Fix overlapping copies in CLI read-buffer coalescing

programs/fileio_asyncio.c

@@ -614,7 +614,8 @@ size_t AIO_ReadPool_fillBuffer(ReadPoolCtx_t* ctx, size_t n) {
      * and coalesce the remaining bytes with the next job's buffer */
     if (ctx->srcBufferLoaded > 0) {
         useCoalesce = 1;
-        memcpy(ctx->coalesceBuffer, ctx->srcBuffer, ctx->srcBufferLoaded);
+        /* Remaining bytes may already be in coalesceBuffer. */
+        memmove(ctx->coalesceBuffer, ctx->srcBuffer, ctx->srcBufferLoaded);
         ctx->srcBuffer = ctx->coalesceBuffer;
     }
 

tests/cli-tests/decompression/skippable-overlap.sh

@@ -0,0 +1,17 @@
+#!/bin/sh
+
+set -eu
+
+# An 8-byte empty skippable frame followed by a 9-byte empty zstd frame.
+# The initial 18-byte header refill moves this 17-byte stream into the
+# coalesce buffer. After consuming the skippable frame, the next refill
+# moves 9 bytes from offset 8 back to offset 0. Run under ASan to detect
+# an overlapping memcpy.
+printf '\120\052\115\030\000\000\000\000\050\265\057\375\040\000\001\000\000' > skippable-overlap.zst
+
+# This is a valid stream and must decode successfully to empty output.
+for io in --asyncio --no-asyncio; do
+    for operation in -t -dc; do
+        zstd -q "$io" "$operation" skippable-overlap.zst
+    done
+done