Fix apc_sma_get_avail_size() to look for a contiguous block of memory to make expunge more reliable (#532)

apc_sma_get_avail_size() is used by apc_cache_default_expunge() to check whether enough memory is available to store a new cache entry after expired entries are removed. If there is not enough memory available, a real expunge is performed to be able to store the new entry.

The problem is that apc_sma_get_avail_size() only checks the total amount of available memory, which is usually divided into several fragments of memory. However, we need a contiguous block of memory to store a cache entry.

This pull request fixes apc_sma_get_avail_size() to check if a continuous block of memory is available to store a new entry, which solves the problem.

Prior to this fix, the result was that in situations with fragmentation, new cache entries cannot be saved and PHP functions such as apcu_store() return false, which is not the expected behavior.

Author: madmajestro

apc_sma.c

@@ -631,11 +631,29 @@ PHP_APCU_API zend_bool apc_sma_get_avail_size(apc_sma_t* sma, size_t size) {
 	int32_t i;
 
 	for (i = 0; i < sma->num; i++) {
-		sma_header_t* header = SMA_HDR(sma, i);
-		if (header->avail > size) {
-			return 1;
+		sma_header_t *shmaddr = SMA_HDR(sma, i);
+
+		/* If total size of available memory is too small, we can skip the contiguous-block check */
+		if (shmaddr->avail < size) {
+			continue;
+		}
+
+		SMA_LOCK(sma, i);
+		block_t *cur = BLOCKAT(ALIGNWORD(sizeof(sma_header_t)));
+
+		/* Look for a contiguous block of memory */
+		while (cur->fnext) {
+			cur = BLOCKAT(cur->fnext);
+
+			if (cur->size >= size) {
+				SMA_UNLOCK(sma, i);
+				return 1;
+			}
 		}
+
+		SMA_UNLOCK(sma, i);
 	}
+
 	return 0;
 }
 

apc_sma.h

@@ -138,7 +138,7 @@ PHP_APCU_API void apc_sma_free_info(apc_sma_t* sma, apc_sma_info_t* info);
 PHP_APCU_API size_t apc_sma_get_avail_mem(apc_sma_t* sma);
 
 /*
-* apc_sma_api_get_avail_size will return true if at least size bytes are available to the sma
+* apc_sma_api_get_avail_size will return true if at least size contiguous bytes are available to the sma
 */
 PHP_APCU_API zend_bool apc_sma_get_avail_size(apc_sma_t* sma, size_t size);
 

tests/apc_020.phpt

@@ -17,7 +17,7 @@ apc.shm_size=1M
 apcu_store("no_ttl_unaccessed", 12);
 apcu_store("no_ttl_accessed", 24);
 apcu_store("ttl", 42, 3);
-apcu_store("dummy", "xxx");
+apcu_store("dummy", str_repeat('x', 1000));
 
 apcu_inc_request_time(1);
 apcu_fetch("no_ttl_accessed");