Remove 'unlimited' setting for the max-cache-size

Since TTL-based cache cleaning has been removed, an unlimited
max-cache-size would eventually exhaust system memory.

Both 'max-cache-size unlimited;' and 'max-cache-size 0;' now fall
back to the default value (90% of physical memory for recursive
views).

bin/named/server.c

@@ -3872,17 +3872,27 @@ configure_view(dns_view_t *view, dns_viewlist_t *viewlist, cfg_obj_t *config,
 		}
 	}
 
+	if (max_cache_size == 0) {
+		cfg_obj_log(obj, ISC_LOG_WARNING,
+			    "'max-cache-size' can't be zero or unlimited; "
+			    "falling back to default");
+		max_cache_size = SIZE_AS_PERCENT;
+		max_cache_size_percent = 90;
+	}
+
 	if (max_cache_size == SIZE_AS_PERCENT) {
 		uint64_t totalphys = isc_meminfo_totalphys();
 
-		max_cache_size =
-			(size_t)(totalphys * max_cache_size_percent / 100);
 		if (totalphys == 0) {
-			cfg_obj_log(obj, ISC_LOG_WARNING,
+			cfg_obj_log(obj, ISC_LOG_ERROR,
 				    "Unable to determine amount of physical "
-				    "memory, setting 'max-cache-size' to "
-				    "unlimited");
+				    "memory, setting 'max-cache-size' to the "
+				    "minimum value");
+			max_cache_size = 1;
 		} else {
+			max_cache_size = (size_t)(totalphys *
+						  max_cache_size_percent / 100);
+
 			cfg_obj_log(obj, ISC_LOG_INFO,
 				    "'max-cache-size %d%%' "
 				    "- setting to %" PRIu64 "MB "

doc/arm/reference.rst

@@ -3832,9 +3832,12 @@ system.
      - 2 MB for views with :any:`recursion` set to ``no``.
 
    Any positive value smaller than 2 MB is ignored and reset to 2 MB.
-   The keyword ``unlimited``, or the value ``0``, places no limit on the
-   cache size; records are then purged from the cache only when they
-   expire (according to their TTLs).
+
+   .. warning::
+
+       Previously, the keyword ``unlimited``, or the value ``0``, placed
+       no limit on the cache size; this is no longer permitted as
+       TTL-based cleaning has been removed from :iscman:`named`.
 
    .. note::
 
@@ -3847,7 +3850,7 @@ system.
    .. note::
 
        :any:`max-cache-size` does not work reliably for a maximum
-       amount of memory of 100 MB or lower.
+       amount of memory of 256 MB or lower.
 
    Upon startup and reconfiguration, caches with a limited size
    preallocate a small amount of memory (less than 1% of
@@ -3856,10 +3859,13 @@ system.
    internal cache structures.
 
    On systems where detection of the amount of physical memory is not
-   supported, percentage-based values fall back to ``unlimited``. Note
-   that the amount of physical memory available is only detected on
-   startup, so :iscman:`named` does not adjust the cache size limits if the
-   amount of physical memory is changed at runtime.
+   supported, the :iscman:`named` will fail to start.
+
+   .. note::
+
+       The amount of physical memory available is only detected on startup, so
+       :iscman:`named` does not adjust the cache size limits if the amount of
+       physical memory is changed at runtime.
 
    On Linux, the system administrator can use `cgroup`_ (Control Group)
    mechanism to limit the amount of available memory to the process.  This limit

lib/dns/cache.c

@@ -213,11 +213,7 @@ static void
 updatewater(dns_cache_t *cache) {
 	size_t hi = cache->size - (cache->size >> 3); /* ~ 7/8ths. */
 	size_t lo = cache->size - (cache->size >> 2); /* ~ 3/4ths. */
-	if (cache->size == 0U || hi == 0U || lo == 0U) {
-		isc_mem_clearwater(cache->tmctx);
-	} else {
-		isc_mem_setwater(cache->tmctx, hi, lo);
-	}
+	isc_mem_setwater(cache->tmctx, hi, lo);
 }
 
 void
@@ -228,7 +224,7 @@ dns_cache_setcachesize(dns_cache_t *cache, size_t size) {
 	 * Impose a minimum cache size; pathological things happen if there
 	 * is too little room.
 	 */
-	if (size != 0U && size < DNS_CACHE_MINSIZE) {
+	if (size < DNS_CACHE_MINSIZE) {
 		size = DNS_CACHE_MINSIZE;
 	}
 

lib/dns/include/dns/cache.h

@@ -130,7 +130,7 @@ dns_cache_getname(dns_cache_t *cache);
 void
 dns_cache_setcachesize(dns_cache_t *cache, size_t size);
 /*%<
- * Set the maximum cache size.  0 means unlimited.
+ * Set the maximum cache size.
  */
 
 size_t