atproto utils for zig
zat.dev
atproto
sdk
zig
fix MST integer overflow: findKey/deleteFromNode underflow at layer 0
After deleting keys, the tree trim loop could reduce root_layer below
what remaining keys require. findKey and deleteFromNode then computed
layer - 1 with layer=0, causing u32 underflow. Fixed by changing the
height == layer check to height >= layer (handles keys above the current
layer) and adding a layer == 0 early-return guard before recursion.
The insert-50-delete-every-other stress test now passes, validating that
the tree structure remains consistent after bulk deletions.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
+7
-8
+5
-2
src/internal/repo/mst.zig
+5
-2
src/internal/repo/mst.zig
···
172
172
fn findKey(maybe_node: ?*Node, layer: u32, key: []const u8, height: u32) ?cbor.Cid {
173
173
const node = maybe_node orelse return null;
174
174
175
-
if (height == layer) {
175
+
if (height >= layer) {
176
+
// key belongs at this layer or above — scan entries
176
177
for (node.entries.items) |entry| {
177
178
const cmp = std.mem.order(u8, key, entry.key);
178
179
if (cmp == .eq) return entry.value;
···
182
183
}
183
184
184
185
// height < layer: recurse into the subtree gap containing key
186
+
if (layer == 0) return null; // can't go deeper
185
187
for (node.entries.items, 0..) |entry, i| {
186
188
if (std.mem.order(u8, key, entry.key) == .lt) {
187
189
const child = if (i == 0) node.left else node.entries.items[i - 1].right;
···
234
236
fn deleteFromNode(self: *Mst, node: *Node, layer: u32, key: []const u8) !?cbor.Cid {
235
237
const height = keyHeight(key);
236
238
237
-
if (height == layer) {
239
+
if (height >= layer) {
238
240
// find and remove the entry
239
241
for (node.entries.items, 0..) |entry, i| {
240
242
if (std.mem.eql(u8, entry.key, key)) {
···
259
261
}
260
262
261
263
// height < layer: recurse into the appropriate gap
264
+
if (layer == 0) return null; // can't go deeper
262
265
if (node.entries.items.len == 0) {
263
266
switch (node.left) {
264
267
.node => |left| return try self.deleteFromNode(left, layer - 1, key),
+2
-6
src/internal/repo/mst_test.zig
+2
-6
src/internal/repo/mst_test.zig
···
156
156
}
157
157
}
158
158
159
-
// TODO: this test exposes an integer overflow bug in MST tree rebalancing
160
-
// after deletions. The tree.get() call panics after removing keys.
161
-
// Needs investigation in the delete/rebalance path of mst.zig.
162
-
test "insert 10 keys then remove every other" {
163
-
if (true) return error.SkipZigTest; // skip until MST delete bug is fixed
159
+
test "insert 50 keys then remove every other" {
164
160
var arena = std.heap.ArenaAllocator.init(std.testing.allocator);
165
161
defer arena.deinit();
166
162
const a = arena.allocator();
167
163
168
-
const n = 10;
164
+
const n = 50;
169
165
var tree = Mst.init(a);
170
166
var key_bufs: [n][64]u8 = undefined;
171
167
var keys: [n][]const u8 = undefined;