Add err global to REPL to store most recent errors (#40642)

Author: BioTurboNick

NEWS.md

@@ -111,6 +111,9 @@ Standard library changes
   argument have a type more specific than `Any`; use SHIFT-TAB instead of TAB
   to allow any compatible methods.
 
+* New `err` global variable in `Main` set when an expression throws an exception, akin to `ans`. Typing `err` reprints
+  the exception information.
+
 #### SparseArrays
 
 #### Dates

base/client.jl

@@ -84,27 +84,33 @@ end
 
 function scrub_repl_backtrace(bt)
     if bt !== nothing && !(bt isa Vector{Any}) # ignore our sentinel value types
-        bt = stacktrace(bt)
+        bt = bt isa Vector{StackFrame} ? copy(bt) : stacktrace(bt)
         # remove REPL-related frames from interactive printing
         eval_ind = findlast(frame -> !frame.from_c && frame.func === :eval, bt)
         eval_ind === nothing || deleteat!(bt, eval_ind:length(bt))
     end
     return bt
 end
+scrub_repl_backtrace(stack::ExceptionStack) =
+    ExceptionStack(Any[(;x.exception, backtrace = scrub_repl_backtrace(x.backtrace)) for x in stack])
 
-function display_error(io::IO, er, bt)
+istrivialerror(stack::ExceptionStack) =
+    length(stack) == 1 && length(stack[1].backtrace) ≤ 1
+    # frame 1 = top level; assumes already went through scrub_repl_backtrace
+
+function display_error(io::IO, stack::ExceptionStack)
     printstyled(io, "ERROR: "; bold=true, color=Base.error_color())
-    bt = scrub_repl_backtrace(bt)
-    showerror(IOContext(io, :limit => true), er, bt, backtrace = bt!==nothing)
+    show_exception_stack(IOContext(io, :limit => true), stack)
     println(io)
 end
-function display_error(io::IO, stack::ExceptionStack)
+display_error(stack::ExceptionStack) = display_error(stderr, stack)
+
+# these forms are depended on by packages outside Julia
+function display_error(io::IO, exception, backtrace)
     printstyled(io, "ERROR: "; bold=true, color=Base.error_color())
-    bt = Any[ (x[1], scrub_repl_backtrace(x[2])) for x in stack ]
-    show_exception_stack(IOContext(io, :limit => true), bt)
+    showerror(IOContext(io, :limit => true), er, bt, backtrace = bt!==nothing)
     println(io)
 end
-display_error(stack::ExceptionStack) = display_error(stderr, stack)
 display_error(er, bt=nothing) = display_error(stderr, er, bt)
 
 function eval_user_input(errio, @nospecialize(ast), show_value::Bool)
@@ -117,6 +123,8 @@ function eval_user_input(errio, @nospecialize(ast), show_value::Bool)
                 print(color_normal)
             end
             if lasterr !== nothing
+                lasterr = scrub_repl_backtrace(lasterr)
+                istrivialerror(lasterr) || ccall(:jl_set_global, Cvoid, (Any, Any, Any), Main, :err, lasterr)
                 invokelatest(display_error, errio, lasterr)
                 errcount = 0
                 lasterr = nothing
@@ -143,7 +151,8 @@ function eval_user_input(errio, @nospecialize(ast), show_value::Bool)
                 @error "SYSTEM: display_error(errio, lasterr) caused an error"
             end
             errcount += 1
-            lasterr = current_exceptions()
+            lasterr = scrub_repl_backtrace(current_exceptions())
+            ccall(:jl_set_global, Cvoid, (Any, Any, Any), Main, :err, lasterr)
             if errcount > 2
                 @error "It is likely that something important is broken, and Julia will not be able to continue normally" errcount
                 break
@@ -260,7 +269,7 @@ function exec_options(opts)
         try
             load_julia_startup()
         catch
-            invokelatest(display_error, current_exceptions())
+            invokelatest(display_error, scrub_repl_backtrace(current_exceptions()))
             !(repl || is_interactive::Bool) && exit(1)
         end
     end
@@ -294,7 +303,7 @@ function exec_options(opts)
         try
             include(Main, PROGRAM_FILE)
         catch
-            invokelatest(display_error, current_exceptions())
+            invokelatest(display_error, scrub_repl_backtrace(current_exceptions()))
             if !is_interactive::Bool
                 exit(1)
             end
@@ -496,7 +505,7 @@ function _start()
     try
         exec_options(JLOptions())
     catch
-        invokelatest(display_error, current_exceptions())
+        invokelatest(display_error, scrub_repl_backtrace(current_exceptions()))
         exit(1)
     end
     if is_interactive && get(stdout, :color, false)

