streaming/buffer: handle and document slide errors

Slide error may happen if the region we're sliding starts to overlap
with the next region. If we can't temporary grow the current region
to merge with the next region, keep the regions separate.

src/util-streaming-buffer.c

@@ -844,10 +844,14 @@ static inline void StreamingBufferSlideToOffsetWithRegions(
         const uint32_t s = slide_offset - to_shift->stream_offset;
         if (s > 0) {
             const uint32_t new_data_size = to_shift->buf_size - s;
-            const uint32_t new_mem_size = ToNextMultipleOf(new_data_size, cfg->buf_size);
+            uint32_t new_mem_size = ToNextMultipleOf(new_data_size, cfg->buf_size);
 
             /* see if after the slide we'd overlap with the next region. If so, we need
-             * to consolidate them into one. */
+             * to consolidate them into one. Error handling is a bit peculiar. We need
+             * to grow a region, move data from another region into it, then free the
+             * other region. This could fail if we're close to the memcap. In this case
+             * we relax our rounding up logic so we only shrink and don't merge the 2
+             * regions after all. */
             if (to_shift->next && slide_offset + new_mem_size >= to_shift->next->stream_offset) {
                 StreamingBufferRegion *start = to_shift;
                 StreamingBufferRegion *next = start->next;
@@ -860,10 +864,17 @@ static inline void StreamingBufferSlideToOffsetWithRegions(
 
                     const uint32_t next_data_offset = next->stream_offset - slide_offset;
                     const uint32_t prev_buf_size = next->buf_size;
-
+                    const uint32_t start_data_offset = slide_offset - start->stream_offset;
+                    DEBUG_VALIDATE_BUG_ON(start_data_offset > start->buf_size);
+                    if (start_data_offset > start->buf_size) {
+                        new_mem_size = new_data_size;
+                        goto just_main;
+                    }
                     /* expand "next" to include relevant part of "start" */
-                    if (GrowRegionToSize(sb, cfg, next, mem_size) != 0)
-                        return; // TODO what to do?
+                    if (GrowRegionToSize(sb, cfg, next, mem_size) != 0) {
+                        new_mem_size = new_data_size;
+                        goto just_main;
+                    }
                     SCLogDebug("region now sized %u", mem_size);
 
                     // slide "next":
@@ -877,8 +888,6 @@ static inline void StreamingBufferSlideToOffsetWithRegions(
                     // start: [ooooookkkkk] (o: old, k: keep)
                     // pre-next     [xxxxxnextnextnext]
                     // post-next    [kkkkknextnextnext]
-                    const uint32_t start_data_offset = slide_offset - start->stream_offset;
-                    BUG_ON(start_data_offset > start->buf_size);
                     const uint32_t start_data_size = start->buf_size - start_data_offset;
                     memcpy(next->buf, start->buf + start_data_offset, start_data_size);
 
@@ -895,10 +904,13 @@ static inline void StreamingBufferSlideToOffsetWithRegions(
                     FREE(cfg, next, sizeof(*next));
                     sb->regions--;
                     BUG_ON(sb->regions == 0);
+                    goto done;
                 } else {
                     /* using "main", expand to include "next" */
-                    if (GrowRegionToSize(sb, cfg, start, mem_size) != 0)
-                        return; // TODO what to do?
+                    if (GrowRegionToSize(sb, cfg, start, mem_size) != 0) {
+                        new_mem_size = new_data_size;
+                        goto just_main;
+                    }
                     SCLogDebug("start->buf now size %u", mem_size);
 
                     // slide "start"
@@ -923,26 +935,29 @@ static inline void StreamingBufferSlideToOffsetWithRegions(
                     FREE(cfg, next, sizeof(*next));
                     sb->regions--;
                     BUG_ON(sb->regions == 0);
+                    goto done;
                 }
-            } else {
-                SCLogDebug("s %u new_data_size %u", s, new_data_size);
-                memmove(to_shift->buf, to_shift->buf + s, new_data_size);
-                /* shrink memory region. If this fails we keep the old */
-                void *ptr = REALLOC(cfg, to_shift->buf, to_shift->buf_size, new_mem_size);
-                if (ptr != NULL) {
-                    to_shift->buf = ptr;
-                    to_shift->buf_size = new_mem_size;
-                    SCLogDebug("new buf_size %u", to_shift->buf_size);
-                }
-                if (s < to_shift->buf_offset)
-                    to_shift->buf_offset -= s;
-                else
-                    to_shift->buf_offset = 0;
-                to_shift->stream_offset = slide_offset;
             }
+
+        just_main:
+            SCLogDebug("s %u new_data_size %u", s, new_data_size);
+            memmove(to_shift->buf, to_shift->buf + s, new_data_size);
+            /* shrink memory region. If this fails we keep the old */
+            void *ptr = REALLOC(cfg, to_shift->buf, to_shift->buf_size, new_mem_size);
+            if (ptr != NULL) {
+                to_shift->buf = ptr;
+                to_shift->buf_size = new_mem_size;
+                SCLogDebug("new buf_size %u", to_shift->buf_size);
+            }
+            if (s < to_shift->buf_offset)
+                to_shift->buf_offset -= s;
+            else
+                to_shift->buf_offset = 0;
+            to_shift->stream_offset = slide_offset;
         }
     }
 
+done:
     SCLogDebug("main: offset %" PRIu64 " buf %p size %u offset %u", sb->region.stream_offset,
             sb->region.buf, sb->region.buf_size, sb->region.buf_offset);
     SCLogDebug("end of slide");