filer: keep a ranged read in random mode through its contiguous tail

A far ReadAt on a fresh ReaderPattern left the sequential counter at -1,
so the next buffer of the same ranged request landed on the frontier and
flipped the verdict straight back to sequential — readChunkSliceAt then
paid a whole-chunk fetch for the remainder of the range. Drop the
counter to -ModeChangeLimit when random mode is entered so the verdict
needs sustained sequential evidence to undo, matching the hysteresis an
established sequential stream already gets.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

weed/filer/reader_pattern.go

@@ -54,10 +54,13 @@ func (rp *ReaderPattern) MonitorReadAt(offset int64, size int) {
 		if counter < ModeChangeLimit {
 			atomic.AddInt64(&rp.isSequentialCounter, 1)
 		}
+	} else if counter <= 0 {
+		// Entering random mode is a strong verdict: drop to the bottom of
+		// the window so the contiguous tail of one ranged request cannot
+		// flip it back on the next buffer read and pay a whole-chunk fetch.
+		atomic.StoreInt64(&rp.isSequentialCounter, -ModeChangeLimit)
 	} else {
-		if counter > -ModeChangeLimit {
-			atomic.AddInt64(&rp.isSequentialCounter, -1)
-		}
+		atomic.AddInt64(&rp.isSequentialCounter, -1)
 	}
 }
 

weed/filer/reader_pattern_test.go

@@ -102,3 +102,24 @@ func TestReaderPatternRecoversFromRandom(t *testing.T) {
 		t.Fatal("sustained near reads must recover sequential mode")
 	}
 }
+
+// A ranged request's first read lands far from the frontier, but its
+// remaining buffer reads are contiguous. The random verdict must stick for
+// them — otherwise the tail of every range >256KiB pays a whole-chunk fetch.
+func TestReaderPatternRangedReadStaysRandom(t *testing.T) {
+	rp := NewReaderPattern()
+	rp.MonitorReadAt(500*mb, 256*1024) // far first read -> -ModeChangeLimit
+	for i := 1; i <= 2; i++ {
+		rp.MonitorReadAt(500*mb+int64(i)*256*1024, 256*1024)
+		if !rp.IsRandomMode() {
+			t.Fatalf("contiguous read %d of a ranged request flipped back to sequential", i+1)
+		}
+	}
+	for i := 3; i < 10; i++ {
+		rp.MonitorReadAt(500*mb+int64(i)*256*1024, 256*1024)
+	}
+	if rp.IsRandomMode() {
+		t.Fatal("sustained sequential reads should restore sequential mode")
+	}
+}
+