add TaskFailedException to propagate backtrace of failed task in `w…

…ait`

NEWS.md

@@ -37,6 +37,10 @@ New library functions
 Standard library changes
 ------------------------
 
+* When `wait` (or `@sync`, or `fetch`) is called on a failing `Task`, the exception is propagated as a
+  `TaskFailedException` wrapping the task.
+  This makes it possible to see the location of the original failure inside the task (as well as the
+  location of the `wait` call, as before) ([#32814]).
 * `Regex` can now be multiplied (`*`) and exponentiated (`^`), like strings ([#23422]).
 * `Cmd` interpolation (``` `$(x::Cmd) a b c` ``` where) now propagates `x`'s process flags
   (environment, flags, working directory, etc) if `x` is the first interpolant and errors

base/condition.jl

@@ -88,8 +88,8 @@ Block the current task until some event occurs, depending on the type of the arg
 * [`Condition`](@ref): Wait for [`notify`](@ref) on a condition.
 * `Process`: Wait for a process or process chain to exit. The `exitcode` field of a process
   can be used to determine success or failure.
-* [`Task`](@ref): Wait for a `Task` to finish. If the task fails with an exception, the
-  exception is propagated (re-thrown in the task that called `wait`).
+* [`Task`](@ref): Wait for a `Task` to finish. If the task fails with an exception, a
+  `TaskFailedException` (which wraps the failed task) is thrown.
 * [`RawFD`](@ref): Wait for changes on a file descriptor (see the `FileWatching` package).
 
 If no argument is passed, the task blocks for an undefined period. A task can only be

base/exports.jl

@@ -125,6 +125,7 @@ export
     KeyError,
     MissingException,
     ProcessFailedException,
+    TaskFailedException,
     SystemError,
     StringIndexError,
 

base/task.jl

@@ -56,6 +56,31 @@ function showerror(io::IO, ex::CompositeException)
     end
 end
 
+"""
+    TaskFailedException
+
+This exception is thrown by a `wait(t)` call when task `t` fails.
+`TaskFailedException` wraps the failed task `t`.
+"""
+struct TaskFailedException <: Exception
+    task::Task
+end
+
+function showerror(io::IO, ex::TaskFailedException)
+    stacks = []
+    while isa(ex.task.exception, TaskFailedException)
+        pushfirst!(stacks, ex.task.backtrace)
+        ex = ex.task.exception
+    end
+    println(io, "TaskFailedException:")
+    showerror(io, ex.task.exception, ex.task.backtrace)
+    if !isempty(stacks)
+        for bt in stacks
+            show_backtrace(io, bt)
+        end
+    end
+end
+
 function show(io::IO, t::Task)
     print(io, "Task ($(t.state)) @0x$(string(convert(UInt, pointer_from_objref(t)), base = 16, pad = Sys.WORD_SIZE>>2))")
 end
@@ -204,9 +229,8 @@ function task_local_storage(body::Function, key, val)
     end
 end
 
-# NOTE: you can only wait for scheduled tasks
-function wait(t::Task)
-    t === current_task() && error("deadlock detected: cannot wait on current task")
+# just wait for a task to be done, no error propagation
+function _wait(t::Task)
     if !istaskdone(t)
         lock(t.donenotify)
         try
@@ -217,8 +241,14 @@ function wait(t::Task)
             unlock(t.donenotify)
         end
     end
+    nothing
+end
+
+function wait(t::Task)
+    t === current_task() && error("deadlock detected: cannot wait on current task")
+    _wait(t)
     if istaskfailed(t)
-        throw(t.exception)
+        throw(TaskFailedException(t))
     end
     nothing
 end
@@ -228,8 +258,9 @@ fetch(@nospecialize x) = x
 """
     fetch(t::Task)
 
