Port Base.current_exceptions() to older Julia versions (#746)

* Port Base.current_exceptions() to older Julia versions

See JuliaLang/julia#29901

This is limited to julia-1.1 and above because earlier versions don't
have the necessary runtime library support (Base.catch_stack() etc).

* Also implement current_exceptions on julia-1.0

* Tweak readme to mention Julia-1.0 limitations

Project.toml

@@ -1,6 +1,6 @@
 name = "Compat"
 uuid = "34da2185-b29b-5c13-b0c7-acf172513d20"
-version = "3.33.0"
+version = "3.34.0"
 
 [deps]
 Base64 = "2a0f44e3-6c83-55bd-87e4-b1978d98bd5f"

README.md

@@ -54,6 +54,11 @@ changes in `julia`.
 
 ## Supported features
 
+* The function `current_exceptions()` has been added to get the current
+  exception stack. Julia-1.0 lacks runtime support for full execption stacks,
+  so we return only the most recent exception in that case. ([#29901]) (since
+  Compat 3.34)
+
 * The methods `Base.include(::Function, ::Module, path)` and
   `Base.include_string(::Function, ::Module, code[, filename])` have been added. ([#34595]) (since Compat 3.33)
 

src/Compat.jl

@@ -1049,6 +1049,59 @@ if VERSION < v"1.5.0-DEV.263"
     end
 end
 
+# https://github.com/JuliaLang/julia/pull/29901
+if VERSION < v"1.7.0-DEV.1106"
+    struct ExceptionStack <: AbstractArray{Any,1}
+        stack
+    end
+
+    if VERSION >= v"1.1"
+        function current_exceptions(task=current_task(); backtrace=true)
+            stack = Base.catch_stack(task, include_bt=backtrace)
+            ExceptionStack(Any[(exception=x[1],backtrace=x[2]) for x in stack])
+        end
+    else
+        # There's no exception stack in 1.0, but we can fall back to returning
+        # the (single) current exception and backtrace instead.
+        @eval function current_exceptions(task=current_task(); backtrace=true)
+            bt = catch_backtrace()
+            # `exc = Expr(:the_exception)` is the lowering for `catch exc`
+            exc = isempty(bt) ? nothing : $(Expr(:the_exception))
+            ExceptionStack(isempty(bt) ? Any[] : Any[(exception=exc, backtrace=bt)])
+        end
+        @eval function the_stack()
+           $(Expr(:the_exception)), catch_backtrace()
+        end
+    end
+
+    Base.size(s::ExceptionStack) = size(s.stack)
+    Base.getindex(s::ExceptionStack, i::Int) = s.stack[i]
+
+    function show_exception_stack(io::IO, stack)
+        # Display exception stack with the top of the stack first.  This ordering
+        # means that the user doesn't have to scroll up in the REPL to discover the
+        # root cause.
+        nexc = length(stack)
+        for i = nexc:-1:1
+            if nexc != i
+                printstyled(io, "\ncaused by: ", color=Base.error_color())
+            end
+            exc, bt = stack[i]
+            showerror(io, exc, bt, backtrace = bt!==nothing)
+            i == 1 || println(io)
+        end
+    end
+
+    function Base.show(io::IO, ::MIME"text/plain", stack::ExceptionStack)
+        nexc = length(stack)
+        printstyled(io, nexc, "-element ExceptionStack", nexc == 0 ? "" : ":\n")
+        show_exception_stack(io, stack)
+    end
+    Base.show(io::IO, stack::ExceptionStack) = show(io, MIME("text/plain"), stack)
+
+    export current_exceptions
+end
+
 include("iterators.jl")
 include("deprecated.jl")
 

test/runtests.jl

@@ -1109,3 +1109,38 @@ end
     @test m.y === 3
     @test !isdefined(m, :x)
 end
+
+# https://github.com/JuliaLang/julia/pull/29901
+@testset "current_exceptions" begin
+    # Display of errors which cause more than one entry on the exception stack
+    excs = try
+        try
+            __not_a_binding__
+        catch
+            1 ÷ 0  # Generate error while handling error
+        end
+    catch
+        current_exceptions()
+    end
+
+    if VERSION >= v"1.1"
+        @test typeof.(first.(excs)) == [UndefVarError, DivideError]
+
+        @test occursin(r"""
+        2-element ExceptionStack:
+        DivideError: integer division error
+        Stacktrace:.*
+
+        caused by: UndefVarError: __not_a_binding__ not defined
+        Stacktrace:.*
+        """s, sprint(show, excs))
+    else
+        # Due to runtime limitations, julia-1.0 only retains the last exception
+        @test typeof.(first.(excs)) == [DivideError]
+        @test occursin(r"""
+        1-element ExceptionStack:
+        DivideError: integer division error
+        Stacktrace:.*
+        """, sprint(show, excs))
+    end
+end