Configurable DB hash seed for SCAN family commands consistency

src/config.c

@@ -3187,6 +3187,15 @@ static int applyClientMaxMemoryUsage(const char **err) {
     return 1;
 }
 
+#define HASH_SEED_MAX_LEN 256
+static int isValidDbHashSeed(sds val, const char **err) {
+    if (sdslen(val) > HASH_SEED_MAX_LEN) {
+        *err = "hash-seed must be less than or equal to " STRINGIFY(HASH_SEED_MAX_LEN) " characters";
+        return 0;
+    }
+    return 1;
+}
+
 standardConfig static_configs[] = {
     /* Bool configs */
     createBoolConfig("rdbchecksum", NULL, IMMUTABLE_CONFIG, server.rdb_checksum, 1, NULL, NULL),
@@ -3277,6 +3286,7 @@ standardConfig static_configs[] = {
     createSDSConfig("primaryauth", "masterauth", MODIFIABLE_CONFIG | SENSITIVE_CONFIG, EMPTY_STRING_IS_NULL, server.primary_auth, NULL, NULL, NULL),
     createSDSConfig("requirepass", NULL, MODIFIABLE_CONFIG | SENSITIVE_CONFIG, EMPTY_STRING_IS_NULL, server.requirepass, NULL, NULL, updateRequirePass),
     createSDSConfig("availability-zone", NULL, MODIFIABLE_CONFIG, ALLOW_EMPTY_STRING, server.availability_zone, "", NULL, NULL),
+    createSDSConfig("hash-seed", NULL, IMMUTABLE_CONFIG, EMPTY_STRING_IS_NULL, server.hash_seed, NULL, isValidDbHashSeed, NULL),
 
     /* Enum Configs */
     createEnumConfig("supervised", NULL, IMMUTABLE_CONFIG, supervised_mode_enum, server.supervised_mode, SUPERVISED_NONE, NULL, NULL),

src/server.c

@@ -7374,8 +7374,13 @@ __attribute__((weak)) int main(int argc, char **argv) {
         sdsfree(options);
     }
     if (server.sentinel_mode) sentinelCheckConfigFile();
+    if (server.hash_seed != NULL) {
+        memset(hashseed, 0, sizeof(hashseed));
+        getHashSeedFromString(hashseed, sizeof(hashseed), server.hash_seed);
+        hashtableSetHashFunctionSeed(hashseed);
+    }
 
-        /* Do system checks */
+    /* Do system checks */
 #ifdef __linux__
     linuxMemoryWarnings();
     sds err_msg = NULL;

src/server.h

@@ -2214,6 +2214,7 @@ struct valkeyServer {
                                                             * dropping packets of a specific type */
     unsigned long cluster_blacklist_ttl;                   /* Duration in seconds that a node is denied re-entry into
                                                             * the cluster after it is forgotten with CLUSTER FORGET. */
+    sds hash_seed;                                         /* Configurable DB hash seed */
     int cluster_slot_stats_enabled;                        /* Cluster slot usage statistics tracking enabled. */
     mstime_t cluster_mf_timeout;                           /* Milliseconds to do a manual failover. */
     unsigned long cluster_slot_migration_log_max_len;      /* Maximum count of migrations to display in the

src/util.c

@@ -1037,6 +1037,21 @@ int ld2string(char *buf, size_t len, long double value, ld2string_mode mode) {
     return 0;
 }
 
+/* Populate the provided seed array by hashing the provided string with SHA256
+ * and copying the first outlen bytes of the digest into the seed buffer. */
+void getHashSeedFromString(unsigned char *seed_array, size_t outlen, const char *value) {
+    SHA256_CTX ctx;
+    unsigned char digest[SHA256_BLOCK_SIZE];
+
+    sha256_init(&ctx);
+    sha256_update(&ctx, (const BYTE *)value, strlen(value));
+    sha256_final(&ctx, digest);
+
+    if (outlen > SHA256_BLOCK_SIZE) outlen = SHA256_BLOCK_SIZE;
+    memcpy(seed_array, digest, outlen);
+}
+
+
 /* Parses a version string on the form "major.minor.patch" and returns an
  * integer on the form 0xMMmmpp. Returns -1 on parse error. */
 int version2num(const char *version) {

