cache: Drop cuckoo hashing

Now we use a simple closed-addressing cache. Cuckoo hashing was creating
too many issues with race conditions. Fuck that.

Let's see what happens performance wise, we may have to roll back or
come up with another way to implement an efficient multi-threaded cache.

src/cache.c

@@ -29,9 +29,6 @@
 #include "thread-utils.h"
 #include "cache.h"
 
-#define GIT_CACHE_OPENADR 3
-
-
 int git_cache_init(git_cache *cache, size_t size, git_cached_obj_freeptr free_ptr)
 {
 	size_t i;
@@ -58,7 +55,6 @@ int git_cache_init(git_cache *cache, size_t size, git_cached_obj_freeptr free_pt
 	for (i = 0; i < (size + 1); ++i) {
 		git_mutex_init(&cache->nodes[i].lock);
 		cache->nodes[i].ptr = NULL;
-		cache->nodes[i].lru = 0;
 	}
 
 	return GIT_SUCCESS;
@@ -81,93 +77,53 @@ void git_cache_free(git_cache *cache)
 void *git_cache_get(git_cache *cache, const git_oid *oid)
 {
 	const uint32_t *hash;
-	size_t i, pos, found = 0;
 	cache_node *node = NULL;
+	void *result = NULL;
 
 	hash = (const uint32_t *)oid->id;
+	node = &cache->nodes[hash[0] & cache->size_mask];
 
-	for (i = 0; !found && i < GIT_CACHE_OPENADR; ++i) {
-		pos = hash[i] & cache->size_mask;
-		node = &cache->nodes[pos];
-
-		git_mutex_lock(&node->lock);
-		{
-			if (node->ptr && git_cached_obj_compare(node->ptr, oid) == 0) {
-				git_cached_obj_incref(node->ptr);
-				node->lru = ++cache->lru_count;
-				found = 1;
-			}
+	git_mutex_lock(&node->lock);
+	{
+		if (node->ptr && git_cached_obj_compare(node->ptr, oid) == 0) {
+			git_cached_obj_incref(node->ptr);
+			result = node->ptr;
 		}
-		git_mutex_unlock(&node->lock);
 	}
+	git_mutex_unlock(&node->lock);
 
-
-	return found ? node->ptr : NULL;
+	return result;
 }
 
 void *git_cache_try_store(git_cache *cache, void *entry)
 {
-	cache_node *nodes[GIT_CACHE_OPENADR], *lru_node;
 	const uint32_t *hash;
 	const git_oid *oid;
-	size_t i, j, node_count;
+	cache_node *node = NULL;
 
 	oid = &((git_cached_obj*)entry)->oid;
 	hash = (const uint32_t *)oid->id;
+	node = &cache->nodes[hash[0] & cache->size_mask];
 
 	/* increase the refcount on this object, because
 	 * the cache now owns it */
 	git_cached_obj_incref(entry);
-
-	node_count = 0;
-	for (i = 0; i < GIT_CACHE_OPENADR; ++i) {
-		size_t pos = hash[i] & cache->size_mask;
-		cache_node *node = &cache->nodes[pos];
-
-		for (j = 0; j < node_count; ++j)
-			if (nodes[j] == node)
-				break;
-
-		if (j == node_count) {
-			nodes[node_count++] = node;
-			git_mutex_lock(node);
-		}
-	}
-
-	lru_node = nodes[0];
-
-	for (i = 0; i < node_count; ++i) {
-
-		if (nodes[i]->ptr == NULL) {
-			nodes[i]->ptr = entry;
-			nodes[i]->lru = ++cache->lru_count;
-			break;
-		} else if (git_cached_obj_compare(nodes[i]->ptr, oid) == 0) {
-			git_cached_obj_decref(entry, cache->free_obj);
-			entry = nodes[i]->ptr;
-			nodes[i]->lru = ++cache->lru_count;
-			break;
-		}
-
-		if (nodes[i]->lru < lru_node->lru)
-			lru_node = nodes[i];
-	}
-
-	if (i == node_count) {
-		void *old_entry = lru_node->ptr;
-		assert(old_entry);
-
-		git_cached_obj_decref(old_entry, cache->free_obj);
-		lru_node->ptr = entry;
-		lru_node->lru = ++cache->lru_count;
+	git_mutex_lock(&node->lock);
+
+	if (node->ptr == NULL) {
+		node->ptr = entry;
+	} else if (git_cached_obj_compare(node->ptr, oid) == 0) {
+		git_cached_obj_decref(entry, cache->free_obj);
+		entry = node->ptr;
+	} else {
+		git_cached_obj_decref(node->ptr, cache->free_obj);
+		node->ptr = entry;
 	}
 
 	/* increase the refcount again, because we are
 	 * returning it to the user */
 	git_cached_obj_incref(entry);
-
-	for (i = 0; i < node_count; ++i)
-		git_mutex_unlock(&nodes[i]->lock);
+	git_mutex_unlock(&node->lock);
 
 	return entry;
 }

src/cache.h

@@ -19,7 +19,6 @@ typedef struct {
 typedef struct {
 	git_cached_obj *ptr;
 	git_mutex lock;
-	unsigned int lru;
 } cache_node;
 
 typedef struct {