bundlespec: make the `stream` case less special

The handling of the stream case seems fragile (does not account for newer parts
and options that will arise) and has special code dedicated to it.

To simplify and strengthen things, we make it use the same mechanism as the other
options. So we make it less special by making it a special value in the common case.

mercurial/bundlecaches.py

@@ -56,6 +56,15 b' class bundlespec:'
         b'tagsfnodescache': True,
         b'revbranchcache': True,
     },
+    b'streamv2': {
+        b'changegroup': False,
+        b'cg.version': b'02',
+        b'obsolescence': False,
+        b'phases': False,
+        b"streamv2": True,
+        b'tagsfnodescache': False,
+        b'revbranchcache': False,
+    },
     b'packed1': {
         b'cg.version': b's1',
     },
@@ -65,14 +74,7 b' class bundlespec:'
 }
 _bundlespeccontentopts[b'bundle2'] = _bundlespeccontentopts[b'v2']
 
-_bundlespecvariants = {
-    b"streamv2": {
-        b"changegroup": False,
-        b"streamv2": True,
-        b"tagsfnodescache": False,
-        b"revbranchcache": False,
-    }
-}
+_bundlespecvariants = {b"streamv2": {}}
 
 # Compression engines allowed in version 1. THIS SHOULD NEVER CHANGE.
 _bundlespecv1compengines = {b'gzip', b'bzip2', b'none'}
@@ -208,12 +210,21 b' def parsebundlespec(repo, spec, strict=T'
             )
 
     # Compute contentopts based on the version
+    if b"stream" in params and params[b"stream"] == b"v2":
+        # That case is fishy as this mostly derails the version selection
+        # mechanism. `stream` bundles are quite specific and used differently
+        # as "normal" bundles.
+        #
+        # So we are pinning this to "v2", as this will likely be
+        # compatible forever. (see the next conditional).
+        #
+        # (we should probably define a cleaner way to do this and raise a
+        # warning when the old way is encounter)
+        version = b"streamv2"
     contentopts = _bundlespeccontentopts.get(version, {}).copy()
-
-    # Process the variants
-    if b"stream" in params and params[b"stream"] == b"v2":
-        variant = _bundlespecvariants[b"streamv2"]
-        contentopts.update(variant)
+    if version == b"streamv2":
+        # streamv2 have been reported as "v2" for a while.
+        version = b"v2"
 
     engine = util.compengines.forbundlename(compression)
     compression, wirecompression = engine.bundletype()