initgroups_dyn allocator optimization

src/libnss.zig

@@ -482,6 +482,7 @@ fn initgroups_dyn(
     const offset = user.additional_gids_offset;
     var vit = compress.varintSliceIteratorMust(db.additional_gids[offset..]);
     var gids = compress.deltaDecompressionIterator(&vit);
+    const remaining = vit.remaining;
 
     // the implementation below is ported from glibc's db-initgroups.c
     // even though we know the size of the groups upfront, I found it too difficult
@@ -493,10 +494,17 @@ fn initgroups_dyn(
                 return c.NSS_STATUS_SUCCESS;
 
             const oldsize = @intCast(usize, size.*);
+            const newsize_want = blk: {
+                // double the oldsize until it is above or equal 'remaining'
+                var res = oldsize;
+                while (res < remaining) res *= 2;
+                break :blk res;
+            };
+
             const newsize: usize = if (limit <= 0)
-                2 * oldsize
+                newsize_want
             else
-                math.min(@intCast(usize, limit), 2 * oldsize);
+                math.min(@intCast(usize, limit), newsize_want);
 
             var buf = groupsp.*[0..oldsize];
             const new_groups = state.initgroups_dyn_allocator.realloc(buf, newsize);