Fix Libdl changes (JuliaLang#28953)

* Update `NEWS.md` for `Libdl.{dlopen,dlsym}` changes

* Fix `dlopen()` and `dlsym()` backwards incompatibility

* Update tests

NEWS.md

@@ -11,11 +11,21 @@ Refer to the [Release Notes for
 v0.7](https://github.com/JuliaLang/julia/blob/master/HISTORY.md) for a
 detailed list of changes from Julia v0.6.
 
+Standard Library Changes
+------------------------
+
+* The `Libdl` module's methods `dlopen()` and `dlsym()` have gained a
+ `throw_error` keyword argument, replacing the now-deprecated `dlopen_e()`
+ and `dlsym_e()` methods. When `throw_error` is `false`, failure to locate
+ a shared library or symbol will return `nothing` rather than `C_NULL`.
+ ([#28888])
+
 Deprecated or removed
 ---------------------
 
-The old package manager (now called `OldPkg`) has been moved to a
-separate repository at https://github.com/JuliaArchive/OldPkg.jl ([#27930])
+* The old package manager (now called `OldPkg`) has been moved to a
+ separate repository at https://github.com/JuliaArchive/OldPkg.jl ([#27930])
 
 <!--- generated by NEWS-update.jl: -->
 [#27930]: https://github.com/JuliaLang/julia/issues/27930
+[#28888]: https://github.com/JuliaLang/julia/issues/28888

stdlib/Libdl/src/Libdl.jl

@@ -66,14 +66,15 @@ end
 """
  dlsym_e(handle, sym)
 
-Look up a symbol from a shared library handle, silently return `NULL` pointer on lookup failure. It is preferred to use dlsym(handle, sym; throw_error=false).
+Look up a symbol from a shared library handle, silently return `C_NULL` on lookup failure.
+This method is now deprecated in favor of `dlsym(handle, sym; throw_error=false)`.
 """
 function dlsym_e(hnd::Ptr, s::Union{Symbol,AbstractString})
- return dlsym(hnd, s; throw_error=false)
+ return something(dlsym(hnd, s; throw_error=false), C_NULL)
 end
 
 """
- dlopen(libfile::AbstractString [, flags::Integer])
+ dlopen(libfile::AbstractString [, flags::Integer]; throw_error:Bool = true)
 
 Load a shared library, returning an opaque handle.
 
@@ -96,7 +97,8 @@ instance `RTLD_LAZY|RTLD_DEEPBIND|RTLD_GLOBAL` allows the library's symbols to b
 for usage in other shared libraries, addressing situations where there are dependencies
 between shared libraries.
 
-If the library cannot be found, this method returns `nothing`.
+If the library cannot be found, this method throws an error, unless the keyword argument
+`throw_error` is set to `false`, in which case this method returns `nothing`.
 """
 function dlopen end
 
@@ -114,10 +116,10 @@ end
 """
  dlopen_e(libfile::AbstractString [, flags::Integer])
 
-Similar to [`dlopen`](@ref), except returns a `NULL` pointer instead of raising errors.
-It is preferred to directly call dlopen(libfile, flags; throw_error=false)`
+Similar to [`dlopen`](@ref), except returns `C_NULL` instead of raising errors.
+This method is now deprecated in favor of `dlsym(handle, sym; throw_error=false)`.
 """
-dlopen_e(args...) = dlopen(args...; throw_error=false)
+dlopen_e(args...) = something(dlopen(args...; throw_error=false), C_NULL)
 
 """
  dlclose(handle)

stdlib/Libdl/test/runtests.jl

@@ -178,9 +178,9 @@ let dl = C_NULL
  @test_throws ErrorException Libdl.dlsym(dl, :foo)
 
  fptr = Libdl.dlsym_e(dl, :set_verbose)
- @test fptr !== nothing
+ @test fptr != C_NULL
  fptr = Libdl.dlsym_e(dl, :foo)
- @test fptr === nothing
+ @test fptr == C_NULL
  finally
  Libdl.dlclose(dl)
  end