modules/kvs: handle treeobj args in kvs.get request

If a treeobj is provided in a kvs.get request, look up
the key relative to this "snapshot".

Return the treeobj used to look up the key in the response,
whether it be the one provided in the request, or one
representing the current root.

This may be useful in a future API that allows the current
version of a directory to be fetched and then walked as
a snapshot.

src/modules/kvs/kvs.c

@@ -775,10 +775,8 @@ static bool lookup (ctx_t *ctx, json_object *root, wait_t *wait,
     json_object *vp, *dirent, *val = NULL;
     int errnum = 0;
 
-    if (root == NULL) {
-        if (!load (ctx, ctx->rootdir, wait, &root))
-            goto stall;
-    }
+    assert (root != NULL);
+
     if (!strcmp (name, ".")) { /* special case root */
         if ((flags & KVS_PROTO_TREEOBJ)) {
             val = dirent_create ("DIRREF", ctx->rootdir);
@@ -874,6 +872,10 @@ static void get_request_cb (flux_t h, flux_msg_handler_t *w,
     int flags;
     const char *key;
     JSON val;
+    JSON root = NULL;
+    JSON root_dirent = NULL;
+    JSON tmp_dirent = NULL;
+    const char *root_ref = ctx->rootdir;
     wait_t *wait = NULL;
     int lookup_errnum = 0;
     int rc = -1;
@@ -884,11 +886,24 @@ static void get_request_cb (flux_t h, flux_msg_handler_t *w,
         errno = EPROTO;
         goto done;
     }
-    if (kp_tget_dec (in, NULL, &key, &flags) < 0)
+    if (kp_tget_dec (in, &root_dirent, &key, &flags) < 0)
         goto done;
     if (!(wait = wait_create_msg_handler (h, w, msg, get_request_cb, arg)))
         goto done;
-    if (!lookup (ctx, NULL, wait, flags, key, &val, &lookup_errnum))
+    /* If root dirent was specified, lookup corresponding 'root' directory.
+     * Otherwise, use the current root.
+     */
+    if (root_dirent) {
+        if (!Jget_str (root_dirent, "DIRREF", &root_ref)) {
+            errno = EINVAL;
+            goto done;
+        }
+    } else {
+        root_dirent = tmp_dirent = dirent_create ("DIRREF", ctx->rootdir);
+    }
+    if (!load (ctx, root_ref, wait, &root))
+        goto stall;
+    if (!lookup (ctx, root, wait, flags, key, &val, &lookup_errnum))
         goto stall;
     if (lookup_errnum != 0) {
         errno = lookup_errnum;
@@ -898,7 +913,7 @@ static void get_request_cb (flux_t h, flux_msg_handler_t *w,
         errno = ENOENT;
         goto done;
     }
-    if (!(out = kp_rget_enc (NULL, val)))
+    if (!(out = kp_rget_enc (Jget (root_dirent), val)))
         goto done;
     rc = 0;
 done:
@@ -909,6 +924,7 @@ static void get_request_cb (flux_t h, flux_msg_handler_t *w,
 stall:
     Jput (in);
     Jput (out);
+    Jput (tmp_dirent);
 }
 
 static bool compare_json (json_object *o1, json_object *o2)
@@ -928,6 +944,7 @@ static void watch_request_cb (flux_t h, flux_msg_handler_t *w,
     JSON in2 = NULL;
     JSON out = NULL;
     JSON oval, val = NULL;
+    JSON root = NULL;
     flux_msg_t *cpy = NULL;
     const char *key;
     int flags;
@@ -946,7 +963,9 @@ static void watch_request_cb (flux_t h, flux_msg_handler_t *w,
         goto done;
     if (!(wait = wait_create_msg_handler (h, w, msg, watch_request_cb, arg)))
         goto done;
-    if (!lookup (ctx, NULL, wait, flags, key, &val, &lookup_errnum))
+    if (!load (ctx, ctx->rootdir, wait, &root))
+        goto stall;
+    if (!lookup (ctx, root, wait, flags, key, &val, &lookup_errnum))
         goto stall;
     if (lookup_errnum) {
         errno = lookup_errnum;