From 967e49d89f73da6ffd3ee13fc903664cf0107a3a Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Wed, 25 Mar 2026 16:42:21 +0300
Subject: [PATCH] builtin: fix map memory management (fixes #24835)

---
 vlib/builtin/map.c.v                   |  1 +
 vlib/builtin/map.v                     | 25 ++++++++++++++
 vlib/builtin/map_delete_reclaim_test.v | 46 ++++++++++++++++++++++++++
 3 files changed, 72 insertions(+)
 create mode 100644 vlib/builtin/map_delete_reclaim_test.v

diff --git a/vlib/builtin/map.c.v b/vlib/builtin/map.c.v
index b65e61284..9d3543cd1 100644
--- a/vlib/builtin/map.c.v
+++ b/vlib/builtin/map.c.v
@@ -131,6 +131,7 @@ fn (mut d DenseArray) zeros_to_end() {
 		d.deletes = 0
 		// TODO: reallocate instead as more deletes are likely
 		free(d.all_deleted)
+		d.all_deleted = nil
 	}
 	d.len = count
 	old_cap := d.cap
diff --git a/vlib/builtin/map.v b/vlib/builtin/map.v
index 55704b869..92df65f83 100644
--- a/vlib/builtin/map.v
+++ b/vlib/builtin/map.v
@@ -119,6 +119,26 @@ fn (d &DenseArray) has_index(i int) bool {
 	return d.deletes == 0 || unsafe { d.all_deleted[i] } == 0
 }
 
+@[inline]
+fn (mut d DenseArray) trim_deleted_tail() {
+	if d.deletes == 0 {
+		return
+	}
+	for d.len > 0 && unsafe { d.all_deleted[d.len - 1] } != 0 {
+		unsafe {
+			d.all_deleted[d.len - 1] = 0
+		}
+		d.deletes--
+		d.len--
+	}
+	if d.deletes == 0 {
+		unsafe {
+			free(d.all_deleted)
+			d.all_deleted = nil
+		}
+	}
+}
+
 // Make space to append an element and return index
 // The growth-factor is roughly 1.125 `(x + (x >> 3))`
 @[inline]
@@ -672,6 +692,11 @@ fn (m &map) exists(key voidptr) bool {
 
 @[inline]
 fn (mut d DenseArray) delete(i int) {
+	if i == d.len - 1 {
+		d.len--
+		d.trim_deleted_tail()
+		return
+	}
 	if d.deletes == 0 {
 		d.all_deleted = vcalloc(d.cap) // sets to 0
 	}
diff --git a/vlib/builtin/map_delete_reclaim_test.v b/vlib/builtin/map_delete_reclaim_test.v
new file mode 100644
index 000000000..5e35d07e4
--- /dev/null
+++ b/vlib/builtin/map_delete_reclaim_test.v
@@ -0,0 +1,46 @@
+struct DenseArrayLayoutForTest {
+	key_bytes   int
+	value_bytes int
+mut:
+	cap         int
+	len         int
+	deletes     u32
+	all_deleted &u8 = unsafe { nil }
+	keys        &u8 = unsafe { nil }
+	values      &u8 = unsafe { nil }
+}
+
+struct MapLayoutForTest {
+	key_bytes   int
+	value_bytes int
+mut:
+	even_index      u32
+	cached_hashbits u8
+	shift           u8
+	key_values      DenseArrayLayoutForTest
+	metas           &u32 = unsafe { nil }
+	extra_metas     u32
+	has_string_keys bool
+	hash_fn         voidptr
+	key_eq_fn       voidptr
+	clone_fn        voidptr
+	free_fn         voidptr
+pub mut:
+	len int
+}
+
+fn test_map_delete_reclaims_dense_array_tail() {
+	mut m := map[string]string{}
+	raw := unsafe { &MapLayoutForTest(&m) }
+	initial_cap := raw.key_values.cap
+	for i in 0 .. 100 {
+		key := i.str()
+		m[key] = key
+		m.delete(key)
+		assert m.len == 0
+		assert raw.key_values.len == 0
+		assert raw.key_values.deletes == 0
+		assert raw.key_values.cap == initial_cap
+		assert raw.key_values.all_deleted == unsafe { nil }
+	}
+}
-- 
2.39.5