[core] stat_cache_entry reference counting

future: should probably create fd cache separate from stat_cache,
        perhaps along w/ http-specific fields like etag and content_type
        and maybe even mmap

src/stat_cache.c

@@ -533,6 +533,7 @@ static stat_cache_entry * stat_cache_entry_init(void) {
     stat_cache_entry *sce = calloc(1, sizeof(*sce));
     force_assert(NULL != sce);
     sce->fd = -1;
+    sce->refcnt = 1;
     return sce;
 }
 
@@ -540,6 +541,8 @@ static void stat_cache_entry_free(void *data) {
     stat_cache_entry *sce = data;
     if (!sce) return;
 
+    if (--sce->refcnt) return;
+
   #ifdef HAVE_FAM_H
     /*(decrement refcnt only;
      * defer cancelling FAM monitor on dir even if refcnt reaches zero)*/
@@ -554,6 +557,15 @@ static void stat_cache_entry_free(void *data) {
     free(sce);
 }
 
+void stat_cache_entry_refchg(void *data, int mod) {
+    /*(expect mod == -1 or mod == 1)*/
+    stat_cache_entry * const sce = data;
+    if (mod < 0 && 1 == sce->refcnt)
+        stat_cache_entry_free(data);
+    else
+        sce->refcnt += mod;
+}
+
 #if defined(HAVE_XATTR) || defined(HAVE_EXTATTR)
 
 static const char *attrname = "Content-Type";
@@ -813,8 +825,15 @@ void stat_cache_update_entry(const char *name, uint32_t len,
             buffer_clear(&sce->content_type);
           #endif
             if (sce->fd >= 0) {
-                close(sce->fd);
-                sce->fd = -1;
+                if (1 == sce->refcnt) {
+                    close(sce->fd);
+                    sce->fd = -1;
+                }
+                else {
+                    --sce->refcnt; /* stat_cache_entry_free(sce); */
+                    (*sptree)->data = sce = stat_cache_entry_init();
+                    buffer_copy_string_len(&sce->name, name, len);
+                }
             }
             sce->st = *st;
         }
@@ -1056,8 +1075,15 @@ stat_cache_entry * stat_cache_get_entry(const buffer *name) {
 	if (sce->fd >= 0) {
 		/* close fd if file changed */
 		if (!stat_cache_stat_eq(&sce->st, &st)) {
-			close(sce->fd);
-			sce->fd = -1;
+			if (1 == sce->refcnt) {
+				close(sce->fd);
+				sce->fd = -1;
+			}
+			else {
+				--sce->refcnt; /* stat_cache_entry_free(sce); */
+				sptree->data = sce = stat_cache_entry_init();
+				buffer_copy_string_len(&sce->name, name->ptr, len);
+			}
 		}
 	}
 

src/stat_cache.h

@@ -12,10 +12,11 @@
 
 typedef struct stat stat_cache_st;
 
-typedef struct {
+typedef struct stat_cache_entry {
     buffer name;
     time_t stat_ts;
     int fd;
+    int refcnt;
 #ifdef HAVE_FAM_H
     void *fam_dir;
 #endif
@@ -35,6 +36,8 @@ int stat_cache_init(struct fdevents *ev, log_error_st *errh);
 __attribute_cold__
 void stat_cache_free(void);
 
+void stat_cache_entry_refchg(void *data, int mod);
+
 __attribute_cold__
 void stat_cache_xattrname (const char *name);