DEBUG: new "ziplist" subcommand added. Dumps a ziplist on stdout.

The commit improves ziplistRepr() and adds a new debugging subcommand so
that we can trigger the dump directly from the Redis API.
This command capability was used while investigating issue #3684.

src/debug.c

@@ -280,6 +280,8 @@ void debugCommand(client *c) {
         blen++; addReplyStatus(c,
         "sdslen <key> -- Show low level SDS string info representing key and value.");
         blen++; addReplyStatus(c,
+        "ziplist <key> -- Show low level info about the ziplist encoding.");
+        blen++; addReplyStatus(c,
         "populate <count> [prefix] -- Create <count> string keys named key:<num>. If a prefix is specified is used instead of the 'key' prefix.");
         blen++; addReplyStatus(c,
         "digest   -- Outputs an hex signature representing the current DB content.");
@@ -418,6 +420,18 @@ void debugCommand(client *c) {
                 (long long) sdsavail(val->ptr),
                 (long long) getStringObjectSdsUsedMemory(val));
         }
+    } else if (!strcasecmp(c->argv[1]->ptr,"ziplist") && c->argc == 3) {
+        robj *o;
+
+        if ((o = objectCommandLookupOrReply(c,c->argv[2],shared.nokeyerr))
+                == NULL) return;
+
+        if (o->encoding != OBJ_ENCODING_ZIPLIST) {
+            addReplyError(c,"Not an sds encoded string.");
+        } else {
+            ziplistRepr(o->ptr);
+            addReplyStatus(c,"Ziplist structure printed on stdout");
+        }
     } else if (!strcasecmp(c->argv[1]->ptr,"populate") &&
                (c->argc == 3 || c->argc == 4)) {
         long keys, j;

src/server.h

@@ -1655,6 +1655,8 @@ robj *lookupKeyWrite(redisDb *db, robj *key);
 robj *lookupKeyReadOrReply(client *c, robj *key, robj *reply);
 robj *lookupKeyWriteOrReply(client *c, robj *key, robj *reply);
 robj *lookupKeyReadWithFlags(redisDb *db, robj *key, int flags);
+robj *objectCommandLookup(client *c, robj *key);
+robj *objectCommandLookupOrReply(client *c, robj *key, robj *reply);
 #define LOOKUP_NONE 0
 #define LOOKUP_NOTOUCH (1<<0)
 void dbAdd(redisDb *db, robj *key, robj *val);

src/ziplist.c

@@ -1029,7 +1029,7 @@ void ziplistRepr(unsigned char *zl) {
 
     printf(
         "{total bytes %d} "
-        "{length %u}\n"
+        "{num entries %u}\n"
         "{tail offset %u}\n",
         intrev32ifbe(ZIPLIST_BYTES(zl)),
         intrev16ifbe(ZIPLIST_LENGTH(zl)),
@@ -1038,16 +1038,15 @@ void ziplistRepr(unsigned char *zl) {
     while(*p != ZIP_END) {
         zipEntry(p, &entry);
         printf(
-            "{"
-                "addr 0x%08lx, "
-                "index %2d, "
-                "offset %5ld, "
-                "rl: %5u, "
-                "hs %2u, "
-                "pl: %5u, "
-                "pls: %2u, "
-                "payload %5u"
-            "} ",
+            "{\n"
+                "\taddr 0x%08lx,\n"
+                "\tindex %2d,\n"
+                "\toffset %5ld,\n"
+                "\thdr+entry len: %5u,\n"
+                "\thdr len%2u,\n"
+                "\tprevrawlen: %5u,\n"
+                "\tprevrawlensize: %2u,\n"
+                "\tpayload %5u\n",
             (long unsigned)p,
             index,
             (unsigned long) (p-zl),
@@ -1056,8 +1055,14 @@ void ziplistRepr(unsigned char *zl) {
             entry.prevrawlen,
             entry.prevrawlensize,
             entry.len);
+        printf("\tbytes: ");
+        for (unsigned int i = 0; i < entry.headersize+entry.len; i++) {
+            printf("%02x|",p[i]);
+        }
+        printf("\n");
         p += entry.headersize;
         if (ZIP_IS_STR(entry.encoding)) {
+            printf("\t[str]");
             if (entry.len > 40) {
                 if (fwrite(p,40,1,stdout) == 0) perror("fwrite");
                 printf("...");
@@ -1066,9 +1071,9 @@ void ziplistRepr(unsigned char *zl) {
                     fwrite(p,entry.len,1,stdout) == 0) perror("fwrite");
             }
         } else {
-            printf("%lld", (long long) zipLoadInteger(p,entry.encoding));
+            printf("\t[int]%lld", (long long) zipLoadInteger(p,entry.encoding));
         }
-        printf("\n");
+        printf("\n}\n");
         p += entry.len;
         index++;
     }

src/ziplist.h

@@ -48,6 +48,7 @@ unsigned int ziplistCompare(unsigned char *p, unsigned char *s, unsigned int sle
 unsigned char *ziplistFind(unsigned char *p, unsigned char *vstr, unsigned int vlen, unsigned int skip);
 unsigned int ziplistLen(unsigned char *zl);
 size_t ziplistBlobLen(unsigned char *zl);
+void ziplistRepr(unsigned char *zl);
 
 #ifdef REDIS_TEST
 int ziplistTest(int argc, char *argv[]);