 @@ function readbytes!(s::LibuvStream, a::Vector{UInt8}, nb::Int)
     if bytesavailable(sbuf) >= nb
         nread = readbytes!(sbuf, a, nb)
     else
+        initsize = length(a)
         newbuf = PipeBuffer(a, maxsize=nb)
         newbuf.size = newbuf.offset # reset the write pointer to the beginning
         nread = try
@@ -951,7 +952,8 @@ function readbytes!(s::LibuvStream, a::Vector{UInt8}, nb::Int)
         finally
             s.buffer = sbuf
         end
-        compact(newbuf)
+        _take!(a, _unsafe_take!(newbuf))
+        length(a) >= initsize || resize!(a, initsize)
     end
     iolock_end()
     return nread

test/read.jl

@@ -268,13 +268,27 @@ for (name, f) in l
             n2 = readbytes!(s2, a2)
             @test n1 == n2
             @test length(a1) == length(a2)
-            @test a1[1:n1] == a2[1:n2]
+            let l = min(l, n)
+                @test a1[1:l] == a2[1:l]
+            end
             @test n <= length(text) || eof(s1)
             @test n <= length(text) || eof(s2)
 
             cleanup()
         end
 
+        # Test growing output array
+        let x = UInt8[],
+            io = io()
+            n = readbytes!(io, x)
+            @test n == 0
+            @test isempty(x)
+            n = readbytes!(io, x, typemax(Int))
+            @test n == length(x)
+            @test x == codeunits(text)
+            cleanup()
+        end
+
         verbose && println("$name read!...")
         l = length(text)
         for n = [1, 2, l-2, l-1, l]
@@ -477,12 +491,6 @@ let s = "qwerty"
     @test read(IOBuffer(s)) == codeunits(s)
     @test read(IOBuffer(s), 10) == codeunits(s)
     @test read(IOBuffer(s), 1) == codeunits(s)[1:1]
-
-    # Test growing output array
-    x = UInt8[]
-    n = readbytes!(IOBuffer(s), x, 10)
-    @test x == codeunits(s)
-    @test n == length(x)
 end