feat(wpcarro/scratch): create a proof-of-concept blockchain server r/3556

> You cannot get educated by this self-propagating system in which people study
> to pass exams, and teach others to pass exams, but nobody knows anything.  You
> learn something by doing it yourself, by asking questions, by thinking, and by
> experimenting.
> - Richard Feynman

In the spirit of learning by doing, I decided to implement a simple blockchain
server. More work remains, but I'm tired after working on this for ~2-3h. I'd
like to reimplement this from memory using a statically typed language like
Haskell. I'd also like to implement node
discovery (https://en.bitcoin.it/wiki/Satoshi_Client_Node_Discovery) because
that is still something I don't quite understand. But I'm signing-off for
now...

Change-Id: I74f424e7f52ffbf81eaad420d7d5205da66d33b5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4802
Tested-by: BuildkiteCI
Reviewed-by: wpcarro <wpcarro@gmail.com>
Autosubmit: wpcarro <wpcarro@gmail.com>

3 files changed, 286 insertions, 0 deletions

diff --git a/users/wpcarro/scratch/blockchain/default.nix b/users/wpcarro/scratch/blockchain/default.nix
new file mode 100644
index 0000000000..745e7a5ab4
--- /dev/null
+++ b/users/wpcarro/scratch/blockchain/default.nix
@@ -0,0 +1,13 @@
+{ pkgs, ... }:
+
+let
+  pypkgs = pkgs.python3Packages;
+in pkgs.python3Packages.buildPythonApplication {
+  pname = "main";
+  src = ./.;
+  version = "0.0.1";
+  propagatedBuildInputs = with pypkgs; [
+    flask
+    requests
+  ];
+}
diff --git a/users/wpcarro/scratch/blockchain/main.py b/users/wpcarro/scratch/blockchain/main.py
new file mode 100644
index 0000000000..e7b6276133
--- /dev/null
+++ b/users/wpcarro/scratch/blockchain/main.py
@@ -0,0 +1,263 @@
+from flask import Flask, jsonify, request
+from hashlib import sha256
+from datetime import datetime
+from urllib.parse import urlparse
+
+import json
+import requests
+import uuid
+
+################################################################################
+# Helper Functions
+################################################################################
+
+def hash(x):
+  return sha256(x).hexdigest()
+
+def is_pow_valid(guess, prev_proof):
+  """
+  Return true if the hash of `guess` + `prev_proof` has 4x leading zeros.
+  """
+  return hash(str(guess + prev_proof).encode("utf8"))[:4] == "0000"
+
+################################################################################
+# Classes
+################################################################################
+
+class Node(object):
+  def __init__(self, host="0.0.0.0", port=8000):
+    self.app = Flask(__name__)
+    self.define_api()
+    self.identifier = str(uuid.uuid4())
+    self.blockchain = Blockchain()
+    self.neighbors = set()
+
+  def add_neighbors(self, urls=None):
+    for url in urls:
+      parsed = urlparse(url)
+      if not parsed.netloc:
+        raise ValueError("Must pass valid URLs for neighbors")
+      self.neighbors.add(parsed.netloc)
+
+  def decode_chain(chain_json):
+    return Blockchain(
+        blocks=[
+            Block(
+                index=block["index"],
+                ts=block["ts"],
+                transactions=[
+                    Transaction(
+                        origin=tx["origin"],
+                        target=tx["target"],
+                        amount=tx["amount"])
+                        for tx in block["ts"]
+                ],
+                proof=block["proof"],
+                prev_hash=block["prev_hash"])
+                for block in chain_json["blocks"]
+        ],
+        transactions=[
+            Transaction(
+                origin=tx["origin"],
+                target=tx["target"],
+                amount=tx["amount"])
+                for tx in chain_json["transactions"]
+        ])
+
+  def resolve_conflicts(self):
+    auth_chain, auth_length = self.blockchain, len(self.blockchain)
+
+    for neighbor in self.neighbors:
+      res = requests.get(f"http://{neighbor}/chain")
+      if res.status_code == 200 and res.json()["length"] > auth_length:
+         decoded_chain = decode_chain(res.json()["chain"])
+         if Blockchain.is_valid(decoded_chain):
+           auth_length = res.json()["length"]
+           auth_chain = decoded_chain
+
+      self.blockchain = auth_chain
+
+  def define_api(self):
+    def msg(x):
+      return jsonify({"message": x})
+
+    ############################################################################
+    # /
+    ############################################################################
+
+    @self.app.route("/healthz", methods={"GET"})
+    def healthz():
+      return "ok"
+
+    @self.app.route("/reset", methods={"GET"})
+    def reset():
+      self.blockchain = Blockchain()
+      return msg("Success")
+
+    @self.app.route("/mine", methods={"GET"})
+    def mine():
+      # calculate POW
+      proof = self.blockchain.prove_work()
+
+      # reward miner
+      self.blockchain.add_transaction(
+          origin="0", # zero signifies that this is a newly minted coin
