Don't get verbose transaction for getTransactionConfirmations

As Esplora/Electrs doesn't support verbose transactions details in the
response, that would contain the confirmations number we need to
workaround it. (See: Blockstream/electrs#36)

The workaround:
1. Get the raw transaction
2. Deserialize the raw transaction
3. Find transaction block height by finding it in a history of
   transactions for the output script included in the transaction.
4. Get the latest block height
5. Calculate number of confirmations by subtracting the transaction
   block height from the latest block height and adding one.

Author: nkuba

typescript/src/electrum.ts

@@ -203,15 +203,81 @@ export class Client implements BitcoinClient {
   getTransactionConfirmations(
     transactionHash: TransactionHash
   ): Promise<number> {
+    // We cannot use `blockchain_transaction_get` with `verbose = true` argument
+    // to get the the transaction details as Esplora/Electrs doesn't support verbose
+    // transactions.
+    // See: https://github.com/Blockstream/electrs/pull/36
+
     return this.withElectrum<number>(async (electrum: any) => {
-      const transaction = await electrum.blockchain_transaction_get(
+      const rawTransaction: string = await electrum.blockchain_transaction_get(
         transactionHash.toString(),
-        true
+        false
       )
 
-      // For unconfirmed transactions `confirmations` property may be undefined, so
-      // we will return 0 instead.
-      return transaction.confirmations ?? 0
+      // Decode the raw transaction.
+      const transaction = bcoin.TX.fromRaw(rawTransaction, "hex")
+
+      // As a workaround for the problem described in https://github.com/Blockstream/electrs/pull/36
+      // we need to calculate the number of confirmations based on the latest
+      // block height and block height of the transaction.
+      // Electrum protocol doesn't expose a function to get the transaction's block
+      // height (other that the `GetTransaction` that is unsupported by Esplora/Electrs).
+      // To get the block height of the transaction we query the history of transactions
+      // for the output script hash, as the history contains the transaction's block
+      // height.
+
+      // Initialize txBlockHeigh with minimum int32 value to identify a problem when
+      // a block height was not found in a history of any of the script hashes.
+      //
+      // The history is expected to return a block height for confirmed transaction.
+      // If a transaction is unconfirmed (is still in the mempool) the height will
+      // have a value of `0` or `-1`.
+      let txBlockHeight: number = Math.min()
+      for (const output of transaction.outputs) {
+        const scriptHash: Buffer = output.script.sha256()
+
+        type HistoryEntry = {
+          // eslint-disable-next-line camelcase
+          tx_hash: string
+          height: number
+        }
+
+        const scriptHashHistory: HistoryEntry[] =
+          await electrum.blockchain_scripthash_getHistory(
+            scriptHash.reverse().toString("hex")
+          )
+
+        const tx = scriptHashHistory.find(
+          (t) => t.tx_hash === transactionHash.toString()
+        )
+
+        if (tx) {
+          txBlockHeight = tx.height
+          break
+        }
+      }
+
+      // History querying didn't come up with the transaction's block height. Return
+      // an error.
+      if (txBlockHeight === Math.min()) {
+        throw new Error(
+          "failed to find the transaction block height in script hashes' histories"
+        )
+      }
+
+      // If the block height is greater than `0` the transaction is confirmed.
+      if (txBlockHeight > 0) {
+        const latestBlockHeight: number = await this.latestBlockHeight()
+
+        if (latestBlockHeight >= txBlockHeight) {
+          // Add `1` to the calculated difference as if the transaction block
+          // height equals the latest block height the transaction is already
+          // confirmed, so it has one confirmation.
+          return latestBlockHeight - txBlockHeight + 1
+        }
+      }
+
+      return 0
     })
   }