Fix memory leak of old curr_reg on san_bump_grow_locked failure

When san_bump_grow_locked fails, it sets sba->curr_reg to NULL.
The old curr_reg (saved in to_destroy) was never freed or restored,
leaking the virtual memory extent. Restore sba->curr_reg from
to_destroy on failure so the old region remains usable.

test/unit/san_bump.c

@@ -4,6 +4,50 @@
 #include "jemalloc/internal/arena_structs.h"
 #include "jemalloc/internal/san_bump.h"
 
+static extent_hooks_t *san_bump_default_hooks;
+static extent_hooks_t  san_bump_hooks;
+static bool            fail_retained_alloc;
+static unsigned        retained_alloc_fail_calls;
+
+static void *
+san_bump_fail_alloc_hook(extent_hooks_t *UNUSED extent_hooks, void *new_addr,
+    size_t size, size_t alignment, bool *zero, bool *commit,
+    unsigned arena_ind) {
+	if (fail_retained_alloc && new_addr == NULL
+	    && size >= SBA_RETAINED_ALLOC_SIZE) {
+		retained_alloc_fail_calls++;
+		return NULL;
+	}
+	return san_bump_default_hooks->alloc(san_bump_default_hooks, new_addr,
+	    size, alignment, zero, commit, arena_ind);
+}
+
+static void
+install_san_bump_fail_alloc_hooks(unsigned arena_ind) {
+	size_t          hooks_mib[3];
+	size_t          hooks_miblen = sizeof(hooks_mib) / sizeof(size_t);
+	size_t          old_size = sizeof(extent_hooks_t *);
+	size_t          new_size = sizeof(extent_hooks_t *);
+	extent_hooks_t *new_hooks;
+	extent_hooks_t *old_hooks;
+
+	expect_d_eq(
+	    mallctlnametomib("arena.0.extent_hooks", hooks_mib, &hooks_miblen),
+	    0, "Unexpected mallctlnametomib() failure");
+	hooks_mib[1] = (size_t)arena_ind;
+	expect_d_eq(mallctlbymib(hooks_mib, hooks_miblen, (void *)&old_hooks,
+	                &old_size, NULL, 0),
+	    0, "Unexpected extent_hooks error");
+
+	san_bump_default_hooks = old_hooks;
+	san_bump_hooks = *old_hooks;
+	san_bump_hooks.alloc = san_bump_fail_alloc_hook;
+	new_hooks = &san_bump_hooks;
+	expect_d_eq(mallctlbymib(hooks_mib, hooks_miblen, NULL, NULL,
+	                (void *)&new_hooks, new_size),
+	    0, "Unexpected extent_hooks install failure");
+}
+
 TEST_BEGIN(test_san_bump_alloc) {
 	test_skip_if(!maps_coalesce || !opt_retain);
 
@@ -69,6 +113,48 @@ TEST_BEGIN(test_san_bump_alloc) {
 }
 TEST_END
 
+TEST_BEGIN(test_failed_grow_preserves_curr_reg) {
+	test_skip_if(!maps_coalesce || !opt_retain);
+
+	tsdn_t *tsdn = tsdn_fetch();
+
+	san_bump_alloc_t sba;
+	san_bump_alloc_init(&sba);
+
+	unsigned arena_ind = do_arena_create(0, 0);
+	assert_u_ne(arena_ind, UINT_MAX, "Failed to create an arena");
+	install_san_bump_fail_alloc_hooks(arena_ind);
+
+	arena_t *arena = arena_get(tsdn, arena_ind, false);
+	pac_t   *pac = &arena->pa_shard.pac;
+
+	size_t   small_alloc_size = PAGE * 16;
+	edata_t *edata = san_bump_alloc(tsdn, &sba, pac, pac_ehooks_get(pac),
+	    small_alloc_size, /* zero */ false);
+	expect_ptr_not_null(edata, "Initial san_bump allocation failed");
+	expect_ptr_not_null(sba.curr_reg,
+	    "Expected retained region remainder after initial allocation");
+
+	fail_retained_alloc = true;
+	retained_alloc_fail_calls = 0;
+
+	edata_t *failed = san_bump_alloc(tsdn, &sba, pac, pac_ehooks_get(pac),
+	    SBA_RETAINED_ALLOC_SIZE, /* zero */ false);
+	expect_ptr_null(failed, "Expected retained grow allocation failure");
+	expect_u_eq(retained_alloc_fail_calls, 1,
+	    "Expected exactly one failed retained allocation attempt");
+
+	edata_t *reused = san_bump_alloc(tsdn, &sba, pac, pac_ehooks_get(pac),
+	    small_alloc_size, /* zero */ false);
+	expect_ptr_not_null(
+	    reused, "Expected allocator to reuse preexisting current region");
+	expect_u_eq(retained_alloc_fail_calls, 1,
+	    "Reuse path should not attempt another retained grow allocation");
+
+	fail_retained_alloc = false;
+}
+TEST_END
+
 TEST_BEGIN(test_large_alloc_size) {
 	test_skip_if(!maps_coalesce || !opt_retain);
 
@@ -105,5 +191,6 @@ TEST_END
 
 int
 main(void) {
-	return test(test_san_bump_alloc, test_large_alloc_size);
+	return test(test_san_bump_alloc, test_failed_grow_preserves_curr_reg,
+	    test_large_alloc_size);
 }