What is the 'root' of a transaction receipt?

Question

web3.eth.getTransactionReceipt returns an object with a root property and is currently undocumented in the wiki.

Result: {
    ...
      "root": "7583254379574ee8eb2943c3ee41582a0041156215e2c7d82e363098c89fe21b",
      "to": "0x91067b439e1be22196a5f64ee61e803670ba5be9",
      "transactionHash": "0xad62c939b2e865f13c61eebcb221d2c9737955e506b69fb624210d3fd4e0035b",
      "transactionIndex": 0
    }

1. What is this root? It is not the same as the receiptRoot from web3.eth.getBlock.

2. What is the relationship between these roots?

Answer 1

It's the hash of the root of the state trie, whereas receiptRoot is the hash of the array of receipts for a given block.

---

root

In GetTransactionReceipt() in api.go there's set of mappings, one of which is:

"root":              common.Bytes2Hex(receipt.PostState),

Looking at receipt.go, PostState is a byte array:

// Receipt represents the results of a transaction.
type Receipt struct {
    // Consensus fields
    PostState         []byte
    CumulativeGasUsed *big.Int
    Bloom             Bloom
    Logs              vm.Logs

This is set in NewReceipt() to a value passed in from state_processor.go:

receipt := types.NewReceipt(statedb.IntermediateRoot().Bytes(), usedGas)

...and IntermediateRoot() is defined in statedb.go as:

// IntermediateRoot computes the current root hash of the state trie.
// It is called in between transactions to get the root hash that
// goes into transaction receipts.

---

receiptRoot

In block.go:

// The values of TxHash, UncleHash, ReceiptHash and Bloom in header
// are ignored and set to values derived from the given txs, uncles
// and receipts.

With the pertinent code in that function being:

b.header.ReceiptHash = DeriveSha(Receipts(receipts))

And in api.go:

"receiptRoot":      b.ReceiptHash(),