output-json: drop eve records that are too long

In the situation where the mem buffer cannot be expanded to the
requested size, drop the log message.

For each JSON log context, a warning will be emitted once with a partial
bit of the log record being dropped to identify what event types may be
leading to large log records.

This also fixes the call to MemBufferExpand which is supposed be
passed the amount to expand by, not the new size required.

Ticket: #7300

src/output-json.c

@@ -980,8 +980,22 @@ int OutputJsonBuilderBuffer(
 
     size_t jslen = jb_len(js);
     DEBUG_VALIDATE_BUG_ON(jb_len(js) > UINT32_MAX);
-    if (MEMBUFFER_OFFSET(*buffer) + jslen >= MEMBUFFER_SIZE(*buffer)) {
-        MemBufferExpand(buffer, (uint32_t)jslen);
+    size_t remaining = MEMBUFFER_SIZE(*buffer) - MEMBUFFER_OFFSET(*buffer);
+    if (jslen >= remaining) {
+        size_t expand_by = jslen + 1 - remaining;
+        if (MemBufferExpand(buffer, (uint32_t)expand_by) < 0) {
+            if (!ctx->too_large_warning) {
+                /* Log a warning once, and include enough of the log
+                 * message to hopefully identify the event_type. */
+                char partial[120];
+                size_t partial_len = MIN(sizeof(partial), jslen);
+                memcpy(partial, jb_ptr(js), partial_len - 1);
+                partial[partial_len - 1] = '\0';
+                SCLogWarning("Formatted JSON EVE record too large, will be dropped: %s", partial);
+                ctx->too_large_warning = true;
+            }
+            return 0;
+        }
     }
 
     MemBufferWriteRaw((*buffer), jb_ptr(js), (uint32_t)jslen);

src/output-json.h

@@ -90,6 +90,7 @@ typedef struct OutputJsonThreadCtx_ {
     OutputJsonCtx *ctx;
     LogFileCtx *file_ctx;
     MemBuffer *buffer;
+    bool too_large_warning;
 } OutputJsonThreadCtx;
 
 json_t *SCJsonString(const char *val);