[mlir][mbr] Improve diagnostics on error with `raise from`.

[ingomueller-net]

Summary

This commit extends the raise statements on errors in user-provided
code with from e clauses that attach the original exception to the one
being raised. This allows to debug the root cause of the error more
easily.

[SaurabhJha]

Thanks for submitting this patch! Added a comment :)

mlir/utils/mbr/mbr/main.py

40–43

How about also including e in error_message string?

[ingomueller-net]

Updating D129762: [mlir][mbr] Improve diagnostics on error with raise from.

Also giving more details about the root exception in the short message
where that hadn't been done yet.

[SaurabhJha]

Looks great, thank you!

mlir/utils/mbr/mbr/main.py

def main(top_level_path, stop_on_error):
     for module in modules:
         benchmark_functions = [
             function for function in
             get_benchmark_functions(module, benchmark_function_name)
         ]
         for benchmark_function in benchmark_functions:
             try:
                 compiler, runner = benchmark_function()
-            except (TypeError, ValueError):
+            except (TypeError, ValueError) as e:
                 error_message = (
-                    f"benchmark_function '{benchmark_function.__name__}'"
-                    f" must return a two tuple value (compiler, runner)."
+                    f"Obtaining compiler and runner failed because of {e}."
+                    f" Benchmark function '{benchmark_function.__name__}'"
+                    f" must return a two-tuple value (compiler, runner)."
                 )
                 if stop_on_error is False:
                     print(error_message, file=sys.stderr)
                     continue
                 else:
-                    raise AssertionError(error_message)
+                    raise AssertionError(error_message) from e
             measurements_ns = np.array([])
             if compiler:
                 start_compile_time_s = time.time()
                 try:
                     compiled_callable = compiler()
                 except Exception as e:
                     error_message = (
                         f"Compilation of {benchmark_function.__name__} failed"
                         f" because of {e}"
                     )
                     if stop_on_error is False:
                         print(error_message, file=sys.stderr)
                         continue
                     else:
-                        raise AssertionError(error_message)
+                        raise AssertionError(error_message) from e
                 total_compile_time_s = time.time() - start_compile_time_s
                 runner_args = (compiled_callable,)
             else:
                 total_compile_time_s = 0
                 runner_args = ()
             while not has_enough_measurements(measurements_ns):
                 try:
                     measurement_ns = runner(*runner_args)
                 except Exception as e:
                     error_message = (
                         f"Runner of {benchmark_function.__name__} failed"
                         f" because of {e}"
                     )
                     if stop_on_error is False:
                         print(error_message, file=sys.stderr)
                         # Recover from runner error by breaking out of this loop
                         # and continuing forward.
                         break
                     else:
-                        raise AssertionError(error_message)
+                        raise AssertionError(error_message) from e
                 if not isinstance(measurement_ns, int):
                     error_message = (
                         f"Expected benchmark runner function"
                         f" to return an int, got {measurement_ns}"
                     )
                     if stop_on_error is False:
                         print(error_message, file=sys.stderr)
                         continue