Fix sbang hook for non-writable files (#27007)

* Fix sbang hook for non-writable files

PR #26793 seems to have broken the sbang hook for files with missing
write permissions. Installing perl now breaks with the following error:
```
==> [2021-10-28-12:09:26.832759] Error: PermissionError: [Errno 13] Permission denied: '$SPACK/opt/spack/linux-fedora34-zen2/gcc-11.2.1/perl-5.34.0-afuweplnhphcojcowsc2mb5ngncmczk4/bin/cpanm'
```

Temporarily add write permissions to the original file so it can be
overwritten with the patched one.

And test that file permissions are preserved in sbang even for non-writable files

Co-authored-by: Harmen Stoppels <harmenstoppels@gmail.com>

lib/spack/spack/hooks/sbang.py

@@ -110,6 +110,10 @@ def filter_shebang(path):
         # Store the file permissions, the patched version needs the same.
         saved_mode = os.stat(path).st_mode
 
+        # Change non-writable files to be writable if needed.
+        if not os.access(path, os.W_OK):
+            os.chmod(path, saved_mode | stat.S_IWUSR)
+
         # No need to delete since we'll move it and overwrite the original.
         patched = tempfile.NamedTemporaryFile('wb', delete=False)
         patched.write(new_sbang_line)

lib/spack/spack/test/sbang.py

@@ -374,3 +374,14 @@ def test_shebang_exceeds_spack_shebang_limit(shebang_limits_system_8_spack_16, t
 
     with open(file, 'rb') as f:
         assert b'sbang' not in f.read()
+
+
+def test_sbang_hook_handles_non_writable_files_preserving_permissions(tmpdir):
+    path = str(tmpdir.join('file.sh'))
+    with open(path, 'w') as f:
+        f.write(long_line)
+    os.chmod(path, 0o555)
+    sbang.filter_shebang(path)
+    with open(path, 'r') as f:
+        assert 'sbang' in f.readline()
+    assert os.stat(path).st_mode & 0o777 == 0o555