os/user: add pure Go LookupGroup when cgo isn't available #18102
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Feel free to add /etc/group parsing code for Unix in the no-cgo case, but the default shouldn't change: when cgo is available, use cgo. There's no way to write a pure Go function, since the configuration of libc's user/groups allows arbitrary shared libraries to be loaded to do the work. |
bradfitz
changed the title
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If we fix LookupGroup, I think it makes sense to fix all of os/user to be
like the net package: if /etc/nsswitch.conf allows, use a pure Go version
rather than cgo.
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But DNS is common. LookupUser and LookGroup aren't, I imagine. |
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Yeah, but parsing /etc/passwd and /etc/group is so much simpler than a DNS
client, so I think it's still worthwhile.
…On Nov 29, 2016 8:08 PM, "Brad Fitzpatrick" ***@***.***> wrote:
But DNS is common. LookupUser and LookGroup aren't, I imagine.
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I've started working on this. Related to @minux's comment, should we respect the group setting in /etc/nsswitch.conf, if it exists? There's code to parse nsswitch.conf in net, but it's not currently exported. |
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The parsing should probably move to a new internal package if it's to be shared. But I'm not sure how much it matters. What's an interesting case that parsing nsswitch.conf solves? It seems that it should either be simple (no cgo, only parsing /etc/group) or complete (cgo). Things in-between seem "meh" unless there's a common use case or common configuration that you're looking to address. What do you have in mind? |
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To be honest I just want to cross compile and find the GID for a groupname. But I figured if I was going to write a patch I could try to match libc, to a degree. I'll ignore it for now. |
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CL https://golang.org/cl/33713 mentions this issue. |
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CL https://golang.org/cl/37497 mentions this issue. |
In another CL, I'll add a pure Go implementation of lookupGroup and lookupGroupId in lookup_unix.go, but attempting that in one CL makes the diff too difficult to read. Updates #18102. Change-Id: If8e26cee5efd30385763430f34304c70165aef32 Reviewed-on: https://go-review.googlesource.com/37497 Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org> Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org>
Please answer these questions before submitting your issue. Thanks!
What version of Go are you using (
go version)?tip
Currently when you cross compile Go for a Unix target, calls to
user.LookupGroupfail withuser: LookupGroup requires cgo. Under the hood,LookupGroupcallsgetgrnam_rfrom C.Is there any interest in replacing this with a pure Go function? We'd probably need a clean rewrite, but here's some Go code that parses /etc/group, so it's at least possible. https://github.com/opencontainers/runc/blob/master/libcontainer/user/user.go#L141
The other consideration (reading through libc) is that apparently getgrnam checks other sources besides /etc/group:
It might be trickier to replicate that other behavior.
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