Make finalizer warning for shredded SecretBuffer async (#33698)

This avoids IO in the finalizer where it's invalid to task switch.

Author: c42f

base/secretbuffer.jl

@@ -166,7 +166,7 @@ function read(io::SecretBuffer, ::Type{UInt8})
 end
 
 function final_shred!(s::SecretBuffer)
-    !isshredded(s) && @warn("a SecretBuffer was `shred!`ed by the GC; use `shred!` manually after use to minimize exposure.")
+    !isshredded(s) && @async @warn("a SecretBuffer was `shred!`ed by the GC; use `shred!` manually after use to minimize exposure.")
     shred!(s)
 end
 

test/secretbuffer.jl

@@ -28,7 +28,15 @@ using Test
         @test !isshredded(secret_b)
 
         # TODO: ideally we'd test that the finalizer warns from GC.gc(), but that is harder
-        @test_logs (:warn, r".*SecretBuffer was `shred!`ed by the GC.*") finalize(secret_b)
+        @test_logs (:warn, r".*SecretBuffer was `shred!`ed by the GC.*") begin
+            finalize(secret_b)
+            # Allow the async task which produces the SecretBuffer warning to run.
+            # This is a hack, but we don't have a way to get a handle to that
+            # task in order to `wait` on it.
+            for i=1:1000
+                yield()
+            end
+        end
         @test isshredded(secret_b)
         secret_b = nothing
         GC.gc()