Bitcoin - How to identify if transaction is Segwit

Question

Below is my logic to parse transaction from Bitcoin blk files

1. Pull Version by getting uint32
2. Read next 2 bytes. If they are 0,1 then it is SegWit
3. Else read again for varint to pull input count
4. If input count = 0, read one byte and check if it is 0x01.
5. If not 0x01 then error out as I do not know what kind of transaction it is with 0 input
6. Else read varint to fetch input count

I know that blk?????.dat files are not in particular order but I did not get this issue in #5 while parsing blocks in first 1000 files.

The above logic seems flawed as I am not able to read block successfully. Can someone who has done this explain

One more question, is there a way to identify if block in blk file is orphaned and should be ignored?

Raw data of block - link. It has ~1400 transactions

Answer 1

Sometimes ago I've allready wrote the script that supports parsing Witness data from the raw database. For example, parsing the file blk01896.dat (from my local database) with my blockchain parser. The parsing results of the first block 0000000000000000000D9C6917E3E865812D419648349D85C40C5A8302842D79 is:

The first transaction in that block BC1A08C90D142E4B475E844D16CDD796C2CFEAD15DC2D4C666131EB308CB739A has parsing results:

transactionVersionNumber = 00000002
Witness activated >>
Inputs count = 01
TX from hash = 0000000000000000000000000000000000000000000000000000000000000000
N output = FFFFFFFF
Input script = 032D470904E181F45D455530322F4254432E434F4D2FFABE6D6D9832F682523CD8FEA4AF9A980D6CAA10B5847FD6CC8D8BE3C9BB5EBB89547F9908000000CEEED33D2425EE84AC19040000000000
sequenceNumber = FFFFFFFF
Outputs count = 3
Value = 000000004B116E87
Output script = 001497CFC76442FE717F2A3F0CC9C175F7561B661997
Value = 0000000000000000
Output script = 6A24AA21A9EDB6B23A8ABBC43D9D9F3005646F9C8FF7307722698A360142E6C40D593FF1931C
Value = 0000000000000000
Output script = 6A2952534B424C4F434B3AF7AD9B2A11A8CE87CF52B80DAF26B1F1AD45FE3CF1BAF3DB9A457874001CADBE
Witness 0 0 32 0000000000000000000000000000000000000000000000000000000000000000
Lock time = 00000000
TX hash = BC1A08C90D142E4B475E844D16CDD796C2CFEAD15DC2D4C666131EB308CB739A

The second transaction EDDCB4AAD8DDC54E2A1B6FCD216CBF6C3BA2D4879AEE3570A0289C499BD8EF1B:

transactionVersionNumber = 00000001
Inputs count = 01
TX from hash = 428AB83113FD33C26020635FC0DCC1B275E6E18DC1E3B4AB8D81610155149968
N output = 000000C2
Input script = 473044022047118E23692DAD799096751526BE098B81328A1763E26FE0AB56CC88691699D002205BF4469E44C975B5DE3971243E5B4925CF5C9A658B049AEA9993202232B88914014104FCF07BB1222F7925F2B7CC15183A40443C578E62EA17100AA3B44BA66905C95D4980AEC4CD2F6EB426D1B1EC45D76724F26901099416B9265B76BA67C8B0B73D
sequenceNumber = FFFFFFFF
Outputs count = 3
Value = 000000000000088E
Output script = 76A914FA0692278AFE508514B5FFEE8FE5E97732CE066988AC
Value = 0000000000000000
Output script = 6A146F6D6E69000000000000000300000000000000E9
Value = 000000000000021C
Output script = A914DE9F32C7F3CBFDCCDF6BF98F596F9B3F830DA56987
Lock time = 00000000
TX hash = EDDCB4AAD8DDC54E2A1B6FCD216CBF6C3BA2D4879AEE3570A0289C499BD8EF1B

The fifth transaction B0E255196D16F244BDF552100C052F84DF304BF405CA2FCCB1FF55CB400A8CB2:

transactionVersionNumber = 00000002
Witness activated >>
Inputs count = 01
TX from hash = C453DD9343B17E9D728F46EA550831334AD342B5AEA2FDBD1E57CF73A97EC280
N output = 00000001
Input script = 160014369C1A58C231BA8311E26B8DDE1624138F68E980
sequenceNumber = FEFFFFFF
Outputs count = 2
Value = 000000000005E498
Output script = 76A914D5AA6FF082BED43D64D890B9AEFFE67C872D0AFE88AC
Value = 00000000046F8F55
Output script = A914FC249B6158D6CC6A6AD1FB3EE71747B86DF76D1187
Witness 0 0 71 012231AA44DEAA9B391EBC81A7419B93CBB0F50292AF9445D0A1AFB114FB5E1927200232BAF193AF7273F3EE1EFDCA873D3BB524E11B39259AAFCBA7B03E2A30DCC76820024430
Witness 0 1 33 FD1F21A7C45348728989BF8CE4C9CF1B17496DD41B834297D1AA53A3E22CF85303
Lock time = 0009472C
TX hash = B0E255196D16F244BDF552100C052F84DF304BF405CA2FCCB1FF55CB400A8CB2

First and fifth TXs has Witness, the second not. So that's the differences. Good luck!

P.S. You can check the parsing script section for transactions and see the differences (which bytes execute witness and where is the witness fields).

Answer 2

It is probable that the issue you are facing is the result of incorrectly parsing or failing to parse the witness data. In segwit, witness data for each input is included after the outputs. The witness data for an input is an array of an array of bytes (each array is length prefixed with a compact size unsigned int). There is one witness data for each input and no length prefix for the witness data section entirely.

Given that you say that you run into a parsing issue for the second transaction of the given block, I am inclined to believe that the root cause is simply not parsing the witness data at all. The coinbase transactions of blocks containing segwit transactions must include a witness consisting of a 32 byte array of 0's for the coinbase input. So it is likely that you are mistakenly parsing these 0's as fields for the next transaction. And given the number of them, it would explain why you see 0 inputs for the second transaction, and also a byte that is not 0x01 for the segwit flag.

Answer 3

After reading the comments on your question, i think you are not parsing the witness data and continuing with parsing a new transaction when you are actually reading witness data.

In a SegWit transaction, the varint that tells you how much inputs there are also tells you how much pieces of witness data there are. For each input one. Each piece of witness data has a varint of how much witness elements there are for that input. P2PKH or other non segwit inputs in a segwit transaction simply have a 00 there to indicate that there are no witness elements for that input. (They use sigScript for unlocking)

If this didn't help you, please post your code. Without it (i think) nobody can help you any further.