misc: add linked list helpers

This provides macros for managing intrusive doubly linked lists.

There are many ways how to do those in a "generic" way in C. For example
Solaris style lists are pretty nice:

https://github.com/illumos/illumos-gate/blob/master/usr/src/uts/common/sys/list.h
https://github.com/illumos/illumos-gate/blob/master/usr/src/common/list/list.c

I even have an independent implementation of this, which could be ISC
licensed. But I think it's easier to vomit ~100 lines of preprocessor
garbage, which has a lower footprint, and I think it wins slightly on
the side of type safety, simplicity, and ease of use, even if it doesn't
look as magically nice.

misc/linked_list.h

@@ -0,0 +1,107 @@
+#pragma once
+
+#include <stddef.h>
+
+/*
+ * Doubly linked list macros. All of these require that each list item is a
+ * struct, that contains a field, that is another struct with prev/next fields:
+ *
+ *  struct example_item {
+ *      struct {
+ *          struct example_item *prev, *next;
+ *      } mylist;
+ *  };
+ *
+ * And a struct somewhere that represents the "list" and has head/tail fields:
+ *
+ *  struct {
+ *      struct example_item *head, *tail;
+ *  } mylist_var;
+ *
+ * Then you can e.g. insert elements like this:
+ *
+ *  struct example_item item;
+ *  LL_APPEND(mylist, &mylist_var, &item);
+ *
+ * The first macro argument is always the name if the field in the item that
+ * contains the prev/next pointers, in this case struct example_item.mylist.
+ * This was done so that a single item can be in multiple lists.
+ *
+ * The list is started/terminated with NULL. Nothing ever points _to_ the
+ * list head, so the list head memory location can be safely moved.
+ *
+ * General rules are:
+ *  - list head is initialized by setting head/tail to NULL
+ *  - list items do not need to be initialized before inserting them
+ *  - next/prev fields of list items are not cleared when they are removed
+ *  - there's no way to know whether an item is in the list or not (unless
+ *    you clear prev/next on init/removal, _and_ check whether items with
+ *    prev/next==NULL are referenced by head/tail)
+ */
+
+// Insert item at the end of the list (list->tail == item).
+// Undefined behavior if item is already in the list.
+#define LL_APPEND(field, list, item)  do {                              \
+    (item)->field.prev = (list)->tail;                                  \
+    (item)->field.next = NULL;                                          \
+    LL_RELINK_(field, list, item)                                       \
+} while (0)
+
+// Insert item enew after eprev (i.e. eprev->next == enew). If eprev is NULL,
+// then insert it as head (list->head == enew).
+// Undefined behavior if enew is already in the list, or eprev isn't.
+#define LL_INSERT_AFTER(field, list, eprev, enew) do {                  \
+    (enew)->field.prev = (eprev);                                       \
+    (enew)->field.next = (eprev) ? (eprev)->field.next : (list)->head;  \
+    LL_RELINK_(field, list, enew)                                       \
+} while (0)
+
+// Insert item at the start of the list (list->head == item).
+// Undefined behavior if item is already in the list.
+#define LL_PREPEND(field, list, item)  do {                             \
+    (item)->field.prev = NULL;                                          \
+    (item)->field.next = (list)->head;                                  \
+    LL_RELINK_(field, list, item)                                       \
+} while (0)
+
+// Insert item enew before enext (i.e. enew->next == enext). If enext is NULL,
+// then insert it as tail (list->tail == enew).
+// Undefined behavior if enew is already in the list, or enext isn't.
+#define LL_INSERT_BEFORE(field, list, enext, enew) do {                 \
+    (enew)->field.prev = (enext) ? (enext)->field.prev : (list)->tail;  \
+    (enew)->field.next = (enext);                                       \
+    LL_RELINK_(field, list, enew)                                       \
+} while (0)
+
+// Remove the item from the list.
+// Undefined behavior if item is not in the list.
+#define LL_REMOVE(field, list, item) do {                               \
+    if ((item)->field.prev) {                                           \
+        (item)->field.prev->field.next = (item)->field.next;            \
+    } else {                                                            \
+        (list)->head = (item)->field.next;                              \
+    }                                                                   \
+    if ((item)->field.next) {                                           \
+        (item)->field.next->field.prev = (item)->field.prev;            \
+    } else {                                                            \
+        (list)->tail = (item)->field.prev;                              \
+    }                                                                   \
+} while (0)
+
+// Remove all items from the list.
+#define LL_CLEAR(field, list) do {                                      \
+    (list)->head = (list)->tail = NULL;                                 \
+} while (0)
+
+// Internal helper.
+#define LL_RELINK_(field, list, item)                                   \
+    if ((item)->field.prev) {                                           \
+        (item)->field.prev->field.next = (item);                        \
+    } else {                                                            \
+        (list)->head = (item);                                          \
+    }                                                                   \
+    if ((item)->field.next) {                                           \
+        (item)->field.next->field.prev = (item);                        \
+    } else {                                                            \
+        (list)->tail = (item);                                          \
+    }