-Wait for a Task to finish, then return its result value. If the task fails with an
-exception, the exception is propagated (re-thrown in the task that called fetch).
+Wait for a Task to finish, then return its result value.
+If the task fails with an exception, a `TaskFailedException` (which wraps the failed task)
+is thrown.
 """
 function fetch(t::Task)
     wait(t)
@@ -240,22 +271,32 @@ end
 ## lexically-scoped waiting for multiple items
 
 function sync_end(refs)
-    c_ex = CompositeException()
+    local c_ex
+    defined = false
     for r in refs
-        try
-            wait(r)
-        catch
-            if !isa(r, Task) || (isa(r, Task) && !istaskfailed(r))
-                rethrow()
+        if isa(r, Task)
+            _wait(r)
+            if istaskfailed(r)
+                if !defined
+                    defined = true
+                    c_ex = CompositeException()
+                end
+                push!(c_ex, TaskFailedException(r))
             end
-        finally
-            if isa(r, Task) && istaskfailed(r)
-                push!(c_ex, CapturedException(task_result(r), r.backtrace))
+        else
+            try
+                wait(r)
+            catch e
+                if !defined
+                    defined = true
+                    c_ex = CompositeException()
+                end
+                push!(c_ex, e)
             end
         end
     end
 
-    if !isempty(c_ex)
+    if defined
         throw(c_ex)
     end
     nothing
@@ -301,6 +342,15 @@ macro async(expr)
     end
 end
 
+# add a wait-able object to the sync pool
+macro sync_add(expr)
+    var = esc(sync_varname)
+    quote
+        local ref = $(esc(expr))
+        push!($var, ref)
+        ref
+    end
+end
 
 function register_taskdone_hook(t::Task, hook)
     tls = get_task_tls(t)
@@ -324,7 +374,7 @@ function task_done_hook(t::Task)
         try
             if !isempty(donenotify.waitq)
                 handled = true
-                notify(donenotify, result, true, err)
+                notify(donenotify)
             end
         finally
             unlock(donenotify)

stdlib/Distributed/src/Distributed.jl

@@ -10,7 +10,7 @@ import Base: getindex, wait, put!, take!, fetch, isready, push!, length,
              hash, ==, kill, close, isopen, showerror
 
 # imports for use
-using Base: Process, Semaphore, JLOptions, AnyDict, buffer_writes,
+using Base: Process, Semaphore, JLOptions, AnyDict, buffer_writes, @sync_add,
             VERSION_STRING, binding_module, atexit, julia_exename,
             julia_cmd, AsyncGenerator, acquire, release, invokelatest,
             shell_escape_posixly, uv_error, something, notnothing, isbuffered
@@ -74,7 +74,7 @@ function _require_callback(mod::Base.PkgId)
         # broadcast top-level (e.g. from Main) import/using from node 1 (only)
         @sync for p in procs()
             p == 1 && continue
-            @async remotecall_wait(p) do
+            @sync_add remotecall(p) do
                 Base.require(mod)
                 nothing
             end

stdlib/Distributed/src/clusterserialize.jl

@@ -243,7 +243,7 @@ An exception is raised if a global constant is requested to be cleared.
 """
 function clear!(syms, pids=workers(); mod=Main)
     @sync for p in pids
-        @async remotecall_wait(clear_impl!, p, syms, mod)
+        @sync_add remotecall(clear_impl!, p, syms, mod)
     end
 end
 clear!(sym::Symbol, pid::Int; mod=Main) = clear!([sym], [pid]; mod=mod)

stdlib/Distributed/src/macros.jl

@@ -220,7 +220,7 @@ function remotecall_eval(m::Module, procs, ex)
             if pid == myid()
                 run_locally += 1
             else
-                @async remotecall_wait(Core.eval, pid, m, ex)
+                @sync_add remotecall(Core.eval, pid, m, ex)
             end
         end
         yield() # ensure that the remotecall_fetch have had a chance to start

stdlib/Distributed/test/distributed_exec.jl

@@ -420,11 +420,11 @@ try
 catch ex
     @test typeof(ex) == CompositeException
     @test length(ex) == 5
-    @test typeof(ex.exceptions[1]) == CapturedException
-    @test typeof(ex.exceptions[1].ex) == ErrorException
+    @test typeof(ex.exceptions[1]) == TaskFailedException
+    @test typeof(ex.exceptions[1].task.exception) == ErrorException
     # test start, next, and done
     for (i, i_ex) in enumerate(ex)
-        @test i == parse(Int, i_ex.ex.msg)
+        @test i == parse(Int, i_ex.task.exception.msg)
     end
     # test showerror
     err_str = sprint(showerror, ex)
@@ -738,7 +738,7 @@ end # full-test
 
 let t = @task 42
     schedule(t, ErrorException(""), error=true)
-    @test_throws ErrorException Base.wait(t)
+    @test_throws TaskFailedException(t) Base.wait(t)
 end
 
