fix(libstore/lock): support users that belong to more than 10 groups

The manpage for `getgrouplist` says:

> If the number of groups of which user is a member is less than or
> equal to *ngroups, then the value *ngroups is returned.
>
> If the user is a member of more than *ngroups groups, then
> getgrouplist() returns -1.  In this case, the value returned in
> *ngroups can be used to resize the buffer passed to a further
> call getgrouplist().

In our original code, however, we allocated a list of size `10` and, if
`getgrouplist` returned `-1` threw an exception. In practice, this
caused the code to fail for any user belonging to more than 10 groups.

While unusual for single-user systems, large companies commonly have a
huge number of POSIX groups users belong to, causing this issue to crop
up and make multi-user Nix unusable in such settings.

The fix is relatively simple, when `getgrouplist` fails, it stores the
real number of GIDs in `ngroups`, so we must resize our list and retry.
Only then, if it errors once more, we can raise an exception.

This should be backported to, at least, 2.9.x.

1 files changed, 18 insertions, 5 deletions

diff --git a/src/libstore/lock.cc b/src/libstore/lock.cc
index f1356fdca..fa718f55d 100644
--- a/src/libstore/lock.cc
+++ b/src/libstore/lock.cc
@@ -67,13 +67,26 @@ bool UserLock::findFreeUser() {
 #if __linux__
             /* Get the list of supplementary groups of this build user.  This
                is usually either empty or contains a group such as "kvm".  */
-            supplementaryGIDs.resize(10);
-            int ngroups = supplementaryGIDs.size();
-            int err = getgrouplist(pw->pw_name, pw->pw_gid,
-                supplementaryGIDs.data(), &ngroups);
+            int ngroups = 32; // arbitrary initial guess
+            supplementaryGIDs.resize(ngroups);
+
+            int err = getgrouplist(pw->pw_name, pw->pw_gid, supplementaryGIDs.data(),
+                                 &ngroups);
+
+            // Our initial size of 32 wasn't sufficient, the correct size has
+            // been stored in ngroups, so we try again.
+            if (err == -1) {
+                supplementaryGIDs.resize(ngroups);
+                err = getgrouplist(pw->pw_name, pw->pw_gid, supplementaryGIDs.data(),
+                                   &ngroups);
+            }
+
+            // If it failed once more, then something must be broken.
             if (err == -1)
-                throw Error("failed to get list of supplementary groups for '%1%'", pw->pw_name);
+                throw Error("failed to get list of supplementary groups for '%1%'",
+                            pw->pw_name);
 
+            // Finally, trim back the GID list to its real size
             supplementaryGIDs.resize(ngroups);
 #endif