test/linked_list.c

@@ -0,0 +1,162 @@
+#include "test_helpers.h"
+
+#include "common/common.h"
+#include "misc/linked_list.h"
+
+struct list_item {
+    int v;
+    struct {
+        struct list_item *prev, *next;
+    } list_node;
+};
+
+struct the_list {
+    struct list_item *head, *tail;
+};
+
+static bool do_check_list(struct the_list *lst, int *c, int num_c)
+{
+    if (!lst->head)
+        assert_true(!lst->tail);
+    if (!lst->tail)
+        assert_true(!lst->head);
+
+    for (struct list_item *cur = lst->head; cur; cur = cur->list_node.next) {
+        if (cur->list_node.prev) {
+            assert_true(cur->list_node.prev->list_node.next == cur);
+            assert_true(lst->head != cur);
+        } else {
+            assert_true(lst->head == cur);
+        }
+        if (cur->list_node.next) {
+            assert_true(cur->list_node.next->list_node.prev == cur);
+            assert_true(lst->tail != cur);
+        } else {
+            assert_true(lst->tail == cur);
+        }
+
+        if (num_c < 1)
+            return false;
+        if (c[0] != cur->v)
+            return false;
+
+        num_c--;
+        c++;
+    }
+
+    if (num_c)
+        return false;
+
+    return true;
+}
+
+static void test_linked_list(void **state)
+{
+    struct the_list lst = {0};
+    struct list_item e1 = {1};
+    struct list_item e2 = {2};
+    struct list_item e3 = {3};
+    struct list_item e4 = {4};
+    struct list_item e5 = {5};
+    struct list_item e6 = {6};
+
+#define check_list(...) \
+        assert_true(do_check_list(&lst, (int[]){__VA_ARGS__}, \
+                        sizeof((int[]){__VA_ARGS__}) / sizeof(int)));
+#define check_list_empty() \
+        assert_true(do_check_list(&lst, NULL, 0));
+
+    check_list_empty();
+    LL_APPEND(list_node, &lst, &e1);
+
+    check_list(1);
+    LL_APPEND(list_node, &lst, &e2);
+
+    check_list(1, 2);
+    LL_APPEND(list_node, &lst, &e4);
+
+    check_list(1, 2, 4);
+    LL_CLEAR(list_node, &lst);
+
+    check_list_empty();
+    LL_PREPEND(list_node, &lst, &e4);
+
+    check_list(4);
+    LL_PREPEND(list_node, &lst, &e2);
+
+    check_list(2, 4);
+    LL_PREPEND(list_node, &lst, &e1);
+
+    check_list(1, 2, 4);
+    LL_CLEAR(list_node, &lst);
+
+    check_list_empty();
+    LL_INSERT_BEFORE(list_node, &lst, (struct list_item *)NULL, &e6);
+
+    check_list(6);
+    LL_INSERT_BEFORE(list_node, &lst, (struct list_item *)NULL, &e1);
+
+    check_list(6, 1);
+    LL_INSERT_BEFORE(list_node, &lst, (struct list_item *)NULL, &e2);
+
+    check_list(6, 1, 2);
+    LL_INSERT_BEFORE(list_node, &lst, &e6, &e3);
+
+    check_list(3, 6, 1, 2);
+    LL_INSERT_BEFORE(list_node, &lst, &e6, &e5);
+
+    check_list(3, 5, 6, 1, 2);
+    LL_INSERT_BEFORE(list_node, &lst, &e2, &e4);
+
+    check_list(3, 5, 6, 1, 4, 2);
+    LL_REMOVE(list_node, &lst, &e6);
+
+    check_list(3, 5, 1, 4, 2);
+    LL_REMOVE(list_node, &lst, &e3);
+
+    check_list(5, 1, 4, 2);
+    LL_REMOVE(list_node, &lst, &e2);
+
+    check_list(5, 1, 4);
+    LL_REMOVE(list_node, &lst, &e4);
+
+    check_list(5, 1);
+    LL_REMOVE(list_node, &lst, &e5);
+
+    check_list(1);
+    LL_REMOVE(list_node, &lst, &e1);
+
+    check_list_empty();
+    LL_APPEND(list_node, &lst, &e2);
+
+    check_list(2);
+    LL_REMOVE(list_node, &lst, &e2);
+
+    check_list_empty();
+    LL_INSERT_AFTER(list_node, &lst, (struct list_item *)NULL, &e1);
+
+    check_list(1);
+    LL_INSERT_AFTER(list_node, &lst, (struct list_item *)NULL, &e2);
+
+    check_list(2, 1);
+    LL_INSERT_AFTER(list_node, &lst, (struct list_item *)NULL, &e3);
+
+    check_list(3, 2, 1);
+    LL_INSERT_AFTER(list_node, &lst, &e3, &e4);
+
+    check_list(3, 4, 2, 1);
+    LL_INSERT_AFTER(list_node, &lst, &e4, &e5);
+
+    check_list(3, 4, 5, 2, 1);
+    LL_INSERT_AFTER(list_node, &lst, &e1, &e6);
+
+    check_list(3, 4, 5, 2, 1, 6);
+}
+
+int main(void) {
+    const struct CMUnitTest tests[] = {
+        cmocka_unit_test(test_linked_list),
+    };
+    return cmocka_run_group_tests(tests, NULL, NULL);
+}
+