Do a better job of capturing the source of errors (#305)

Fixes #216

src/error_handling.jl

@@ -6,9 +6,10 @@ LoaderError("ImageMagick", "Foo not available")
 struct LoaderError <: Exception
     library::String
     msg::String
+    ex
 end
 Base.showerror(io::IO, e::LoaderError) = println(io, e.library, " load error: ",
-                                                 e.msg, "\n  Will try next loader.")
+                                                 e.msg, "\n  due to ", e.ex, "\n  Will try next loader.")
 
 """
 `WriterError` should be thrown when writer library code fails, and other libraries should
@@ -18,10 +19,11 @@ WriterError("ImageMagick", "Foo not available")
 struct WriterError <: Exception
     library::String
     msg::String
+    ex
 end
 Base.showerror(io::IO, e::WriterError) = println(
     io, e.library, " writer error: ",
-    e.msg, "\n  Will try next writer."
+    e.msg, "\n  due to ", e.ex, "\n  Will try next loader."
 )
 
 
@@ -41,7 +43,7 @@ function handle_exceptions(exceptions::Vector, action)
     if multiple
         println("All errors:")
         println("===========================================")
-        for (err, file) in exceptions
+        for (err, file, bt) in exceptions
             showerror(stdout, err)
             println("\n===========================================")
         end
@@ -56,4 +58,4 @@ function handle_exceptions(exceptions::Vector, action)
     end
 end
 
-handle_error(e, q) = throw(e)
+handle_error(e, q, bt) = throw(CapturedException(e, stacktrace(bt)))

src/loadsave.jl

@@ -190,7 +190,7 @@ const fileiofuncs = Dict{Symbol,Function}(:load => load,
 
 function action(call::Symbol, libraries::Vector{ActionSource}, @nospecialize(file::Formatted), args...; options...)
     issave = call ∈ (:save, :savestreaming)
-    failures = Tuple{Any,ActionSource}[]
+    failures = Tuple{Any,ActionSource,Vector}[]
     pkgfuncname = Symbol("fileio_", call)
     local mod
     for library in libraries
@@ -213,9 +213,9 @@ function action(call::Symbol, libraries::Vector{ActionSource}, @nospecialize(fil
         catch e
             if isa(e, MethodError) || isa(e, SpecError)
                 str = "neither $call nor $pkgfuncname is defined"
-                e = issave ? WriterError(string(mod), str) : LoaderError(string(mod), str)
+                e = issave ? WriterError(string(mod), str, e) : LoaderError(string(mod), str, e)
             end
-            push!(failures, (e, library))
+            push!(failures, (e, library, catch_backtrace()))
         end
     end
     handle_exceptions(failures, "$call $(repr(file))")

test/error_handling.jl

@@ -27,7 +27,7 @@ end
     @test_logs (:warn, r"supply `pkg` as a Module or `name=>uuid`") @test_throws(ArgumentError, add_format(format"NotInstalled", (), ".not_installed", [:NotInstalled]))
     # Give it a fake UUID
     add_format(format"NotInstalled", (), ".not_installed", [:NotInstalled=>UUID("79e393ae-7a7b-11eb-1530-bf4d98024096")])
-    @test_throws ArgumentError save("test.not_installed", nothing)
+    @test_throws CapturedException save("test.not_installed", nothing)
 
     # Core.eval(Base, :(is_interactive = true)) # for interactive error handling
     # add_format(format"NotInstalled", (), ".not_installed", [:NotInstalled])
@@ -62,8 +62,14 @@ add_format(format"BROKEN", (), ".brok", [BrokenIO])
 @testset "Absent implementation" begin
     stderr_copy = stderr
     rserr, wrerr = redirect_stderr()
-    @test_throws FileIO.LoaderError load(Stream{format"BROKEN"}(stdin))
-    @test_throws FileIO.WriterError save(Stream{format"BROKEN"}(stdout), nothing)
+    ex = try load(Stream{format"BROKEN"}(stdin)) catch e e end
+    @test isa(ex, CapturedException)
+    @test isa(ex.ex, FileIO.LoaderError)
+    @test isa(ex.ex.ex, FileIO.SpecError)
+    ex = try save(Stream{format"BROKEN"}(stdout), nothing) catch e e end
+    @test isa(ex, CapturedException)
+    @test isa(ex.ex, FileIO.WriterError)
+    @test isa(ex.ex.ex, FileIO.SpecError)
     redirect_stderr(stderr_copy)
     close(rserr);close(wrerr)
 end
@@ -89,6 +95,7 @@ end
     open(tmpfile, "w") do io
         println(io, "dummy content")
     end
-    ret = @test_throws ErrorException load(tmpfile)
-    #@test ret.value.msg == "1" # this is 0.5 only
+    ex = try load(tmpfile) catch e e end
+    @test isa(ex, CapturedException)
+    @test isa(ex.ex, ErrorException)
 end