unittests: add and use _open_meson_log function

Factors out opening the meson log into its own function so that
it can be used in the places where with was done before.

Additionally instead of checking if the file exists before opening it,
try to open it and handle the exception when it is not found.

unittests/baseplatformtests.py

@@ -15,6 +15,7 @@
 from pathlib import PurePath
 from unittest import mock, TestCase, SkipTest
 import json
+import io
 import os
 import re
 import subprocess
@@ -120,13 +121,17 @@ class BasePlatformTests(TestCase):
         newdir = os.path.realpath(newdir)
         self.change_builddir(newdir)
 
-    def _get_meson_log(self) -> T.Optional[str]:
+    def _open_meson_log(self) -> io.TextIOWrapper:
         log = os.path.join(self.logdir, 'meson-log.txt')
-        if not os.path.isfile(log):
-            print(f"{log!r} doesn't exist", file=sys.stderr)
+        return open(log, encoding='utf-8')
+
+    def _get_meson_log(self) -> T.Optional[str]:
+        try:
+            with self._open_meson_log() as f:
+                return f.read()
+        except FileNotFoundError as e:
+            print(f"{e.filename!r} doesn't exist", file=sys.stderr)
             return None
-        with open(log, encoding='utf-8') as f:
-            return f.read()
 
     def _print_meson_log(self) -> None:
         log = self._get_meson_log()
@@ -318,7 +323,7 @@ class BasePlatformTests(TestCase):
         return contents
 
     def get_meson_log(self):
-        with open(os.path.join(self.builddir, 'meson-logs', 'meson-log.txt'), encoding='utf-8') as f:
+        with self._open_meson_log() as f:
             return f.readlines()
 
     def get_meson_log_compiler_checks(self):
@@ -326,19 +331,19 @@ class BasePlatformTests(TestCase):
         Fetch a list command-lines run by meson for compiler checks.
         Each command-line is returned as a list of arguments.
         '''
-        log = self.get_meson_log()
         prefix = 'Command line:'
-        cmds = [l[len(prefix):].split() for l in log if l.startswith(prefix)]
-        return cmds
+        with self._open_meson_log() as log:
+            cmds = [l[len(prefix):].split() for l in log if l.startswith(prefix)]
+            return cmds
 
     def get_meson_log_sanitychecks(self):
         '''
         Same as above, but for the sanity checks that were run
         '''
-        log = self.get_meson_log()
         prefix = 'Sanity check compiler command line:'
-        cmds = [l[len(prefix):].split() for l in log if l.startswith(prefix)]
-        return cmds
+        with self._open_meson_log() as log:
+            cmds = [l[len(prefix):].split() for l in log if l.startswith(prefix)]
+            return cmds
 
     def introspect(self, args):
         if isinstance(args, str):