tapgarden: test fundBatch, external seedling keys

In this commit, we add a unit test that funds a batch before finalize.
This test also provides external keys for some seedlings to ensure that
these are handled correctly during minting. We also update the planter
to reject fund and finalize calls if a batch is already funded.

tapgarden/batch.go

@@ -50,9 +50,6 @@ type MintingBatch struct {
 	// GenesisPacket is the funded genesis packet that may or may not be
 	// fully signed. When broadcast, this will create all assets stored
 	// within this batch.
-	//
-	// NOTE: This field is only set if the state is beyond
-	// BatchStateCommitted.
 	GenesisPacket *tapsend.FundedPsbt
 
 	// RootAssetCommitment is the root Taproot Asset commitment for all the

tapgarden/planter.go

@@ -879,7 +879,13 @@ func (c *ChainPlanter) gardener() {
 				req.Resolve(batches)
 
 			case reqTypeFundBatch:
-				log.Infof("Funding batch")
+				if c.pendingBatch != nil &&
+					c.pendingBatch.IsFunded() {
+
+					req.Error(fmt.Errorf("batch already " +
+						"funded"))
+					break
+				}
 
 				fundReqParams, err :=
 					typedParam[FundParams](req)
@@ -898,6 +904,8 @@ func (c *ChainPlanter) gardener() {
 					break
 				}
 
+				req.Resolve(c.pendingBatch)
+
 			// TODO(jhb): follow-up PR: Implement SealBatch command
 			case reqTypeSealBatch:
 				req.Error(fmt.Errorf("not yet implemented"))
@@ -1190,6 +1198,15 @@ func (c *ChainPlanter) finalizeBatch(params FinalizeParams) (*BatchCaretaker,
 		err     error
 	)
 
+	// Before modifying the pending batch, check if the batch was already
+	// funded. If so, reject any provided parameters, as they would conflict
+	// with those previously used for batch funding.
+	haveParams := params.FeeRate.IsSome() || params.SiblingTapTree.IsSome()
+	if haveParams && c.pendingBatch.IsFunded() {
+		return nil, fmt.Errorf("cannot provide finalize parameters " +
+			"if batch already funded")
+	}
+
 	// Process the finalize parameters.
 	feeRate = params.FeeRate.UnwrapToPtr()
 
@@ -1427,7 +1444,7 @@ func (c *ChainPlanter) prepAssetSeedling(ctx context.Context,
 		req.ScriptKey = asset.NewScriptKeyBip86(scriptKey)
 	}
 
-	// Now that we know the field are valid, we'll check to see if a batch
+	// Now that we know the seedling is valid, we'll check to see if a batch
 	// already exists.
 	switch {
 	// No batch, so we'll create a new one with only this seedling as part