Merge remote-tracking branch 'origin/master' into auto-uid-allocation

1 files changed, 21 insertions, 4 deletions

diff --git a/src/libstore/lock.cc b/src/libstore/lock.cc
index 02ccb1186..cc3977496 100644
--- a/src/libstore/lock.cc
+++ b/src/libstore/lock.cc
@@ -80,13 +80,30 @@ struct SimpleUserLock : UserLock
                 /* Get the list of supplementary groups of this build
                    user.  This is usually either empty or contains a
                    group such as "kvm".  */
-                lock->supplementaryGIDs.resize(10);
-                int ngroups = lock->supplementaryGIDs.size();
-                int err = getgrouplist(pw->pw_name, pw->pw_gid,
-                    lock->supplementaryGIDs.data(), &ngroups);
+                int ngroups = 32; // arbitrary initial guess
+                lock->supplementaryGIDs.resize(ngroups);
+
+                int err = getgrouplist(
+                    pw->pw_name, pw->pw_gid,
+                    lock->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) {
+                    lock->supplementaryGIDs.resize(ngroups);
+                    err = getgrouplist(
+                        pw->pw_name, pw->pw_gid,
+                        lock->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 '%s'", pw->pw_name);
 
+                // Finally, trim back the GID list to its real size.
                 lock->supplementaryGIDs.resize(ngroups);
                 #endif