tests/run-perfbench.py: Don't allow imports from the cwd.

Make tests run in an isolated environment (i.e. `import io` would
otherwise get the `tests/io` directory).

This work was funded through GitHub Sponsors.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

tests/perf_bench/core_import_mpy_multi.py

@@ -47,7 +47,7 @@ class FS:
         pass
 
     def stat(self, path):
-        if path == "__injected.mpy":
+        if path == "/__injected.mpy":
             return tuple(0 for _ in range(10))
         else:
             raise OSError(-2)  # ENOENT
@@ -58,7 +58,7 @@ class FS:
 
 def mount():
     os.mount(FS(), "/__remote")
-    os.chdir("/__remote")
+    sys.path.insert(0, "/__remote")
 
 
 def test(r):

tests/perf_bench/core_import_mpy_single.py

@@ -2,7 +2,7 @@
 # The first import of a module will intern strings that don't already exist, and
 # this test should be representative of what happens in a real application.
 
-import io, os
+import io, os, sys
 
 if not (hasattr(io, "IOBase") and hasattr(os, "mount")):
     print("SKIP")
@@ -102,7 +102,7 @@ class FS:
         pass
 
     def stat(self, path):
-        if path == "__injected.mpy":
+        if path == "/__injected.mpy":
             return tuple(0 for _ in range(10))
         else:
             raise OSError(-2)  # ENOENT
@@ -113,7 +113,7 @@ class FS:
 
 def mount():
     os.mount(FS(), "/__remote")
-    os.chdir("/__remote")
+    sys.path.insert(0, "/__remote")
 
 
 def test():

tests/run-perfbench.py

@@ -109,8 +109,9 @@ def run_benchmarks(args, target, param_n, param_m, n_average, test_list):
             continue
 
         # Create test script
+        test_script = b"import sys\nsys.path.remove('')\n\n"
         with open(test_file, "rb") as f:
-            test_script = f.read()
+            test_script += f.read()
         with open(BENCH_SCRIPT_DIR + "benchrun.py", "rb") as f:
             test_script += f.read()
         test_script += b"bm_run(%u, %u)\n" % (param_n, param_m)