fix: skip exhausted blocks before creating an interval (#8180)

* fix: skip exhausted blocks before creating an interval

* refactor: optimize interval creation and fix logic duplication

* docs: add docstring for LocateData

* refactor: extract moveToNextBlock helper to deduplicate logic

* fix: use int64 for block index comparison to prevent overflow

* test: add unit test for LocateData boundary crossing (issue #8179)

* fix: skip exhausted blocks to prevent negative interval size and panics (issue #8179)

* refactor: apply review suggestions for test maintainability and code style

weed/storage/erasure_coding/ec_locate.go

@@ -12,10 +12,23 @@ type Interval struct {
 	LargeBlockRowsCount int
 }
 
+// LocateData finds the intervals of data within erasure coding blocks for a given offset and size.
 func LocateData(largeBlockLength, smallBlockLength int64, shardDatSize int64, offset int64, size types.Size) (intervals []Interval) {
 	blockIndex, isLargeBlock, nLargeBlockRows, innerBlockOffset := locateOffset(largeBlockLength, smallBlockLength, shardDatSize, offset)
 
 	for size > 0 {
+		blockRemaining := largeBlockLength - innerBlockOffset
+		if !isLargeBlock {
+			blockRemaining = smallBlockLength - innerBlockOffset
+		}
+
+		if blockRemaining <= 0 {
+			// move to next block
+			blockIndex, isLargeBlock = moveToNextBlock(blockIndex, isLargeBlock, nLargeBlockRows)
+			innerBlockOffset = 0
+			continue
+		}
+
 		interval := Interval{
 			BlockIndex:          blockIndex,
 			InnerBlockOffset:    innerBlockOffset,
@@ -23,11 +36,6 @@ func LocateData(largeBlockLength, smallBlockLength int64, shardDatSize int64, of
 			LargeBlockRowsCount: int(nLargeBlockRows),
 		}
 
-		blockRemaining := largeBlockLength - innerBlockOffset
-		if !isLargeBlock {
-			blockRemaining = smallBlockLength - innerBlockOffset
-		}
-
 		if int64(size) <= blockRemaining {
 			interval.Size = size
 			intervals = append(intervals, interval)
@@ -37,17 +45,23 @@ func LocateData(largeBlockLength, smallBlockLength int64, shardDatSize int64, of
 		intervals = append(intervals, interval)
 
 		size -= interval.Size
-		blockIndex += 1
-		if isLargeBlock && blockIndex == interval.LargeBlockRowsCount*DataShardsCount {
-			isLargeBlock = false
-			blockIndex = 0
-		}
+		blockIndex, isLargeBlock = moveToNextBlock(blockIndex, isLargeBlock, nLargeBlockRows)
 		innerBlockOffset = 0
 
 	}
 	return
 }
 
+func moveToNextBlock(blockIndex int, isLargeBlock bool, nLargeBlockRows int64) (int, bool) {
+	nextBlockIndex := blockIndex + 1
+	nextIsLargeBlock := isLargeBlock
+	if isLargeBlock && int64(nextBlockIndex) == nLargeBlockRows*DataShardsCount {
+		nextIsLargeBlock = false
+		nextBlockIndex = 0
+	}
+	return nextBlockIndex, nextIsLargeBlock
+}
+
 func locateOffset(largeBlockLength, smallBlockLength int64, shardDatSize int64, offset int64) (blockIndex int, isLargeBlock bool, nLargeBlockRows int64, innerBlockOffset int64) {
 	largeRowSize := largeBlockLength * DataShardsCount
 	nLargeBlockRows = (shardDatSize - 1) / largeBlockLength

weed/storage/erasure_coding/ec_test.go

@@ -236,3 +236,20 @@ func TestLocateData3(t *testing.T) {
 		{BlockIndex: 8876, InnerBlockOffset: 912752, Size: 112568, IsLargeBlock: false, LargeBlockRowsCount: 2},
 	})
 }
+
+func TestLocateData_Issue8179(t *testing.T) {
+	large := int64(10000)
+	small := int64(100)
+	shardSize := int64(259092) // Resulting in nLargeBlockRows = 25 as seen in panic log
+
+	// Testing range through the large-to-small transition boundary
+	nLargeBlockRows := (shardSize - 1) / large
+	largeAreaSize := nLargeBlockRows * int64(DataShardsCount) * large
+
+	for offset := largeAreaSize - 500; offset < largeAreaSize+500; offset++ {
+		intervals := LocateData(large, small, shardSize, offset, 200)
+		for _, interval := range intervals {
+			assert.True(t, interval.Size > 0, "Interval size must be positive at offset %d, got %+v", offset, interval)
+		}
+	}
+}