attempt to avoid content differences due to paths in keywords

Some RCS keywords, such has "Header", contain absolute file paths
derived from the on-disk filesystem path of the CVS repository.

When we fetch files over the pserver protocol such keywords are
expanded by the CVS server. But when using the rsync protocol we
will first copy the CVS repository to local disk and the path to
this local copy will correspond to some temporary directory.

Try to avoid file content differences between pserver and rsync
access methods by deriving a likely server-side path from path
information found in the rsync:// origin URL.
This will work as expected as long as the CVS server-side setup
exposes the same path to the CVS repository over both access
methods, which is the case for GNU savannah for example.

In general, we should recommend treating pserver and rsync as distinct
origins and not rely on them to be interchangable and always produce
the same conversion result. But we can still try our best to avoid
needless differences in content hashes.

swh/loader/cvs/loader.py

@@ -137,6 +137,7 @@ class CvsLoader(BaseLoader):
         self, k: ChangeSetKey, f: FileRevision, rcsfile: rcsparse.rcsfile
     ) -> None:
         assert self.cvsroot_path
+        assert self.server_style_cvsroot
         path = file_path(self.cvsroot_path, f.path)
         wtpath = os.path.join(self.worktree_path, path)
         self.log.info("rev %s state %s file %s" % (f.rev, f.state, f.path))
@@ -151,7 +152,26 @@ class CvsLoader(BaseLoader):
             if not rcsfile:
                 rcsfile = rcsparse.rcsfile(f.path)
             rcs = RcsKeywords()
-            contents = rcs.expand_keyword(f.path, rcsfile, f.rev)
+
+            # We try our best to generate the same commit hashes over both pserver
+            # and rsync. To avoid differences in file content due to expansion of
+            # RCS keywords which contain absolute file paths (such as "Header"),
+            # attempt to expand such paths in the same way as a regular CVS server
+            # would expand them.
+            # Whether this will avoid content differences depends on pserver and
+            # rsync servers exposing the same server-side path to the CVS repository.
+            # However, this is the best we can do, and only matters if an origin can
+            # be fetched over both pserver and rsync. Each will still be treated as
+            # a distinct origin, but will hopefully point at the same SWH snapshot.
+            # In any case, an absolute path based on the origin URL looks nicer than
+            # an absolute path based on a temporary directory used by the CVS loader.
+            server_style_path = f.path.replace(
+                self.cvsroot_path, self.server_style_cvsroot
+            )
+            if server_style_path[0] != "/":
+                server_style_path = "/" + server_style_path
+
+            contents = rcs.expand_keyword(server_style_path, rcsfile, f.rev)
             os.makedirs(os.path.dirname(wtpath), exist_ok=True)
             outfile = open(wtpath, mode="wb")
             outfile.write(contents)
@@ -293,6 +313,7 @@ class CvsLoader(BaseLoader):
         if not url.path:
             raise NotFound("Invalid CVS origin URL '%s'" % self.origin_url)
         self.cvs_module_name = os.path.basename(url.path)
+        self.server_style_cvsroot = os.path.dirname(url.path)
         os.mkdir(os.path.join(self.worktree_path, self.cvs_module_name))
         if url.scheme == "file" or url.scheme == "rsync":
             # local CVS repository conversion