fix(disk-transform): fix getBlockIndexesCount

introduced by #10058

the current method assumes that source block are full but they can
be sparse. In the worst case getBlockIndexesCount can be the same as
the source

@xen-orchestra/disk-transform/src/DiskLargerBlock.mts

@@ -67,22 +67,24 @@ export class DiskLargerBlock extends RandomAccessDisk {
 
   getBlockIndexes(): Array<number> {
     const maxBlock = Math.ceil(this.getVirtualSize() / this.getBlockSize())
-    const blockRatio = this.getBlockSize() / this.source.getBlockSize()
     const indexes = []
     for (let i = 0; i < maxBlock; i++) {
-      for (let j = 0; j < blockRatio; j++) {
-        if (this.source.hasBlock(i * blockRatio + j)) {
-          indexes.push(i)
-          break
-        }
+      if (this.hasBlock(i)) {
+        indexes.push(i)
       }
     }
     return indexes
   }
 
   getBlockIndexesCount(): number {
-    let blockRatio = this.getBlockSize() / this.source.getBlockSize()
-    return this.#source.getBlockIndexesCount() / blockRatio
+    let count = 0
+    const maxBlock = Math.ceil(this.getVirtualSize() / this.getBlockSize())
+    for (let i = 0; i < maxBlock; i++) {
+      if (this.hasBlock(i)) {
+        count++
+      }
+    }
+    return count
   }
 
   hasBlock(index: number): boolean {

@xen-orchestra/disk-transform/src/DiskLargerBlock.test.mts

@@ -237,6 +237,34 @@ test('close propagation', async () => {
   await disk.close()
   assert.strictEqual(source.closed, true)
 })
+test('getBlockIndexesCount', async () => {
+  const block = createPatternBuffer(512, 'BLOCK')
+  let source = new MockDisk(512, 4096, [
+    [2, block],
+    [3, block],
+  ])
+  await source.init()
+
+  let disk = new DiskLargerBlock(source, 1024)
+  await disk.init()
+  assert.strictEqual(disk.getBlockIndexesCount(), 1)
+  assert.strictEqual(disk.getBlockIndexesCount(), disk.getBlockIndexes().length)
+
+  await disk.close()
+
+  source = new MockDisk(512, 4096, [
+    [0, block],
+    [3, block],
+  ])
+  await source.init()
+
+  disk = new DiskLargerBlock(source, 1024)
+  await disk.init()
+  assert.strictEqual(disk.getBlockIndexesCount(), 2)
+  assert.strictEqual(disk.getBlockIndexesCount(), disk.getBlockIndexes().length)
+
+  await disk.close()
+})
 
 test('generator', async () => {
   const source = new MockDisk(512, 1024)