stop updating filesystems that are dynamically removed from the text

fixes brndnmtthws#208 hopefully properly by tracking which fs_stats entries are
actually being used

as an aside, also fixes the niche but possible issue of slowly running out of
fs_stats entries despite never having too many at one time

src/common.cc

@@ -301,6 +301,9 @@ void update_stuff() {
   /* if you registered a callback with conky::register_cb, this will run it */
   conky::run_all_callbacks();
 
+  /* reset fs_stats in_use flags */
+  clear_fs_stats();
+
 #if !defined(__linux__)
   /* XXX: move the following into the update_meminfo() functions? */
   if (no_buffers.get(*state)) {

src/fs.cc

@@ -79,6 +79,10 @@ int update_fs_stats() {
   if (current_update_time - last_fs_update < 13) { return 0; }
 
   for (i = 0; i < MAX_FS_STATS; ++i) {
+    /* stop monitoring the fs if isn't in_use */
+    if (fs_stats[i].set != 0 && !fs_stats[i].in_use) {
+      memset(&fs_stats[i], 0, sizeof(struct fs_stat));
+    }
     if (fs_stats[i].set != 0) { update_fs_stat(&fs_stats[i]); }
   }
   last_fs_update = current_update_time;
@@ -88,7 +92,7 @@ int update_fs_stats() {
 void clear_fs_stats() {
   unsigned i;
   for (i = 0; i < MAX_FS_STATS; ++i) {
-    memset(&fs_stats[i], 0, sizeof(struct fs_stat));
+    fs_stats[i].in_use = false;
   }
 }
 
@@ -100,6 +104,7 @@ struct fs_stat *prepare_fs_stat(const char *s) {
   for (i = 0; i < MAX_FS_STATS; ++i) {
     if (fs_stats[i].set != 0) {
       if (strncmp(fs_stats[i].path, s, DEFAULT_TEXT_BUFFER_SIZE) == 0) {
+        fs_stats[i].in_use = true;
         return &fs_stats[i];
       }
     } else {
@@ -111,6 +116,7 @@ struct fs_stat *prepare_fs_stat(const char *s) {
     NORM_ERR("too many fs stats");
     return nullptr;
   }
+
   strncpy(next->path, s, DEFAULT_TEXT_BUFFER_SIZE);
   next->set = 1;
   update_fs_stat(next);

src/fs.h

@@ -40,6 +40,7 @@ struct fs_stat {
   long long avail;
   long long free;
   char set;
+  bool in_use;
 };
 
 /* forward declare to make gcc happy (fs.h <-> text_object.h include) */