src/util.h

@@ -91,6 +91,7 @@ int trimDoubleString(char *buf, size_t len);
 int d2string(char *buf, size_t len, double value);
 int fixedpoint_d2string(char *dst, size_t dstlen, double dvalue, int fractional_digits);
 int ld2string(char *buf, size_t len, long double value, ld2string_mode mode);
+void getHashSeedFromString(unsigned char *seed_array, size_t len, const char *value);
 int double2ll(double d, long long *out);
 int version2num(const char *version);
 int yesnotoi(char *s);

tests/integration/scan-family-consistency.tcl

@@ -0,0 +1,60 @@
+test {scan family consistency with configured hash seed} {
+    start_server {tags {"external:skip"}} {
+
+        set fixed_seed "aabbccddeeffgghh"
+        set shared_overrides [list appendonly no save "" hash-seed $fixed_seed activedefrag no hz 1]
+
+        start_server [list overrides $shared_overrides] {
+            set primary_host [srv 0 host]
+            set primary_port [srv 0 port]
+
+            start_server [list overrides $shared_overrides] {
+                set primary [srv -1 client]
+                set replica [srv 0 client]
+
+                $primary flushall
+                $replica replicaof $primary_host $primary_port
+                wait_for_sync $replica
+
+                set n 50
+                for {set i 0} {$i < $n} {incr i} {
+                    $primary set "k:$i" x
+                    $primary hset h "f:$i" $i
+                    $primary sadd s "m:$i"
+                    $primary zadd z $i "m:$i"
+                }
+
+                wait_for_condition 200 50 {
+                    [$replica dbsize] == [$primary dbsize]
+                } else {
+                    fail "replica did not catch up dbsize (primary=[$primary dbsize], replica=[$replica dbsize])"
+                }
+                set cursor {{0} {}}
+                while {1} {
+                    set primary_cursor_next [$primary scan [lindex $cursor 0]]
+                    set replica_cursor_next [$replica scan [lindex $cursor 0]]
+                    assert_equal $primary_cursor_next $replica_cursor_next
+                    if {[lindex $primary_cursor_next 0] eq "0"} {
+                        assert_equal "0" [lindex $replica_cursor_next 0]
+                        break
+                    }
+                    set cursor $primary_cursor_next
+                }
+
+                foreach {cmd key} {hscan h sscan s zscan z} {
+                    set cursor {{0} {}}
+                    while {1} {
+                        set primary_cursor_next [$primary $cmd $key [lindex $cursor 0]]
+                        set replica_cursor_next [$replica $cmd $key [lindex $cursor 0]]
+                        assert_equal $primary_cursor_next $replica_cursor_next
+                        if {[lindex $primary_cursor_next 0] eq "0"} {
+                            assert_equal "0" [lindex $replica_cursor_next 0]
+                            break
+                        }
+                        set cursor $primary_cursor_next
+                    }
+                }
+            }
+        }
+    }
+}

tests/unit/introspection.tcl

@@ -1225,6 +1225,7 @@ start_server {tags {"introspection"}} {
             disable-thp
             aclfile
             unixsocket
+            hash-seed
             pidfile
             syslog-ident
             appendfilename
@@ -1956,3 +1957,11 @@ test {CONFIG REWRITE handles alias config properly} {
         assert_equal [r config get hash-max-listpack-entries] {hash-max-listpack-entries 100}
     }
 } {} {external:skip}
+
+test {CONFIG hash-seed is immutable and settable at startup} {
+    start_server {tags {"introspection"} overrides {hash-seed aabbccddeeffgghh}} {
+        assert_error "ERR CONFIG SET failed (possibly related to argument 'hash-seed') - can't set immutable config*" {
+            r config set hash-seed newseed
+        }
+    }
+} {} {external:skip}

valkey.conf

@@ -527,6 +527,13 @@ locale-collate ""
 #
 # availability-zone "zone-name"
 
+# Use a fixed hash seed for hashtable instead of a random one.
+# Setting this option makes commands like SCAN return keys in a consistent
+# order across restarts and failovers. The seed can be any string up to 256 characters.
+# The value is immutable and must be provided only at server startup.
+#
+# hash-seed example-seed-val
+
 ################################ SNAPSHOTTING  ################################
 
 # Save the DB to disk.