GH-120754: Add a strace helper and test set of syscalls for open().read() (#121143)

Lib/test/test_fileio.py

@@ -11,7 +11,7 @@ from functools import wraps
 
 from test.support import (
     cpython_only, swap_attr, gc_collect, is_emscripten, is_wasi,
-    infinite_recursion,
+    infinite_recursion, strace_helper
 )
 from test.support.os_helper import (
     TESTFN, TESTFN_ASCII, TESTFN_UNICODE, make_bad_fd,
@@ -24,6 +24,9 @@ import _io  # C implementation of io
 import _pyio # Python implementation of io
 
 
+_strace_flags=["--trace=%file,%desc"]
+
+
 class AutoFileTests:
     # file tests for which a test file is automatically set up
 
@@ -359,6 +362,94 @@ class AutoFileTests:
         a = array('b', b'x'*10)
         f.readinto(a)
 
+    @strace_helper.requires_strace()
+    def test_syscalls_read(self):
+        """Check that the set of system calls produced by the I/O stack is what
+        is expected for various read cases.
+
+        It's expected as bits of the I/O implementation change, this will need
+        to change. The goal is to catch changes that unintentionally add
+        additional systemcalls (ex. additional calls have been looked at in
+        bpo-21679 and gh-120754).
+        """
+        self.f.write(b"Hello, World!")
+        self.f.close()
+
+
+        def check_readall(name, code, prelude="", cleanup=""):
+            with self.subTest(name=name):
+                syscalls = strace_helper.get_syscalls(code, _strace_flags,
+                                                      prelude=prelude,
+                                                      cleanup=cleanup)
+
+                # There are a number of related syscalls used to implement
+                # behaviors in a libc (ex. fstat, newfstatat, open, openat).
+                # Allow any that use the same substring.
+                def count_similarname(name):
+                    return len([sc for sc in syscalls if name in sc])
+
+                # Should open and close the file exactly once
+                self.assertEqual(count_similarname('open'), 1)
+                self.assertEqual(count_similarname('close'), 1)
+
+                # Should only have one fstat (bpo-21679, gh-120754)
+                self.assertEqual(count_similarname('fstat'), 1)
+
+
+        # "open, read, close" file using different common patterns.
+        check_readall(
+            "open builtin with default options",
+            f"""
+            f = open('{TESTFN}')
+            f.read()
+            f.close()
+            """
+        )
+
+        check_readall(
+            "open in binary mode",
+            f"""
+            f = open('{TESTFN}', 'rb')
+            f.read()
+            f.close()
+            """
+        )
+
+        check_readall(
+            "open in text mode",
+            f"""
+            f = open('{TESTFN}', 'rt')
+            f.read()
+            f.close()
+            """
+        )
+
+        check_readall(
+            "pathlib read_bytes",
+            "p.read_bytes()",
+            prelude=f"""from pathlib import Path; p = Path("{TESTFN}")"""
+
+        )
+
+        check_readall(
+            "pathlib read_text",
+            "p.read_text()",
+            prelude=f"""from pathlib import Path; p = Path("{TESTFN}")"""
+        )
+
+        # Focus on just `read()`.
+        calls = strace_helper.get_syscalls(
+            prelude=f"f = open('{TESTFN}')",
+            code="f.read()",
+            cleanup="f.close()",
+            strace_flags=_strace_flags
+        )
+        # One to read all the bytes
+        # One to read the EOF and get a size 0 return.
+        self.assertEqual(calls.count("read"), 2)
+
+
+
 class CAutoFileTests(AutoFileTests, unittest.TestCase):
     FileIO = _io.FileIO
     modulename = '_io'