From dc61904aeea9fe10cf9c3f67b7a2f0b1ee97d148 Mon Sep 17 00:00:00 2001
From: Jake Lishman <jake@binhbar.com>
Date: Thu, 13 Apr 2023 12:54:25 +0100
Subject: [PATCH] Wrap io::Error in QASM2ParseError on failed file read

Previously, if Python thought the target file for `qasm2.load` existed,
but Rust was unable to open it, the result `io:Error` would be
propagated up to Python space verbatim, and these situations usually
have confusing error messages.  This could happen, for example, if the
target was a directory on Windows.
---
 crates/qasm2/src/lib.rs                | 12 +++++++++++-
 test/python/qasm2/test_parse_errors.py |  5 +++++
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/crates/qasm2/src/lib.rs b/crates/qasm2/src/lib.rs
index 1ae817364d11..fab26c5b8e93 100644
--- a/crates/qasm2/src/lib.rs
+++ b/crates/qasm2/src/lib.rs
@@ -13,6 +13,8 @@
 use pyo3::prelude::*;
 use pyo3::Python;
 
+use crate::error::QASM2ParseError;
+
 mod bytecode;
 mod error;
 mod expr;
@@ -94,6 +96,7 @@ fn bytecode_from_string(
 /// without loading the entire token and parse tree into memory at once.
 #[pyfunction]
 fn bytecode_from_file(
+    py: Python<'_>,
     path: std::ffi::OsString,
     include_path: Vec<std::path::PathBuf>,
     custom_instructions: Vec<CustomInstruction>,
@@ -101,7 +104,14 @@ fn bytecode_from_file(
     strict: bool,
 ) -> PyResult<bytecode::BytecodeIterator> {
     bytecode::BytecodeIterator::new(
-        lex::TokenStream::from_path(path, strict)?,
+        lex::TokenStream::from_path(&path, strict).map_err(|err| {
+            let exc = QASM2ParseError::new_err(format!(
+                "failed to read a token stream from file '{}'",
+                path.to_string_lossy()
+            ));
+            exc.set_cause(py, Some(err.into()));
+            exc
+        })?,
         include_path,
         &custom_instructions,
         &custom_classical,
diff --git a/test/python/qasm2/test_parse_errors.py b/test/python/qasm2/test_parse_errors.py
index 801559aac7a1..1cc90cdaef3c 100644
--- a/test/python/qasm2/test_parse_errors.py
+++ b/test/python/qasm2/test_parse_errors.py
@@ -220,6 +220,11 @@ def test_eof(self, statement):
         with self.assertRaisesRegex(qiskit.qasm2.QASM2ParseError, "unexpected end-of-file"):
             qiskit.qasm2.loads(full)
 
+    def test_loading_directory(self):
+        """Test that the correct error is raised when a file fails to open."""
+        with self.assertRaisesRegex(qiskit.qasm2.QASM2ParseError, "failed to read"):
+            qiskit.qasm2.load(".")
+
 
 class TestVersion(QiskitTestCase):
     def test_invalid_version(self):