+          target=self.identifier,
+          amount=1)
+
+      # publish new block
+      self.blockchain.add_block(proof=proof)
+      return msg("Success")
+
+    ############################################################################
+    # /transactions
+    ############################################################################
+
+    @self.app.route("/transactions/new", methods={"POST"})
+    def new_transaction():
+      payload = request.get_json()
+
+      self.blockchain.add_transaction(
+          origin=payload["origin"],
+          target=payload["target"],
+          amount=payload["amount"])
+      return msg("Success")
+
+    ############################################################################
+    # /blocks
+    ############################################################################
+
+    @self.app.route("/chain", methods={"GET"})
+    def view_blocks():
+      return jsonify({
+          "length": len(self.blockchain),
+          "chain": self.blockchain.dictify(),
+      })
+
+    ############################################################################
+    # /nodes
+    ############################################################################
+    @self.app.route("/node/neighbors", methods={"GET"})
+    def view_neighbors():
+      return jsonify({"neighbors": list(self.neighbors)})
+
+    @self.app.route("/node/register", methods={"POST"})
+    def register_nodes():
+      payload = request.get_json()["neighbors"]
+      payload = set(payload) if payload else set()
+      self.add_neighbors(payload)
+      return msg("Success")
+
+    @self.app.route("/node/resolve", methods={"GET"})
+    def resolve_nodes():
+      self.resolve_conflicts()
+      return msg("Success")
+
+  def run(self):
+    self.app.run(host="0.0.0.0", port=8000)
+
+
+class Blockchain(object):
+  def __init__(self, blocks=None, transactions=None):
+    self.blocks = blocks or []
+    self.transactions = transactions or []
+    self.add_block()
+
+  def __len__(self):
+    return len(self.blocks)
+
+  def __iter__(self):
+    for block in self.blocks:
+      yield block
+
+  def prove_work(self):
+    guess, prev_proof = 0, self.blocks[-1].proof or 0
+    while not is_pow_valid(guess, prev_proof):
+      guess += 1
+    return guess
+
+  def add_block(self, prev_hash=None, proof=None):
+    b = Block(
+        index=len(self),
+        transactions=self.transactions,
+        prev_hash=self.blocks[-1].hash() if self.blocks else None,
+        proof=proof)
+    self.blocks.append(b)
+    return b
+
+  def adopt_blocks(self, json_blocks):
+    pass
+
+  def add_transaction(self, origin=None, target=None, amount=None):
+    tx = Transaction(origin=origin, target=target, amount=amount)
+    self.transactions.append(tx)
+
+  @staticmethod
+  def is_valid(chain):
+    prev_block = next(chain)
+
+    for block in chain:
+      if block.prev_hash != prev_block.hash() or not is_pow_valid(prev_block.proof, block.proof):
+        return False
+      prev_block = block
+
+    return True
+
+  def dictify(self):
+    return {
+        "blocks": [block.dictify() for block in self.blocks],
+        "transactions": [tx.dictify() for tx in self.transactions],
+    }
+
+
+class Block(object):
+  def __init__(self, index=None, ts=None, transactions=None, proof=None, prev_hash=None):
+    self.index = index
+    self.ts = ts or str(datetime.now())
+    self.transactions = transactions
+    self.proof = proof
+    self.prev_hash = prev_hash
+
+  def hash(self):
+    return sha256(self.jsonify().encode()).hexdigest()
+
+  def dictify(self):
+    return {
+        "index": self.index,
+        "ts": self.ts,
+        "transactions": [tx.dictify() for tx in self.transactions],
+        "proof": self.proof,
+        "prev_hash": self.prev_hash,
+    }
+
+  def jsonify(self):
+    return json.dumps(self.dictify(), sort_keys=True)
+
+class Transaction(object):
+  def __init__(self, origin=None, target=None, amount=None):
+    if None in {origin, target, amount}:
+      raise ValueError("To create a Transaction, you must provide origin, target, and amount")
+
+    self.origin = origin
+    self.target = target
+    self.amount = amount
+
+  def dictify(self):
+    return {
+        "origin": self.origin,
+        "target": self.target,
+        "amount": self.amount,
+    }
+
+  def jsonify(self):
+    return json.dumps(self.dictify(), sort_keys=True)
+
+################################################################################
+# Main
+################################################################################
+
+def run():
+  Node(host="0.0.0.0", port=8000).run()
+
+if __name__ == "__main__":
+  run()
diff --git a/users/wpcarro/scratch/blockchain/setup.py b/users/wpcarro/scratch/blockchain/setup.py
new file mode 100644
index 0000000000..e5310565db
--- /dev/null
+++ b/users/wpcarro/scratch/blockchain/setup.py
@@ -0,0 +1,10 @@
+from setuptools import setup
+
+setup(
+    name='main',
+    version='0.0.1',
+    py_modules=['main'],
+    entry_points={
+      'console_scripts': ['main = main:run']
+    },
+)