chore(qwp): read the SF recovery scan via pread instead of the mapping

core/src/main/java/io/questdb/client/cutlass/qwp/client/sf/cursor/SegmentRing.java

@@ -197,7 +197,7 @@ public static SegmentRing openExisting(String sfDir, long maxBytesPerSegment) {
                             // CAUTION: only unlink when the file is genuinely
                             // empty past the header. If frame[0] failed CRC
                             // (bit-rot, partial-page-write at crash, etc.) but
-                            // valid frames followed, scanFrames returns
+                            // valid frames followed, the recovery scan returns
                             // lastGood=HEADER_SIZE and frameCount=0 -- yet
                             // tornTailBytes is non-zero. Treating that as
                             // "empty hot-spare" would silently destroy every
@@ -269,8 +269,9 @@ public static SegmentRing openExisting(String sfDir, long maxBytesPerSegment) {
             // FSNs after recovery. A gap means a segment went missing (a
             // manual deletion) or a sealed segment under-recovered -- its tail
             // was cut short by a sparse/unbacked page or a mid-file media error
-            // (bad sector), the same class of fault scanFrames tolerates on the
-            // active segment but which corrupts the range on a sealed one.
+            // (bad sector), the same unreadable-region boundary the recovery
+            // scan tolerates on the active segment but which corrupts the
+            // range on a sealed one.
             for (int i = 1, n = opened.size(); i < n; i++) {
                 MmapSegment prev = opened.get(i - 1);
                 MmapSegment curr = opened.get(i);

core/src/main/java/io/questdb/client/std/FilesFacade.java

@@ -91,15 +91,13 @@ public interface FilesFacade {
      * Stat length of the file at {@code path}, in bytes. Default delegates to
      * {@link Files#length(String)}.
      *
-     * <p>Test injection point: {@code MmapSegment.openExisting} maps the file to
-     * this length and scans straight out of the mapping, so a wrapping facade
-     * that returns a value <em>larger</em> than the real file makes the mapping
-     * extend past end-of-file. A read of a page beyond real EOF raises SIGBUS on
-     * <em>every</em> filesystem (which HotSpot translates to a catchable
-     * {@code InternalError} at an {@code Unsafe} intrinsic site) — the same fault
-     * a genuinely unbacked/sparse page raises on ZFS, but reproduced
-     * deterministically on ext4/xfs too. That is what lets recovery's mmap-fault
-     * guard be regression-tested on any CI runner rather than only on ZFS.
+     * <p>Test injection point: {@code MmapSegment.openExisting} treats this
+     * length as the file's extent and preads its recovery scan against it, so a
+     * wrapping facade that returns a value <em>larger</em> than the real file
+     * makes the scan read past end-of-file. Those reads come back short, which
+     * recovery must treat as the boundary of recoverable data — the same
+     * outcome as a file whose size metadata survived a crash its data blocks
+     * did not, reproduced deterministically on any filesystem and CI runner.
      */
     long length(String path);