Make IndexInput#prefetch take an offset. (apache#13363)

This makes `IndexInput#prefetch` take an offset instead of being relative to
the current position. This avoids requiring callers to seek only to call
`prefetch()`.

lucene/core/src/java/org/apache/lucene/store/IndexInput.java

@@ -199,7 +199,8 @@ public String toString() {
    *
    * <p>The default implementation is a no-op.
    *
+   * @param offset start offset
    * @param length the number of bytes to prefetch
    */
-  public void prefetch(long length) throws IOException {}
+  public void prefetch(long offset, long length) throws IOException {}
 }

lucene/core/src/java21/org/apache/lucene/store/MemorySegmentIndexInput.java

@@ -313,38 +313,37 @@ public void seek(long pos) throws IOException {
   }
 
   @Override
-  public void prefetch(long length) throws IOException {
+  public void prefetch(long offset, long length) throws IOException {
     ensureOpen();
 
-    Objects.checkFromIndexSize(getFilePointer(), length, length());
+    Objects.checkFromIndexSize(offset, length, length());
 
     if (NATIVE_ACCESS.isEmpty()) {
       return;
     }
     final NativeAccess nativeAccess = NATIVE_ACCESS.get();
 
-    // If at the boundary between two chunks, move to the next one.
-    seek(getFilePointer());
     try {
+      final MemorySegment segment = segments[(int) (offset >> chunkSizePower)];
+      offset &= chunkSizeMask;
       // Compute the intersection of the current segment and the region that should be prefetched.
-      long offset = curPosition;
-      if (offset + length > curSegment.byteSize()) {
+      if (offset + length > segment.byteSize()) {
         // Only prefetch bytes that are stored in the current segment. There may be bytes on the
         // next segment but this case is rare enough that we don't try to optimize it and keep
         // things simple instead.
-        length = curSegment.byteSize() - curPosition;
+        length = segment.byteSize() - offset;
       }
       // Now align offset with the page size, this is required for madvise.
       // Compute the offset of the current position in the OS's page.
-      final long offsetInPage = (curSegment.address() + offset) % nativeAccess.getPageSize();
+      final long offsetInPage = (segment.address() + offset) % nativeAccess.getPageSize();
       offset -= offsetInPage;
       length += offsetInPage;
       if (offset < 0) {
         // The start of the page is outside of this segment, ignore.
         return;
       }
 
-      final MemorySegment prefetchSlice = curSegment.asSlice(offset, length);
+      final MemorySegment prefetchSlice = segment.asSlice(offset, length);
       nativeAccess.madviseWillNeed(prefetchSlice);
     } catch (
         @SuppressWarnings("unused")

lucene/test-framework/src/java/org/apache/lucene/tests/store/BaseDirectoryTestCase.java

@@ -1540,36 +1540,32 @@ private void doTestPrefetch(int startOffset) throws IOException {
           in = orig.slice("slice", startOffset, totalLength - startOffset);
         }
         for (int i = 0; i < 10_000; ++i) {
-          final int startPointer = (int) in.getFilePointer();
-          assertTrue(startPointer < in.length());
+          int offset = TestUtil.nextInt(random(), 0, (int) in.length() - 1);
           if (random().nextBoolean()) {
-            final long prefetchLength = TestUtil.nextLong(random(), 1, in.length() - startPointer);
-            in.prefetch(prefetchLength);
+            final long prefetchLength = TestUtil.nextLong(random(), 1, in.length() - offset);
+            in.prefetch(offset, prefetchLength);
           }
-          assertEquals(startPointer, in.getFilePointer());
+          in.seek(offset);
+          assertEquals(offset, in.getFilePointer());
           switch (random().nextInt(100)) {
             case 0:
-              assertEquals(arr[startOffset + startPointer], in.readByte());
+              assertEquals(arr[startOffset + offset], in.readByte());
               break;
             case 1:
-              if (in.length() - startPointer >= Long.BYTES) {
+              if (in.length() - offset >= Long.BYTES) {
                 assertEquals(
-                    (long) BitUtil.VH_LE_LONG.get(arr, startOffset + startPointer), in.readLong());
+                    (long) BitUtil.VH_LE_LONG.get(arr, startOffset + offset), in.readLong());
               }
               break;
             default:
               final int readLength =
-                  TestUtil.nextInt(
-                      random(), 1, (int) Math.min(temp.length, in.length() - startPointer));
+                  TestUtil.nextInt(random(), 1, (int) Math.min(temp.length, in.length() - offset));
               in.readBytes(temp, 0, readLength);
               assertArrayEquals(
                   ArrayUtil.copyOfSubArray(
-                      arr, startOffset + startPointer, startOffset + startPointer + readLength),
+                      arr, startOffset + offset, startOffset + offset + readLength),
                   ArrayUtil.copyOfSubArray(temp, 0, readLength));
           }
-          if (in.getFilePointer() == in.length() || random().nextBoolean()) {
-            in.seek(TestUtil.nextInt(random(), 0, (int) in.length() - 1));
-          }
         }
       }
     }

lucene/test-framework/src/java/org/apache/lucene/tests/store/MockIndexInputWrapper.java

@@ -131,9 +131,9 @@ public void seek(long pos) throws IOException {
   }
 
   @Override
-  public void prefetch(long length) throws IOException {
+  public void prefetch(long offset, long length) throws IOException {
     ensureOpen();
-    in.prefetch(length);
+    in.prefetch(offset, length);
   }
 
   @Override