base/task.jl

@@ -489,13 +489,13 @@ function errormonitor(t::Task)
             try # try to display the failure atomically
                 errio = IOContext(PipeBuffer(), errs::IO)
                 emphasize(errio, "Unhandled Task ")
-                display_error(errio, current_exceptions(t))
+                display_error(errio, scrub_repl_backtrace(current_exceptions(t)))
                 write(errs, errio)
             catch
                 try # try to display the secondary error atomically
                     errio = IOContext(PipeBuffer(), errs::IO)
                     print(errio, "\nSYSTEM: caught exception while trying to print a failed Task notice: ")
-                    display_error(errio, current_exceptions())
+                    display_error(errio, scrub_repl_backtrace(current_exceptions()))
                     write(errs, errio)
                     flush(errs)
                     # and then the actual error, as best we can

contrib/generate_precompile.jl

@@ -32,9 +32,7 @@ precompile(Tuple{typeof(Base.recursive_prefs_merge), Base.Dict{String, Any}})
 precompile(Tuple{typeof(isassigned), Core.SimpleVector, Int})
 precompile(Tuple{typeof(getindex), Core.SimpleVector, Int})
 precompile(Tuple{typeof(Base.Experimental.register_error_hint), Any, Type})
-precompile(Tuple{typeof(Base.display_error), MethodError, Vector{Union{Ptr{Nothing}, Base.InterpreterIP}}})
-precompile(Tuple{typeof(Base.display_error), ErrorException})
-precompile(Tuple{typeof(Base.display_error), BoundsError})
+precompile(Tuple{typeof(Base.display_error), Base.ExceptionStack})
 precompile(Tuple{Core.kwftype(typeof(Type)), NamedTuple{(:sizehint,), Tuple{Int}}, Type{IOBuffer}})
 precompile(Base.CoreLogging.current_logger_for_env, (Base.CoreLogging.LogLevel, String, Module))
 precompile(Base.CoreLogging.current_logger_for_env, (Base.CoreLogging.LogLevel, Symbol, Module))

stdlib/REPL/src/REPL.jl

@@ -284,6 +284,8 @@ function print_response(errio::IO, response, show_value::Bool, have_color::Bool,
         try
             Base.sigatomic_end()
             if iserr
+                val = Base.scrub_repl_backtrace(val)
+                Base.istrivialerror(val) || ccall(:jl_set_global, Cvoid, (Any, Any, Any), Main, :err, val)
                 Base.invokelatest(Base.display_error, errio, val)
             else
                 if val !== nothing && show_value
@@ -305,7 +307,9 @@ function print_response(errio::IO, response, show_value::Bool, have_color::Bool,
                 println(errio) # an error during printing is likely to leave us mid-line
                 println(errio, "SYSTEM (REPL): showing an error caused an error")
                 try
-                    Base.invokelatest(Base.display_error, errio, current_exceptions())
+                    excs = Base.scrub_repl_backtrace(current_exceptions())
+                    ccall(:jl_set_global, Cvoid, (Any, Any, Any), Main, :err, excs)
+                    Base.invokelatest(Base.display_error, errio, excs)
                 catch e
                     # at this point, only print the name of the type as a Symbol to
                     # minimize the possibility of further errors.

stdlib/REPL/test/repl.jl

@@ -885,13 +885,13 @@ end
 
 # Test containers in error messages are limited #18726
 let io = IOBuffer()
-    Base.display_error(io,
-        try
+    Base.display_error(io, Base.ExceptionStack(Any[(exception =
+        (try
             [][trues(6000)]
             @assert false
         catch e
             e
-        end, [])
+        end), backtrace = [])]))
     @test length(String(take!(io))) < 1500
 end
 
@@ -1366,3 +1366,40 @@ end
         @test isempty(mods)
     end
 end
+
+# err should reprint error if deeper than top-level
+fake_repl() do stdin_write, stdout_read, repl
+    repltask = @async begin
+        REPL.run_repl(repl)
+    end
+    # initialize `err` to `nothing`
+    write(stdin_write, "global err = nothing\n")
+    readline(stdout_read)
+    readline(stdout_read) == "\e[0m"
+    readuntil(stdout_read, "julia> ", keep=true)
+    # generate top-level error
+    write(stdin_write, "foobar\n")
+    readline(stdout_read)
+    @test readline(stdout_read) == "\e[0mERROR: UndefVarError: foobar not defined"
+    @test readline(stdout_read) == ""
+    readuntil(stdout_read, "julia> ", keep=true)
+    # check that top-level error did not change `err`
+    write(stdin_write, "err\n")
+    readline(stdout_read)
+    @test readline(stdout_read) == "\e[0m"
+    readuntil(stdout_read, "julia> ", keep=true)
+    # generate deeper error
+    write(stdin_write, "foo() = foobar\n")
+    readline(stdout_read)
+    readuntil(stdout_read, "julia> ", keep=true)
+    write(stdin_write, "foo()\n")
+    readline(stdout_read)
+    @test readline(stdout_read) == "\e[0mERROR: UndefVarError: foobar not defined"
+    readuntil(stdout_read, "julia> ", keep=true)
+    # check that deeper error did set `err`
+    write(stdin_write, "err\n")
+    readline(stdout_read)
+    @test readline(stdout_read) == "\e[0m1-element ExceptionStack:"
+    @test readline(stdout_read) == "UndefVarError: foobar not defined"
+    @test readline(stdout_read) == "Stacktrace:"
+end