cvsimport: strip all inappropriate tag strings

Certain characters such as "?" can be present in a CVS tag name, but
git does not allow these characters in tags. If git-cvsimport
encounters a CVS tag that git cannot handle, cvsimport will error and
refuse to continue the import beyond that point.

When importing CVS tags, strip all the inappropriate strings from the
tag names as we translate them to git tag names.

Provide more debugging information to the user if we've altered the
tag and the "git tag" command still fails. Also, warn the user if we
end up skipping an (unusable) tag altogether.

Signed-off-by: Ken Dreyer <ktdreyer@ktdreyer.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-cvsimport.perl

@@ -889,10 +889,37 @@ sub commit {
 		$xtag =~ s/\s+\*\*.*$//; # Remove stuff like ** INVALID ** and ** FUNKY **
 		$xtag =~ tr/_/\./ if ( $opt_u );
 		$xtag =~ s/[\/]/$opt_s/g;
-		$xtag =~ s/\[//g;
 
-		system('git' , 'tag', '-f', $xtag, $cid) == 0
-			or die "Cannot create tag $xtag: $!\n";
+		# See refs.c for these rules.
+		# Tag cannot contain bad chars. (See bad_ref_char in refs.c.)
+		$xtag =~ s/[ ~\^:\\\*\?\[]//g;
+		# Other bad strings for tags:
+		# (See check_refname_component in refs.c.)
+		1 while $xtag =~ s/
+			(?: \.\.        # Tag cannot contain '..'.
+			|   \@{         # Tag cannot contain '@{'.
+			| ^ -           # Tag cannot begin with '-'.
+			|   \.lock $    # Tag cannot end with '.lock'.
+			| ^ \.          # Tag cannot begin...
+			|   \. $        # ...or end with '.'
+			)//xg;
+		# Tag cannot be empty.
+		if ($xtag eq '') {
+			warn("warning: ignoring tag '$tag'",
+			" with invalid tagname\n");
+			return;
+		}
+
+		if (system('git' , 'tag', '-f', $xtag, $cid) != 0) {
+			# We did our best to sanitize the tag, but still failed
+			# for whatever reason. Bail out, and give the user
+			# enough information to understand if/how we should
+			# improve the translation in the future.
+			if ($tag ne $xtag) {
+				print "Translated '$tag' tag to '$xtag'\n";
+			}
+			die "Cannot create tag $xtag: $!\n";
+		}
 
 		print "Created tag '$xtag' on '$branch'\n" if $opt_v;
 	}