fix #40337, no error info from serialized TaskFailedException (#40395)

Author: JeffBezanson

base/task.jl

@@ -79,7 +79,12 @@ end
 function show_task_exception(io::IO, t::Task; indent = true)
     stack = catch_stack(t)
     b = IOBuffer()
-    show_exception_stack(IOContext(b, io), stack)
+    if isempty(stack)
+        # exception stack buffer not available; probably a serialized task
+        showerror(IOContext(b, io), t.result)
+    else
+        show_exception_stack(IOContext(b, io), stack)
+    end
     str = String(take!(b))
     if indent
         str = replace(str, "\n" => "\n    ")

stdlib/Distributed/src/clusterserialize.jl

@@ -102,19 +102,6 @@ function serialize(s::ClusterSerializer, t::Core.TypeName)
     nothing
 end
 
-function serialize(s::ClusterSerializer, t::Task)
-    serialize_cycle(s, t) && return
-    if istaskstarted(t) && !istaskdone(t)
-        error("cannot serialize a running Task")
-    end
-    writetag(s.io, TASK_TAG)
-    serialize(s, t.code)
-    serialize(s, t.storage)
-    serialize(s, t._state)
-    serialize(s, t.result)
-    serialize(s, t._isexception)
-end
-
 function serialize(s::ClusterSerializer, g::GlobalRef)
     # Record if required and then invoke the default GlobalRef serializer.
     sym = g.name
@@ -244,17 +231,6 @@ function deserialize(s::ClusterSerializer, t::Type{<:CapturedException})
     return CapturedException(capex, bt)
 end
 
-function deserialize(s::ClusterSerializer, ::Type{Task})
-    t = Task(nothing)
-    deserialize_cycle(s, t)
-    t.code = deserialize(s)
-    t.storage = deserialize(s)
-    t._state = deserialize(s)::UInt8
-    t.result = deserialize(s)
-    t._isexception = deserialize(s)
-    t
-end
-
 """
     clear!(syms, pids=workers(); mod=Main)
 

stdlib/Distributed/test/distributed_exec.jl

@@ -1675,7 +1675,7 @@ let e = @test_throws RemoteException pmap(1) do _
     es = sprint(showerror, e.value)
     @test contains(es, ":\nTaskFailedException\nStacktrace:\n")
     @test contains(es, "\n\n    nested task error:")
-    @test_broken contains(es, "\n\n    nested task error: 42\n")
+    @test contains(es, "\n\n    nested task error: 42\n")
 end
 
 # issue #27429, propagate relative `include` path to workers

stdlib/Serialization/src/Serialization.jl

@@ -458,11 +458,19 @@ function serialize(s::AbstractSerializer, t::Task)
     if istaskstarted(t) && !istaskdone(t)
         error("cannot serialize a running Task")
     end
-    state = [t.code, t.storage, t.state, t.result, t._isexception]
     writetag(s.io, TASK_TAG)
-    for fld in state
-        serialize(s, fld)
+    serialize(s, t.code)
+    serialize(s, t.storage)
+    serialize(s, t.state)
+    if t._isexception && (stk = Base.catch_stack(t); !isempty(stk))
+        # the exception stack field is hidden inside the task, so if there
+        # is any information there make a CapturedException from it instead.
+        # TODO: Handle full exception chain, not just the first one.
+        serialize(s, CapturedException(stk[1][1], stk[1][2]))
+    else
+        serialize(s, t.result)
     end
+    serialize(s, t._isexception)
 end
 
 function serialize(s::AbstractSerializer, g::GlobalRef)