Make String(::Memory) copy (#54457)

A more targeted fix of #54369 than #54372

Preserves the performance improvements added in #53962 by creating a new
internal `_unsafe_takestring!(v::Memory{UInt8})` function that does what
`String(::Memory{UInt8})` used to do.

base/gmp.jl

@@ -11,7 +11,7 @@ import .Base: *, +, -, /, <, <<, >>, >>>, <=, ==, >, >=, ^, (~), (&), (|), xor,
              bin, oct, dec, hex, isequal, invmod, _prevpow2, _nextpow2, ndigits0zpb,
              widen, signed, unsafe_trunc, trunc, iszero, isone, big, flipsign, signbit,
              sign, hastypemax, isodd, iseven, digits!, hash, hash_integer, top_set_bit,
-             clamp
+             clamp, unsafe_takestring
 
 if Clong == Int32
     const ClongMax = Union{Int8, Int16, Int32}
@@ -761,7 +761,7 @@ function string(n::BigInt; base::Integer = 10, pad::Integer = 1)
         sv[i] = '0' % UInt8
     end
     isneg(n) && (sv[1] = '-' % UInt8)
-    String(sv)
+    unsafe_takestring(sv)
 end
 
 function digits!(a::AbstractVector{T}, n::BigInt; base::Integer = 10) where {T<:Integer}

base/intfuncs.jl

@@ -792,7 +792,7 @@ function bin(x::Unsigned, pad::Int, neg::Bool)
         i -= 1
     end
     neg && (@inbounds a[1] = 0x2d) # UInt8('-')
-    String(a)
+    unsafe_takestring(a)
 end
 
 function oct(x::Unsigned, pad::Int, neg::Bool)
@@ -806,7 +806,7 @@ function oct(x::Unsigned, pad::Int, neg::Bool)
         i -= 1
     end
     neg && (@inbounds a[1] = 0x2d) # UInt8('-')
-    String(a)
+    unsafe_takestring(a)
 end
 
 # 2-digit decimal characters ("00":"99")
@@ -876,7 +876,7 @@ function dec(x::Unsigned, pad::Int, neg::Bool)
     a = StringMemory(n)
     append_c_digits_fast(n, x, a, 1)
     neg && (@inbounds a[1] = 0x2d) # UInt8('-')
-    String(a)
+    unsafe_takestring(a)
 end
 
 function hex(x::Unsigned, pad::Int, neg::Bool)
@@ -897,7 +897,7 @@ function hex(x::Unsigned, pad::Int, neg::Bool)
         @inbounds a[i] = d + ifelse(d > 0x9, 0x57, 0x30)
     end
     neg && (@inbounds a[1] = 0x2d) # UInt8('-')
-    String(a)
+    unsafe_takestring(a)
 end
 
 const base36digits = UInt8['0':'9';'a':'z']
@@ -922,7 +922,7 @@ function _base(base::Integer, x::Integer, pad::Int, neg::Bool)
         i -= 1
     end
     neg && (@inbounds a[1] = 0x2d) # UInt8('-')
-    String(a)
+    unsafe_takestring(a)
 end
 
 split_sign(n::Integer) = unsigned(abs(n)), n < 0
@@ -998,7 +998,7 @@ function bitstring(x::T) where {T}
         x = lshr_int(x, 4)
         i -= 4
     end
-    return String(str)
+    return unsafe_takestring(str)
 end
 
 """

base/strings/string.jl

@@ -61,12 +61,7 @@ by [`take!`](@ref) on a writable [`IOBuffer`](@ref) and by calls to
 In other cases, `Vector{UInt8}` data may be copied, but `v` is truncated anyway
 to guarantee consistent behavior.
 """
-String(v::AbstractVector{UInt8}) = String(copyto!(StringMemory(length(v)), v))
-function String(v::Memory{UInt8})
-    len = length(v)
-    len == 0 && return ""
-    return ccall(:jl_genericmemory_to_string, Ref{String}, (Any, Int), v, len)
-end
+String(v::AbstractVector{UInt8}) = unsafe_takestring(copyto!(StringMemory(length(v)), v))
 function String(v::Vector{UInt8})
     #return ccall(:jl_array_to_string, Ref{String}, (Any,), v)
     len = length(v)
@@ -83,6 +78,12 @@ function String(v::Vector{UInt8})
     return str
 end
 
+"Create a string re-using the memory, if possible.
+Mutating or reading the memory after calling this function is undefined behaviour."
+function unsafe_takestring(m::Memory{UInt8})
+    isempty(m) ? "" : ccall(:jl_genericmemory_to_string, Ref{String}, (Any, Int), m, length(m))
+end
+
 """
     unsafe_string(p::Ptr{UInt8}, [length::Integer])
 

base/strings/util.jl

@@ -1217,7 +1217,7 @@ function bytes2hex(itr)
         b[2i - 1] = hex_chars[1 + x >> 4]
         b[2i    ] = hex_chars[1 + x & 0xf]
     end
-    return String(b)
+    return unsafe_takestring(b)
 end
 
 function bytes2hex(io::IO, itr)

base/uuid.jl

@@ -98,7 +98,7 @@ let groupings = [36:-1:25; 23:-1:20; 18:-1:15; 13:-1:10; 8:-1:1]
             u >>= 4
         end
         @inbounds a[24] = a[19] = a[14] = a[9] = '-'
-        return String(a)
+        return unsafe_takestring(a)
     end
 end
 

stdlib/FileWatching/src/pidfile.jl

@@ -304,7 +304,7 @@ function open_exclusive(path::String;
 end
 
 function _rand_filename(len::Int=4) # modified from Base.Libc
-    slug = Base.StringMemory(len)
+    slug = Base.StringVector(len)
     chars = b"0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"
     for i = 1:len
         slug[i] = chars[(Libc.rand() % length(chars)) + 1]

test/strings/basic.jl

@@ -1093,6 +1093,17 @@ let v = [0x40,0x41,0x42]
     @test String(view(v, 2:3)) == "AB"
 end
 
+# issue #54369
+let v = Base.StringMemory(3)
+    v .= [0x41,0x42,0x43]
+    s = String(v)
+    @test s == "ABC"
+    @test v == [0x41,0x42,0x43]
+    v[1] = 0x43
+    @test s == "ABC"
+    @test v == [0x43,0x42,0x43]
+end
+
 # make sure length for identical String and AbstractString return the same value, PR #25533
 let rng = MersenneTwister(1), strs = ["∀εa∀aε"*String(rand(rng, UInt8, 100))*"∀εa∀aε",
                                    String(rand(rng, UInt8, 200))]