Add a note to conf about the dangers of modifying dir at runtime (#887)

We've had security issues in the past with it, which is why
we marked it as PROTECTED. But, modifying during runtime
is also a dangerous action. For example, when child processes
are running, persistent temp files and log files may have
unexpected effects.

A scenario for modifying dir at runtime is to migrate a disk
failure, such as using disk-based replication to migrate a node,
writing nodes.conf to save the cluster configuration.

We decided to leave it as is and add a note in the conf
about the dangers of modifying dir at runtime.

Signed-off-by: Binbin <[email protected]>

tests/unit/introspection.tcl

@@ -980,6 +980,13 @@ start_server {tags {"introspection"}} {
         }
     } {} {external:skip}
 
+    test {valkey-server command line arguments - dir multiple times} {
+        start_server {config "default.conf" args {--dir "./" --dir "./"}} {
+            r config get dir
+            assert_equal {PONG} [r ping]
+        }
+    } {} {external:skip}
+
     # Config file at this point is at a weird state, and includes all
     # known keywords. Might be a good idea to avoid adding tests here.
 }

valkey.conf

@@ -582,6 +582,9 @@ rdb-del-sync-files no
 
 # The working directory.
 #
+# The server log is written relative this directory, if the 'logfile'
+# configuration directive is a relative path.
+#
 # The DB will be written inside this directory, with the filename specified
 # above using the 'dbfilename' configuration directive.
 #
@@ -591,6 +594,9 @@ rdb-del-sync-files no
 # 'cluster-config-file' configuration directive is a relative path.
 #
 # Note that you must specify a directory here, not a file name.
+# Note that modifying 'dir' during runtime may have unexpected behavior,
+# for example when a child process is running, related file operations may
+# have unexpected effects.
 dir ./
 
 ################################# REPLICATION #################################