Use os.devnull instead of '/dev/null' in the testsuite driver.

The later caused issues on windows by being translated into
"\\dev\\null" and python then trying to open this non-existant file.

So we now use os.devnull inside python and convert it to "/dev/null"
when calling out to the shell, which is bound to run in a unix like
environment.

This fixes an issue a test producing unexpected stderr output failed
with a framework failure instead of showing a diff of the output.

1 files changed, 26 insertions, 4 deletions

diff --git a/testsuite/driver/testlib.py b/testsuite/driver/testlib.py
index e03a2c54aa..36aaf38076 100644
--- a/testsuite/driver/testlib.py
+++ b/testsuite/driver/testlib.py
@@ -1817,7 +1817,8 @@ def compare_outputs(way: WayName,
         expected_normalised_path = in_testdir(expected_normalised_file)
     else:
         expected_str = ''
-        expected_normalised_path = Path('/dev/null')
+        # See Note [Null device handling]
+        expected_normalised_path = Path(os.devnull)
 
     actual_raw = read_no_crs(actual_path)
     actual_str = normaliser(actual_raw)
@@ -1829,7 +1830,8 @@ def compare_outputs(way: WayName,
         if config.verbose >= 1 and _expect_pass(way):
             print('Actual ' + kind + ' output differs from expected:')
 
-        if expected_normalised_path != '/dev/null':
+        # See Note [Null device handling]
+        if expected_normalised_path != Path(os.devnull):
             write_file(expected_normalised_path, expected_str)
 
         actual_normalised_path = add_suffix(actual_path, 'normalised')
@@ -1837,7 +1839,7 @@ def compare_outputs(way: WayName,
 
         if config.verbose >= 1 and _expect_pass(way):
             # See Note [Output comparison].
-            r = runCmd('diff -uw "{0}" "{1}"'.format(expected_normalised_path,
+            r = runCmd('diff -uw "{0}" "{1}"'.format(null2unix_null(expected_normalised_path),
                                                         actual_normalised_path),
                         stdout=diff_file,
                         print_output=True)
@@ -1845,7 +1847,7 @@ def compare_outputs(way: WayName,
             # If for some reason there were no non-whitespace differences,
             # then do a full diff
             if r == 0:
-                r = runCmd('diff -u "{0}" "{1}"'.format(expected_normalised_path,
+                r = runCmd('diff -u "{0}" "{1}"'.format(null2unix_null(expected_normalised_path),
                                                            actual_normalised_path),
                            stdout=diff_file,
                            print_output=True)
@@ -1930,6 +1932,26 @@ def grep_output(normaliser: OutputNormalizer, pattern_file, actual_file, is_subs
 #    on the `diff` program to ignore whitespace changes as much as
 #    possible (#10152).
 
+# Note [Null device handling]
+#
+# On windows the null device is 'nul' instead of '/dev/null'.
+# This can in principle be easily solved by using os.devnull.
+# Not doing so causes issues when python tries to read/write/open
+# the null device.
+#
+# However this still leads to a problem when executing shell
+# commands in the msys environment. Which again expect '/dev/null'.
+#
+# So what we do is use os.devnull and convert it to the string
+# '/dev/null' for shell commands which are bound to run in a
+# unix-like environment.
+
+def null2unix_null(f: Path) -> str:
+    if f == Path(os.devnull):
+        return ('/dev/null')
+    else:
+        return f.as_posix()
+
 def normalise_whitespace(s: str) -> str:
     # Merge contiguous whitespace characters into a single space.
     return ' '.join(s.split())