\crossref{coinbasetransactions} effectively defined a coinbase transaction as the first

transaction in a block. This wording was copied from the Bitcoin Developer Reference
(https://developer.bitcoin.org/reference/transactions.html#coinbase-input-the-input-of-the-first-transaction-in-a-block),
but it does not match the implementation in zcashd that was inherited from Bitcoin Core.

Instead, a coinbase transaction should be, and now is, defined as a transaction with a
single null prevout. The specifications of consensus rules have been clarified and adjusted
(without any actual consensus change) to take this into account, as follows:

 * a block MUST have at least one transaction;
 * the first transaction in a block MUST be a coinbase transaction, and subsequent
   transactions MUST NOT be coinbase transactions;
 * a transparent input in a non-coinbase transaction MUST NOT have a null prevout;
 * every non-null prevout MUST point to a unique UTXO in either a preceding block, or a
   *previous* transaction in the same block (this rule was previously not given explicitly
   because it was assumed to be inherited from Bitcoin);
 * the rule that "A coinbase transaction MUST NOT have any transparent inputs with non-null
   prevout fields" is removed as an explicit consensus rule because it is implied by the
   corrected definition of coinbase transaction.

Signed-off-by: Daira Hopwood <daira@jacaranda.org>

protocol/protocol.tex

@@ -3677,9 +3677,11 @@ The calculations are described in \crossref{subsidies}.
 
 \lsubsection{Coinbase Transactions}{coinbasetransactions}
 
-The first (and only the first) \transaction in a block is a \defining{\coinbaseTransaction},
-which collects and spends any \minerSubsidy and \transactionFees paid by \transactions
-included in this \block.
+A \transaction that has a single \transparentInput with a null \prevoutField{} field, is
+called a \defining{\coinbaseTransaction}. Every \block has a single \coinbaseTransaction
+as the first \transaction in the \block. The purpose of this \coinbaseTransaction is to
+collect and spend any \minerSubsidy, and \transactionFees paid by other \transactions included
+in the \block.
 
 \precanopy{As described in \crossref{foundersreward}, the \coinbaseTransaction \MUST also pay
 the \foundersReward.}
@@ -12601,13 +12603,9 @@ in \cite{ZIP-239}.
   \item The total value in \zatoshi of \transparentOutputs from a \coinbaseTransaction\heartwood{, minus
         $\vBalance{Sapling}$,}\nufive{ minus $\vBalance{Orchard}$,} \MUSTNOT be greater than the value in
         \zatoshi of \blockSubsidy plus the \transactionFees paid by \transactions in this \block.
-  \item A \coinbaseTransaction \MUSTNOT have any \transparentInputs with non-null $\prevout$ fields,\notheartwood{
-        \joinSplitDescriptions\sapling{, \spendDescriptions, or \outputDescriptions}.
-}
-\notbeforeheartwood{
-        \joinSplitDescriptions\sapling{, or \spendDescriptions}.
-  \preheartwooditem{\sapling{A \coinbaseTransaction also \MUSTNOT have any \outputDescriptions.}}
-}
+  \item A \coinbaseTransaction \MUSTNOT have any \joinSplitDescriptions.
+  \item \sapling{A \coinbaseTransaction \MUSTNOT have any \spendDescriptions.}
+  \preheartwooditem{\sapling{A \coinbaseTransaction \MUST have any \outputDescriptions.}}
   \nufiveonwarditem{In a version 5 \coinbaseTransaction, the \enableSpendsOrchard{} flag \MUST be $0$.}
   \nufiveonwarditem{In a version 5 \transaction, the reserved bits $\barerange{2}{7}$ of the
         \flagsOrchard{} field \MUST be zero.}
@@ -12621,6 +12619,9 @@ in \cite{ZIP-239}.
         the encoding used by \Bitcoin in the implementation of \cite{BIP-34} (but the description here
         is to be considered normative).
   \item A \coinbaseTransaction script \MUST have length in $\range{2}{100}$ bytes.
+  \item A \transparentInput in a non-coinbase \transaction \MUSTNOT have a null \prevout.
+  \item Every non-null \prevout \MUST point to a unique \utxo in either a preceding \block, or a
+        \emph{previous} \transaction in the same \block.
   \item %\consensuslink{bad-txns-premature-spend-of-coinbase}
         A \transaction \MUSTNOT spend a \transparentOutput of a \coinbaseTransaction
         from a \block less than 100 \blocks prior to the spend. Note that \transparentOutputs of
@@ -13100,6 +13101,9 @@ preceding \blocks if there are fewer than $\PoWMedianBlockSpan$). The \medianTim
   \nufiveonwarditem{$\hashBlockCommitments$ \MUST be set to the value of $\hashBlockCommitments$
         for this \block, as specified in \cite{ZIP-244}.}
 }
+  \item A \block \MUST have at least one \transaction.
+  \item The first \transaction in a \block \MUST be a \coinbaseTransaction, and subsequent
+        \transactions \MUSTNOT be \coinbaseTransactions.
   \item \todo{Other rules inherited from \Bitcoin.}
 \end{consensusrules}
 
@@ -14627,6 +14631,29 @@ Peter Newell's illustration of the Jubjub bird, from \cite{Carroll1902}.
 } %sapling
     \item Document the consensus rule that coinbase script length \MUST be
           $\range{2}{100}$ bytes.
+    \item \crossref{coinbasetransactions} effectively defined a \coinbaseTransaction
+          as the first \transaction in a \block. This wording was copied from the
+          Bitcoin Developer Reference \cite{Bitcoin-CbInput}, but it does not match
+          the implementation in \zcashd that was inherited from \BitcoinCore.
+          Instead, a \coinbaseTransaction should be, and now is, defined as a
+          \transaction with a single null \prevout.
+          The specifications of consensus rules have been clarified and adjusted
+          (without any actual consensus change) to take this into account, as follows:
+          \begin{itemize}
+            \item a \block \MUST have at least one \transaction;
+            \item the first \transaction in a \block \MUST be a \coinbaseTransaction,
+                  and subsequent \transactions \MUSTNOT be \coinbaseTransactions;
+            \item a \transparentInput in a non-coinbase \transaction \MUSTNOT have
+                  a null \prevout;
+            \item every non-null \prevout \MUST point to a unique \utxo in either
+                  a preceding \block, or a \emph{previous} \transaction in the
+                  same \block (this rule was previously not given explicitly
+                  because it was assumed to be inherited from \Bitcoin);
+            \item the rule that ``A \coinbaseTransaction \MUSTNOT have any
+                  \transparentInputs with non-null \prevoutField{} fields'' is
+                  removed as an explicit consensus rule because it is implied by
+                  the corrected definition of \coinbaseTransaction.
+          \end{itemize}
 \end{itemize}
 
 

protocol/zcash.bib

@@ -990,6 +990,13 @@ Last revised February~5, 2018.}
    urldate={2020-07-13}
 }
 
+@misc{Bitcoin-CbInput,
+   presort={Bitcoin-CoinbaseIn},
+   title={Coinbase Input --- {B}itcoin {D}eveloper {R}eference},
+   url={https://developer.bitcoin.org/reference/transactions.html#coinbase-input-the-input-of-the-first-transaction-in-a-block},
+   urldate={2022-03-17}
+}
+
 @misc{Bitcoin-Multisig,
    presort={Bitcoin-Multisig},
    title={Transactions: {M}ultisig --- {B}itcoin {D}eveloper {G}uide},