scripts: dbgflags.py: Added -d/--diff, lineno, better find reuse

This started with adding -d/--diff support to dbgflags.py, which is very
useful for comparing flags during test failure.

The flag asserts in our tests generally look like this:

  tests/test_mount.toml:171:assert: assert failed with 33570064,
  expected eq 33570576
      assert(fsinfo.flags == (

Which can now be quickly compared with dbgflags.py:

  $ ./scripts/dbgflags.py +i 33570064 -d 33570576
   LFS3_I_GBMAP         0x02000000  Global on-disk block-map in use
   LFS3_I_REVPERTURB    0x00000010  Mounted with LFS3_M_REVPERTURB
   LFS3_I_MKCONSISTENT  0x00000100  Filesystem needs mkconsistent to write
  -LFS3_I_LOOKAHEAD     0x00000200  Lookahead buffer is not full
   LFS3_I_PREERASE      0x00000400  Blocks can be pre-erased
   LFS3_I_COMPACT       0x00000800  Filesystem may have uncompacted metadata
   LFS3_I_CKMETA        0x00001000  Metadata checksums not checked recently
   LFS3_I_CKDATA        0x00002000  Data checksums not checked recently

The assert print is a bit more annoying than it needs to be, as it only
prints in decimal. But, since our prettyasserts.py only works at the
syntax layer, it's not possible to make it any smarter.

---

To make this diffing work required a couple more features in our
self-parsing Flag class:

- Keep track of lineno, mainly for ordering things
- Moved find logic into a staticmethod on all classes
- Added _sentinel based defaults to find functions
- Allowed self to be non-class in line functions to deduplicate "Unknown
  flag" messages

I went ahead and extended these to the other self-parsing classes (Err
and Tag) in case they're useful in the future.

scripts/dbgbmap.py

@@ -167,11 +167,13 @@ TAG_GCKSUMDELTA = 0x3300    ##  v-11 --11 ++++ ++++
 
 # self-parsing tag repr
 class Tag:
-    def __init__(self, name, tag, encoding, help):
+    def __init__(self, name, tag, encoding, help, *,
+            lineno=0):
         self.name = name
         self.tag = tag
         self.encoding = encoding
         self.help = help
+        self.lineno = lineno
         # derive mask from encoding
         self.mask = sum(
                 (1 if x in 'v-01' else 0) << len(self.encoding)-1-i
@@ -240,29 +242,37 @@ class Tag:
         tag_pattern = re.compile(
             '^(?P<name>TAG_[^ ]*) *= *(?P<tag>[^#]*?) *'
                 '#+ *(?P<encoding>(?:[^ ] *?){16}) *(?P<help>.*)$')
-        for line in (inspect.getsource(
-                    inspect.getmodule(inspect.currentframe()))
-                .replace('\\\n', '')
-                .splitlines()):
+        for i, line in enumerate(
+                inspect.getsource(inspect.getmodule(inspect.currentframe()))
+                    .replace('\\\n', '')
+                    .splitlines()):
             m = tag_pattern.match(line)
             if m:
                 tags.append(Tag(
                         m.group('name'),
                         globals()[m.group('name')],
                         m.group('encoding').replace(' ', ''),
-                        m.group('help')))
+                        m.group('help'),
+                        lineno=1+i))
         return tags
 
     # find best matching tag
+    _sentinel = object()
     @staticmethod
-    def find(tag):
+    def find(tag, *, default=_sentinel):
         # find tags, note this is cached
         tags__ = Tag.tags()
 
         # find the most specific matching tag, ignoring valid bits
-        return max((t for t in tags__ if t.matches(tag & 0x7fff)),
+        t = max((t for t in tags__ if t.matches(tag & 0x7fff)),
                 key=lambda t: t.specificity(),
                 default=None)
+        if t is not None:
+            return t
+        elif default is Tag._sentinel:
+            raise KeyError(tag)
+        else:
+            return default
 
     # human readable tag repr
     @staticmethod
@@ -270,7 +280,9 @@ class Tag:
             global_=False,
             toff=None):
         # find the most specific matching tag, ignoring the shrub bit
-        t = Tag.find(tag & ~(TAG_SHRUB if tag & 0x7000 == TAG_SHRUB else 0))
+        t = Tag.find(
+                tag & ~(TAG_SHRUB if tag & 0x7000 == TAG_SHRUB else 0),
+                default=None)
 
         # build repr
         r = []