os/user: add illumos getgroups support

[bradfitz]

Solaris is broken after the os/user groups change:

http://build.golang.org/log/26a14fbc0126ab1ed65eaeffe18362689b13a54f

...
net
os/user
# os/user
Undefined           first referenced
 symbol                 in file
getgrouplist                        $WORK/os/user/_obj/lookup_unix.cgo2.o
ld: fatal: symbol referencing errors. No output written to $WORK/os/user/_obj/_cgo_.o
collect2: error: ld returned 1 exit status
crypto/x509
net/textproto
log/syslog
...

Let me know if you need help testing on Solaris. It's not yet a trybot, due to lack of time.

[bradfitz]

Apparently Solaris didn't have getgrouplist until recently. I found elsewhere that "getgrouplist() is provided by the latest Solaris 11.2 SRU 11.2.10.5.0, released 15 May 2015. It will be available in all future updates/releases." But apparently our builders don't have that.

@4ad, advice?

[bradfitz]

Actually, we should just not try to build it & test it. I'll send out a CL. We can keep this bug open to add support later.

[bradfitz]

Or rather, I'll wait for @zombiezen to finish his pending CLs, since my change would necessarily split lookup_unix.go into two halves probably (user lookup vs group lookup, with different build tags), or maybe add a group_solaris.go file with an import "C" and a fake getgrouplist impl in it just for Solaris.

Ross, let me know if you prefer to tackle this.

[4ad]

We will have to implement getgrouplist for Solaris ourselves. In the meantime we can stub it.

[zombiezen]

I can roll in the stub as part of https://golang.org/cl/20348, since I'm already splitting out the OS-specific functionality.

[bradfitz]

Great, thanks.

[gopherbot]

CL https://golang.org/cl/20348 mentions this issue.

[zombiezen]

Just to point out, the other group-related functionality should still work on Solaris (let me know if it doesn't). getgrouplist is annoyingly non-standard. I'm contemplating whether the internal listGroups function should just parse /etc/groups itself in the future because it will ironically be more portable.

[bradfitz]

Parsing /etc/groups as a fallback seems fine too.

[bradfitz]

Repurposing this bug to add Solaris support, since this is the bug number references in the t.Skip text now.