Modules: initial pool allocator and a LEFTPAD usage example.

src/modules/helloworld.c

@@ -2,6 +2,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <ctype.h>
+#include <string.h>
 
 /* HELLO.SIMPLE is among the simplest commands you can implement.
  * It just returns the currently selected DB id, a functionality which is
@@ -448,6 +449,58 @@ int HelloHCopy_RedisCommand(RedisModuleCtx *ctx, RedisModuleString **argv, int a
     return REDISMODULE_OK;
 }
 
+/* HELLO.LEFTPAD str len ch
+ * This is an implementation of the infamous LEFTPAD function, that
+ * was at the center of an issue with the npm modules system in March 2016.
+ *
+ * LEFTPAD is a good example of using a Redis Modules API called
+ * "pool allocator", that was a famous way to allocate memory in yet another
+ * open source project, the Apache web server.
+ *
+ * The concept is very simple: there is memory that is useful to allocate
+ * only in the context of serving a request, and must be freed anyway when
+ * the callback implementing the command returns. So in that case the module
+ * does not need to retain a reference to these allocations, it is just
+ * required to free the memory before returning. When this is the case the
+ * module can call RedisModule_PoolAlloc() instead, that works like malloc()
+ * but will automatically free the memory when the module callback returns.
+ *
+ * Note that PoolAlloc() does not necessarily require AutoMemory to be
+ * active. */
+int HelloLeftPad_RedisCommand(RedisModuleCtx *ctx, RedisModuleString **argv, int argc) {
+    RedisModule_AutoMemory(ctx); /* Use automatic memory management. */
+    long long padlen;
+
+    if (argc != 4) return RedisModule_WrongArity(ctx);
+
+    if ((RedisModule_StringToLongLong(argv[2],&padlen) != REDISMODULE_OK) ||
+        (padlen< 0)) {
+        return RedisModule_ReplyWithError(ctx,"ERR invalid padding length");
+    }
+    size_t strlen, chlen;
+    const char *str = RedisModule_StringPtrLen(argv[1], &strlen);
+    const char *ch = RedisModule_StringPtrLen(argv[3], &chlen);
+
+    /* If the string is already larger than the target len, just return
+     * the string itself. */
+    if (strlen >= padlen)
+        return RedisModule_ReplyWithString(ctx,argv[1]);
+
+    /* Padding must be a single character in this simple implementation. */
+    if (chlen != 1)
+        return RedisModule_ReplyWithError(ctx,
+            "ERR padding must be a single char");
+
+    /* Here we use our pool allocator, for our throw-away allocation. */
+    padlen -= strlen;
+    char *buf = RedisModule_PoolAlloc(ctx,padlen+strlen);
+    for (size_t j = 0; j < padlen; j++) buf[j] = *ch;
+    memcpy(buf+padlen,str,strlen);
+
+    RedisModule_ReplyWithStringBuffer(ctx,buf,padlen+strlen);
+    return REDISMODULE_OK;
+}
+
 /* This function must be present on each Redis module. It is used in order to
  * register the commands into the Redis server. */
 int RedisModule_OnLoad(RedisModuleCtx *ctx) {
@@ -515,5 +568,9 @@ int RedisModule_OnLoad(RedisModuleCtx *ctx) {
         HelloHCopy_RedisCommand,"write deny-oom",1,1,1) == REDISMODULE_ERR)
         return REDISMODULE_ERR;
 
+    if (RedisModule_CreateCommand(ctx,"hello.leftpad",
+        HelloLeftPad_RedisCommand,"",1,1,1) == REDISMODULE_ERR)
+        return REDISMODULE_ERR;
+
     return REDISMODULE_OK;
 }