libioencode: allow NULL data parameter in iodecode()

Problem: There is no way to query the decoded length of data using
iodecode() without unpacking the data itself, resulting in unnecessary
allocation and memory copying.

Allow data to be NULL when len is not. In this case unpacking the data
is skipped, but the length is still assigned to to the lenp parameter.

Author: grondo

src/common/libioencode/ioencode.c

@@ -102,11 +102,16 @@ static int decode_data_base64 (char *src,
 {
     ssize_t rc;
     size_t size = base64_decoded_length (srclen);
-    if (!(*datap = malloc (size)))
-        return -1;
-    if ((rc = base64_decode (*datap, size, src, srclen)) < 0)
-        return -1;
-    *lenp = rc;
+    if (datap) {
+        if (!(*datap = malloc (size)))
+            return -1;
+        if ((rc = base64_decode (*datap, size, src, srclen)) < 0)
+            return -1;
+        if (lenp)
+            *lenp = rc;
+    }
+    else if (lenp)
+        *lenp = size;
     return 0;
 }
 
@@ -154,18 +159,21 @@ int iodecode (json_t *o,
         (*streamp) = stream;
     if (rankp)
         (*rankp) = rank;
-    if (datap) {
-        *datap = NULL;
+    if (datap || lenp) {
+        if (datap)
+            *datap = NULL;
         if (data) {
             if (encoding && strcmp (encoding, "base64") == 0) {
                 if (decode_data_base64 (data, len, datap, &bin_len) < 0)
                     goto cleanup;
             }
             else {
                 bin_len = len;
-                if (!(*datap = malloc (bin_len)))
-                    goto cleanup;
-                memcpy (*datap, data, bin_len);
+                if (datap) {
+                    if (!(*datap = malloc (bin_len)))
+                        goto cleanup;
+                    memcpy (*datap, data, bin_len);
+                }
             }
         }
     }

src/common/libioencode/ioencode.h

@@ -29,6 +29,7 @@ json_t *ioencode (const char *stream,
  * - both data and EOF can be available
  * - if no data available, data set to NULL and len to 0
  * - data must be freed after return
+ * - data can be NULL and len non-NULL to retrieve data length
  */
 int iodecode (json_t *o,
               const char **stream,