Introduce repl_slave_ignore_maxmemory flag internally.

Note: this breaks backward compatibility with Redis 4, since now slaves
by default are exact copies of masters and do not try to evict keys
independently.

src/evict.c

@@ -444,6 +444,10 @@ int getMaxmemoryState(size_t *total, size_t *logical, size_t *tofree, float *lev
  * Otehrwise if we are over the memory limit, but not enough memory
  * was freed to return back under the limit, the function returns C_ERR. */
 int freeMemoryIfNeeded(void) {
+    /* By default slaves should ignore maxmemory and just be masters excat
+     * copies. */
+    if (server.masterhost && server.repl_slave_ignore_maxmemory) return C_OK;
+
     size_t mem_reported, mem_tofree, mem_freed;
     mstime_t latency, eviction_latency;
     long long delta;

src/server.c

@@ -1649,6 +1649,7 @@ void initServerConfig(void) {
     server.repl_syncio_timeout = CONFIG_REPL_SYNCIO_TIMEOUT;
     server.repl_serve_stale_data = CONFIG_DEFAULT_SLAVE_SERVE_STALE_DATA;
     server.repl_slave_ro = CONFIG_DEFAULT_SLAVE_READ_ONLY;
+    server.repl_slave_ignore_maxmemory = CONFIG_DEFAULT_SLAVE_IGNORE_MAXMEMORY;
     server.repl_slave_lazy_flush = CONFIG_DEFAULT_SLAVE_LAZY_FLUSH;
     server.repl_down_since = 0; /* Never connected, repl is down since EVER. */
     server.repl_disable_tcp_nodelay = CONFIG_DEFAULT_REPL_DISABLE_TCP_NODELAY;

src/server.h

@@ -131,6 +131,7 @@ typedef long long mstime_t; /* millisecond time type. */
 #define CONFIG_DEFAULT_REPL_DISKLESS_SYNC_DELAY 5
 #define CONFIG_DEFAULT_SLAVE_SERVE_STALE_DATA 1
 #define CONFIG_DEFAULT_SLAVE_READ_ONLY 1
+#define CONFIG_DEFAULT_SLAVE_IGNORE_MAXMEMORY 1
 #define CONFIG_DEFAULT_SLAVE_ANNOUNCE_IP NULL
 #define CONFIG_DEFAULT_SLAVE_ANNOUNCE_PORT 0
 #define CONFIG_DEFAULT_REPL_DISABLE_TCP_NODELAY 0
@@ -1157,6 +1158,7 @@ struct redisServer {
     time_t repl_transfer_lastio; /* Unix time of the latest read, for timeout */
     int repl_serve_stale_data; /* Serve stale data when link is down? */
     int repl_slave_ro;          /* Slave is read only? */
+    int repl_slave_ignore_maxmemory;    /* If true slaves do not evict. */
     time_t repl_down_since; /* Unix time at which link with master went down */
     int repl_disable_tcp_nodelay;   /* Disable TCP_NODELAY after SYNC? */
     int slave_priority;             /* Reported in INFO and used by Sentinel. */