Merge pull request #4555 from facebook/fix4553

Fix #4553

Author: Cyan4973

lib/legacy/zstd_v05.c

@@ -3972,8 +3972,15 @@ size_t ZBUFFv05_decompressContinue(ZBUFFv05_DCtx* zbc, void* dst, size_t* maxDst
                 zbc->outStart += flushedSize;
                 if (flushedSize == toFlushSize) {
                     zbc->stage = ZBUFFv05ds_read;
-                    if (zbc->outStart + BLOCKSIZE > zbc->outBuffSize)
-                        zbc->outStart = zbc->outEnd = 0;
+                    if (zbc->outStart + BLOCKSIZE > zbc->outBuffSize) {
+                        /* Not enough room for next block - need to wrap buffer.
+                         * Preserve history: copy the last windowSize bytes to the
+                         * beginning so that back-references can still find valid data. */
+                        size_t const windowSize = (size_t)1 << zbc->params.windowLog;
+                        size_t const preserveSize = MIN(zbc->outEnd, windowSize);
+                        memmove(zbc->outBuff, zbc->outBuff + zbc->outEnd - preserveSize, preserveSize);
+                        zbc->outStart = zbc->outEnd = preserveSize;
+                    }
                     break;
                 }
                 /* cannot flush everything */