 # issue #8207
@@ -964,13 +964,15 @@ let (p, p2) = filter!(p -> p != myid(), procs())
             if procs isa Int
                 ex = Any[excpt]
             else
-                ex = Any[ (ex::CapturedException).ex for ex in (excpt::CompositeException).exceptions ]
+                ex = (excpt::CompositeException).exceptions
             end
             for (p, ex) in zip(procs, ex)
                 local p
                 if procs isa Int || p != myid()
                     @test (ex::RemoteException).pid == p
                     ex = ((ex::RemoteException).captured::CapturedException).ex
+                else
+                    ex = (ex::TaskFailedException).task.exception
                 end
                 @test (ex::ErrorException).msg == msg
             end
@@ -1165,7 +1167,7 @@ for (addp_testf, expected_errstr, env) in testruns
         close(stdout_in)
         @test isempty(fetch(stdout_txt))
         @test isa(ex, CompositeException)
-        @test ex.exceptions[1].ex.msg == expected_errstr
+        @test ex.exceptions[1].task.exception.msg == expected_errstr
     end
 end
 

stdlib/Serialization/test/runtests.jl

@@ -361,12 +361,12 @@ end
 struct MyErrorTypeTest <: Exception end
 create_serialization_stream() do s # user-defined type array
     t = Task(()->throw(MyErrorTypeTest()))
-    @test_throws MyErrorTypeTest Base.wait(schedule(t))
+    @test_throws TaskFailedException(t) Base.wait(schedule(t))
+    @test isa(t.exception, MyErrorTypeTest)
     serialize(s, t)
     seek(s, 0)
     r = deserialize(s)
     @test r.state == :failed
-    @test isa(t.exception, MyErrorTypeTest)
 end
 
 # corner case: undefined inside immutable struct

test/channels.jl

@@ -323,12 +323,12 @@ end
     ct = current_task()
     testerr = ErrorException("expected")
     @async Base.throwto(t, testerr)
-    @test try
+    @test (try
         Base.wait(t)
         false
     catch ex
         ex
-    end === testerr
+    end).task.exception === testerr
 end
 
 @testset "Timer / AsyncCondition triggering and race #12719" begin

test/errorshow.jl

@@ -562,3 +562,19 @@ let buf = IOBuffer()
     Base.show_method_candidates(buf, Base.MethodError(sin, Tuple{NoMethodsDefinedHere}))
     @test length(take!(buf)) !== 0
 end
+
+# pr #32814
+let t1 = @async(error(1)),
+    t2 = @async(wait(t1))
+    local e
+    try
+        wait(t2)
+    catch e_
+        e = e_
+    end
+    buf = IOBuffer()
+    showerror(buf, e)
+    s = String(take!(buf))
+    @test length(findall("Stacktrace:", s)) == 2
+    @test occursin("[1] error(::Int", s)
+end

test/exceptions.jl

@@ -274,12 +274,12 @@ end
     @test catch_stack(t, include_bt=false) == [ErrorException("A"), ErrorException("B")]
     # Exception stacks for tasks which never get the chance to start
     t = @task nothing
-    @test try
+    @test (try
         @async Base.throwto(t, ErrorException("expected"))
         wait(t)
     catch e
         e
-    end == ErrorException("expected")
+    end).task.exception == ErrorException("expected")
     @test length(catch_stack(t)) == 1
     @test length(catch_stack(t)[1][2]) > 0 # backtrace is nonempty
     # Exception stacks should not be accessed on concurrently running tasks

test/worlds.jl

@@ -139,9 +139,10 @@ h265() = true
 loc_h265 = "$(@__FILE__):$(@__LINE__() - 1)"
 @test h265()
 @test_throws MethodError put_n_take!(h265, ())
-@test_throws MethodError fetch(t265)
+@test_throws TaskFailedException(t265) fetch(t265)
 @test istaskdone(t265)
 let ex = t265.exception
+    @test ex isa MethodError
     @test ex.f == h265
     @test ex.args == ()
     @test ex.world == wc265