[thiell]

Braces list alternatives for a part of a node name: server[5-9]-{mds,oss} is server[5-9]-mds,server[5-9]-oss, and node[1-3]{,-ib} adds the -ib names of node[1-3]. Shell brace expansion is the usual workaround today, but it breaks clush -w (the shell splits the argument and clush runs the wrong command) and does not help in group sources, host files or the Python API.

• {a,b} expands a term once per alternative; the union is one operand of the extended pattern operators, so node[1-9]&node{2,4} is node[2,4] (a textual rewrite would be wrong for & and ^)
• several braces in a term form a cartesian product; alternatives may be empty and surrounding whitespace is ignored
• alternatives are literal strings: ranges, groups, wildcards, operators and nested braces are rejected inside braces, but keep working elsewhere in the same term (@rack{1,2}, server-{mds,oss}[0-3], n{1,2}*)
• output never contains braces, so results stay usable as -w arguments everywhere
• { and } become reserved characters and join ILLEGAL_GROUP_CHARS
• strings without braces follow the same code path as before; parse time is unchanged within measurement noise
• docs: new section in the cluset (or nodeset) tool pages and man pages, a mention for clush -w, the guide, and a warning about quoting braces on the command line