raft.h: Future-proof raft_message.

Author: Mathieu Borderé

include/raft.h

@@ -215,6 +215,7 @@ struct raft_request_vote
     raft_index last_log_term;  /* Term of log entry at last_log_index. */
     bool disrupt_leader;       /* True if current leader should be discarded. */
     bool pre_vote;             /* True if this is a pre-vote request. */
+    uint64_t reserved[8];      /* Future use */
 };
 
 /**
@@ -225,6 +226,7 @@ struct raft_request_vote_result
     raft_term term;        /* Receiver's current term (candidate updates itself). */
     bool vote_granted;     /* True means candidate received vote. */
     raft_tribool pre_vote; /* The response to a pre-vote RequestVote or not. */
+    uint64_t reserved[8];  /* Future use */
 };
 
 /**
@@ -241,6 +243,7 @@ struct raft_append_entries
     raft_index leader_commit;   /* Leader's commit index. */
     struct raft_entry *entries; /* Log entries to append. */
     unsigned n_entries;         /* Size of the log entries array. */
+    uint64_t reserved[8];       /* Future use */
 };
 
 /**
@@ -251,6 +254,7 @@ struct raft_append_entries_result
     raft_term term;            /* Receiver's current_term. */
     raft_index rejected;       /* If non-zero, the index that was rejected. */
     raft_index last_log_index; /* Receiver's last log entry index, as hint. */
+    uint64_t reserved[8];      /* Future use */
 };
 
 /**
@@ -264,6 +268,7 @@ struct raft_install_snapshot
     struct raft_configuration conf; /* Config as of last_index. */
     raft_index conf_index;          /* Commit index of conf. */
     struct raft_buffer data;        /* Raw snapshot data. */
+    uint64_t reserved[8];           /* Future use */
 };
 
 /**
@@ -277,6 +282,7 @@ struct raft_timeout_now
     raft_term term;            /* Leader's term. */
     raft_index last_log_index; /* Index of leader's last log entry. */
     raft_index last_log_term;  /* Term of log entry at last_log_index. */
+    uint64_t reserved[8];      /* Future use */
 };
 
 /**
@@ -293,6 +299,22 @@ enum {
 
 /**
  * A single RPC message that can be sent or received over the network.
+ *
+ * Updating to a new version of an RPC message is done based on the encoded size
+ * of the message, see e.g. `sizeofRequestVoteV1` in uv_encoding.c
+ *
+ * The RPC message types all have a `reserved` field. When extending such a
+ * struct, and thus shrinking the `reserved` array, one should be careful not to
+ * give meaning the `zero` value because the raft implementation needs to know
+ * whether to other side knows the new field or not. `zero` means the other side
+ * does not know the new field/functionality (this is why raft_tribool exists).
+ * Implementers MUST always zero out unknown fields before passing them to raft.
+ *
+ * Adding a new message to the union is supported, as long as its size is
+ * smaller than the largest struct in the union, which should be plenty.
+ * TODO: Add a compile-time check to verify this once a new message is actually
+ * needed.
+ *
  */
 struct raft_message
 {
@@ -307,6 +329,7 @@ struct raft_message
         struct raft_install_snapshot install_snapshot;
         struct raft_timeout_now timeout_now;
     };
+    uint64_t reserved[8];       /* Future use */
 };
 
 /**

src/fixture.c

@@ -376,7 +376,7 @@ static void ioFlushSend(struct io *io, struct send *send)
         goto out;
     }
 
-    transmit = raft_malloc(sizeof *transmit);
+    transmit = raft_calloc(1, sizeof *transmit);
     assert(transmit != NULL);
 
     transmit->type = TRANSMIT;

src/uv_encoding.c

@@ -486,6 +486,8 @@ int uvDecodeMessage(const unsigned long type,
         return RAFT_INVALID;
     }
 
+    memset(message, 0, sizeof(*message));
+
     message->type = (unsigned short)type;
 
     *payload_len = 0;