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Audit use of GC.@preserve #391

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Audit use of GC.@preserve #391

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17 changes: 16 additions & 1 deletion src/lazy.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -270,6 +270,7 @@ function applyobject(keyvalfunc, x::LazyValues)
b == UInt8('}') && return pos + 1
while true
# parsestring returns key as a PtrString
# TODO: Key contains an invalid pointer here
key, pos = @inline parsestring(LazyValue(buf, pos, JSONTypes.STRING, opts, false))
@nextbyte
if b != UInt8(':')
Expand Down Expand Up @@ -440,7 +441,18 @@ function Base.convert(::Type{T}, x::PtrString) where {T <: Enum}
throw(ArgumentError("invalid `$T` string value: \"$sym\""))
end

Base.:(==)(x::PtrString, y::AbstractString) = x.len == sizeof(y) && ccall(:memcmp, Cint, (Ptr{UInt8}, Ptr{UInt8}, Csize_t), x.ptr, pointer(y), x.len) == 0
function Base.:(==)(x::PtrString, y::AbstractString)
# If the string is backed by UInt8 codeunits, the `sizeof` computation below is correct
return if codeunit(y) == UInt8
x.len == sizeof(y) || return false
ref = Base.cconvert(Ptr{UInt8}, y)
cmp = GC.@preserve ref ccall(:memcmp, Cint, (Ptr{UInt8}, Ptr{UInt8}, Csize_t), x.ptr, Base.unsafe_convert(Ptr{UInt8}, ref), x.len)
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@yuyichao yuyichao Oct 6, 2025

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The manual preserve here seems to be exactly equivalent to simply passing y to ccall directly.

iszero(cmp)
else
x == String(y)::String
end
end

Base.:(==)(x::PtrString, y::PtrString) = x.len == y.len && ccall(:memcmp, Cint, (Ptr{UInt8}, Ptr{UInt8}, Csize_t), x.ptr, y.ptr, x.len) == 0
Base.isequal(x::PtrString, y::AbstractString) = x == y
Base.isequal(x::PtrString, y::PtrString) = x == y
Expand Down Expand Up @@ -482,6 +494,8 @@ function parsestring(x::LazyValue)
end
@nextbyte(false)
end
# TODO: The pointer escapes here, but callers of this function do not use GC.@preserve.
# Hence, the pointer is invalid by the time it's returned.
str = PtrString(pointer(buf, spos), pos - spos, escaped)
return str, pos + 1

Expand Down Expand Up @@ -732,6 +746,7 @@ function Base.show(io::IO, x::LazyValue)
show(io, MIME"text/plain"(), la)
end
elseif T == JSONTypes.STRING
# TODO: str contains an invalid pointer here
str, _ = parsestring(x)
Base.print(io, "JSON.LazyValue(", repr(convert(String, str)), ")")
elseif T == JSONTypes.NULL
Expand Down
2 changes: 2 additions & 0 deletions src/parse.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -277,6 +277,7 @@ function applyvalue(f, x::LazyValues, null)
f(arr)
return pos
elseif type == JSONTypes.STRING
# TODO: str contains an invalid pointer here
str, pos = parsestring(x)
f(convert(String, str))
return pos
Expand Down Expand Up @@ -344,6 +345,7 @@ end
function StructUtils.lift(style::StructStyle, ::Type{T}, x::LazyValues, tags=(;)) where {T}
type = gettype(x)
if type == JSONTypes.STRING
# TODO: ptrstr contains an invalid pointer here
ptrstr, pos = parsestring(x)
str, _ = StructUtils.lift(style, T, ptrstr, tags)
return str, pos
Expand Down
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