server.c: Fix undefined behaviour on >1 clients

server.c kept an array of all of its active clients, calling realloc(3)
everytime its size had to be modified. However, reallocating this array
had the undesired consequence of moving other active clients to other
memory locations.

Potentially, this would result in dangling pointers from other
components that also kept pointers to struct server_client instances
e.g.: handler.c.

For this reason, the array-based approach has been completely dropped,
in favour of a doubly-linked list.

server.c

@@ -22,9 +22,8 @@ struct server
     {
         int fd;
         bool write;
+        struct server_client *prev, *next;
     } *c;
-
-    size_t n;
 };
 
 int server_close(struct server *const s)
@@ -44,13 +43,11 @@ int server_client_close(struct server *const s, struct server_client *const c)
 {
     int ret = 0;
 
-    for (size_t i = 0; i < s->n; i++)
+    for (struct server_client *ref = s->c; ref; ref = ref->next)
     {
-        struct server_client *const ref = &s->c[i];
-
         if (c == ref)
         {
-            const size_t n = s->n - 1;
+            struct server_client *const next = ref->next;
 
             if ((ret = close(c->fd)))
             {
@@ -58,28 +55,15 @@ int server_client_close(struct server *const s, struct server_client *const c)
                     __func__, strerror(errno));
                 return -1;
             }
-            else if (n)
-            {
-                memcpy(ref, ref + 1, (s->n - i - 1) * sizeof *ref);
-
-                struct server_client *const c = realloc(s->c, n * sizeof *s->c);
-
-                if (!c)
-                {
-                    fprintf(stderr, "%s: realloc(3): %s\n",
-                        __func__, strerror(errno));
-                    return -1;
-                }
-
-                s->c = c;
-            }
+            else if (ref->prev)
+                ref->prev->next = next;
             else
-            {
-                free(s->c);
-                s->c = NULL;
-            }
+                s->c = next;
 
-            s->n = n;
+            if (next)
+                next->prev = ref->prev;
+
+            free(ref);
             break;
         }
     }
@@ -136,24 +120,30 @@ static struct server_client *alloc_client(struct server *const s)
         return NULL;
     }
 
-    const size_t n = s->n + 1;
-    struct server_client *const clients = realloc(s->c, n * sizeof *s->c);
+    struct server_client *const c = malloc(sizeof *c);
 
-    if (!clients)
+    if (!c)
     {
         fprintf(stderr, "%s: realloc(3): %s\n", __func__, strerror(errno));
         return NULL;
     }
 
-    struct server_client *const c = &clients[s->n];
-
     *c = (const struct server_client)
     {
         .fd = fd
     };
 
-    s->c = clients;
-    s->n = n;
+    if (!s->c)
+        s->c = c;
+    else
+        for (struct server_client *ref = s->c; ref; ref = ref->next)
+            if (!ref->next)
+            {
+                ref->next = c;
+                c->prev = ref;
+                break;
+            }
+
     return c;
 }
 
@@ -180,11 +170,22 @@ static void handle_signal(const int signum)
     }
 }
 
+static size_t get_clients(const struct server *const s)
+{
+    size_t ret = 0;
+
+    for (const struct server_client *c = s->c; c; c = c->next)
+        ret++;
+
+    return ret;
+}
+
 struct server_client *server_poll(struct server *const s, bool *const io,
     bool *const exit)
 {
     struct server_client *ret = NULL;
-    const nfds_t n = s->n + 1;
+    const size_t n_clients = get_clients(s);
+    const nfds_t n = n_clients + 1;
     struct pollfd *const fds = malloc(n * sizeof *fds);
 
     if (!fds)
@@ -202,11 +203,11 @@ struct server_client *server_poll(struct server *const s, bool *const io,
         .events = POLLIN
     };
 
-    for (size_t i = 0, j = 1; i < s->n; i++, j++)
+    for (struct {const struct server_client *c; size_t j;}
+        _ = {.c = s->c, .j = 1}; _.c; _.c = _.c->next, _.j++)
     {
-        struct pollfd *const p = &fds[j];
-        const struct server_client *const c = &s->c[i];
-        const int fd = c->fd;
+        struct pollfd *const p = &fds[_.j];
+        const int fd = _.c->fd;
 
         *p = (const struct pollfd)
         {
@@ -214,7 +215,7 @@ struct server_client *server_poll(struct server *const s, bool *const io,
             .events = POLLIN
         };
 
-        if (c->write)
+        if (_.c->write)
             p->events |= POLLOUT;
     }
 
@@ -251,22 +252,21 @@ again:
         fprintf(stderr, "%s: poll(2) returned zero\n", __func__);
         goto end;
     }
-
-    if (sfd->revents)
+    else if (sfd->revents)
     {
         ret = alloc_client(s);
         goto end;
     }
 
-    for (size_t i = 0, j = 1; i < s->n; i++, j++)
+    for (struct {struct server_client *c; size_t j;}
+        _ = {.c = s->c, .j = 1}; _.c; _.c = _.c->next, _.j++)
     {
-        const struct pollfd *const p = &fds[j];
-        struct server_client *const c = &s->c[i];
+        const struct pollfd *const p = &fds[_.j];
 
         if (p->revents)
         {
             *io = true;
-            ret = c;
+            ret = _.c;
             goto end;
         }
     }