test: Add test_framework/bdb.py module for inspecting bdb files

For upgrade tests and possibly other tests, it is useful to inspect the
bdb file for the wallet (i.e. the wallet.dat file).
test_framework/bdb.py is an implementation of bdb file deserialization
specific for Bitcoin Core's usage.

test/functional/test_framework/bdb.py

@@ -0,0 +1,152 @@
+#!/usr/bin/env python3
+# Copyright (c) 2020 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+"""
+Utilities for working directly with the wallet's BDB database file
+
+This is specific to the configuration of BDB used in this project:
+    - pagesize: 4096 bytes
+    - Outer database contains single subdatabase named 'main'
+    - btree
+    - btree leaf pages
+
+Each key-value pair is two entries in a btree leaf. The first is the key, the one that follows
+is the value. And so on. Note that the entry data is itself not in the correct order. Instead
+entry offsets are stored in the correct order and those offsets are needed to then retrieve
+the data itself.
+
+Page format can be found in BDB source code dbinc/db_page.h
+This only implements the deserialization of btree metadata pages and normal btree pages. Overflow
+pages are not implemented but may be needed in the future if dealing with wallets with large
+transactions.
+
+`db_dump -da wallet.dat` is useful to see the data in a wallet.dat BDB file
+"""
+
+import binascii
+import struct
+
+# Important constants
+PAGESIZE = 4096
+OUTER_META_PAGE = 0
+INNER_META_PAGE = 2
+
+# Page type values
+BTREE_INTERNAL = 3
+BTREE_LEAF = 5
+BTREE_META = 9
+
+# Some magic numbers for sanity checking
+BTREE_MAGIC = 0x053162
+DB_VERSION = 9
+
+# Deserializes a leaf page into a dict.
+# Btree internal pages have the same header, for those, return None.
+# For the btree leaf pages, deserialize them and put all the data into a dict
+def dump_leaf_page(data):
+    page_info = {}
+    page_header = data[0:26]
+    _, pgno, prev_pgno, next_pgno, entries, hf_offset, level, pg_type = struct.unpack('QIIIHHBB', page_header)
+    page_info['pgno'] = pgno
+    page_info['prev_pgno'] = prev_pgno
+    page_info['next_pgno'] = next_pgno
+    page_info['entries'] = entries
+    page_info['hf_offset'] = hf_offset
+    page_info['level'] = level
+    page_info['pg_type'] = pg_type
+    page_info['entry_offsets'] = struct.unpack('{}H'.format(entries), data[26:26 + entries * 2])
+    page_info['entries'] = []
+
+    if pg_type == BTREE_INTERNAL:
+        # Skip internal pages. These are the internal nodes of the btree and don't contain anything relevant to us
+        return None
+
+    assert pg_type == BTREE_LEAF, 'A non-btree leaf page has been encountered while dumping leaves'
+
+    for i in range(0, entries):
+        offset = page_info['entry_offsets'][i]
+        entry = {'offset': offset}
+        page_data_header = data[offset:offset + 3]
+        e_len, pg_type = struct.unpack('HB', page_data_header)
+        entry['len'] = e_len
+        entry['pg_type'] = pg_type
+        entry['data'] = data[offset + 3:offset + 3 + e_len]
+        page_info['entries'].append(entry)
+
+    return page_info
+
+# Deserializes a btree metadata page into a dict.
+# Does a simple sanity check on the magic value, type, and version
+def dump_meta_page(page):
+    # metadata page
+    # general metadata
+    metadata = {}
+    meta_page = page[0:72]
+    _, pgno, magic, version, pagesize, encrypt_alg, pg_type, metaflags, _, free, last_pgno, nparts, key_count, record_count, flags, uid = struct.unpack('QIIIIBBBBIIIIII20s', meta_page)
+    metadata['pgno'] = pgno
+    metadata['magic'] = magic
+    metadata['version'] = version
+    metadata['pagesize'] = pagesize
+    metadata['encrypt_alg'] = encrypt_alg
+    metadata['pg_type'] = pg_type
+    metadata['metaflags'] = metaflags
+    metadata['free'] = free
+    metadata['last_pgno'] = last_pgno
+    metadata['nparts'] = nparts
+    metadata['key_count'] = key_count
+    metadata['record_count'] = record_count
+    metadata['flags'] = flags
+    metadata['uid'] = binascii.hexlify(uid)
+
+    assert magic == BTREE_MAGIC, 'bdb magic does not match bdb btree magic'
+    assert pg_type == BTREE_META, 'Metadata page is not a btree metadata page'
+    assert version == DB_VERSION, 'Database too new'
+
+    # btree metadata
+    btree_meta_page = page[72:512]
+    _, minkey, re_len, re_pad, root, _, crypto_magic, _, iv, chksum = struct.unpack('IIIII368sI12s16s20s', btree_meta_page)
+    metadata['minkey'] = minkey
+    metadata['re_len'] = re_len
+    metadata['re_pad'] = re_pad
+    metadata['root'] = root
+    metadata['crypto_magic'] = crypto_magic
+    metadata['iv'] = binascii.hexlify(iv)
+    metadata['chksum'] = binascii.hexlify(chksum)
+    return metadata
+
+# Given the dict from dump_leaf_page, get the key-value pairs and put them into a dict
+def extract_kv_pairs(page_data):
+    out = {}
+    last_key = None
+    for i, entry in enumerate(page_data['entries']):
+        # By virtue of these all being pairs, even number entries are keys, and odd are values
+        if i % 2 == 0:
+            out[entry['data']] = b''
+            last_key = entry['data']
+        else:
+            out[last_key] = entry['data']
+    return out
+
+# Extract the key-value pairs of the BDB file given in filename
+def dump_bdb_kv(filename):
+    # Read in the BDB file and start deserializing it
+    pages = []
+    with open(filename, 'rb') as f:
+        data = f.read(PAGESIZE)
+        while len(data) > 0:
+            pages.append(data)
+            data = f.read(PAGESIZE)
+
+    # Sanity check the meta pages
+    dump_meta_page(pages[OUTER_META_PAGE])
+    dump_meta_page(pages[INNER_META_PAGE])
+
+    # Fetch the kv pairs from the leaf pages
+    kv = {}
+    for i in range(3, len(pages)):
+        info = dump_leaf_page(pages[i])
+        if info is not None:
+            info_kv = extract_kv_pairs(info)
+            kv = {**kv, **info_kv}
+    return kv