Merge bitcoin/bitcoin#30604: doc, chainparams: 29775 release notes and follow-ups

92c1d7d validation: Use MAX_TIMEWARP constant as testnet4 timewarp defense delta (Fabian Jahr)
4b2fad5 doc: Add release notes for 29775 (Fabian Jahr)
f7cc973 doc: Align deprecation warnings (Fabian Jahr)
1163b08 chainparams: Add initial minimum chain work for Testnet4 (Fabian Jahr)

Pull request description:

  This completes follow-ups left open in #29775.

  - Adds release notes
  - Addresses the [misalignment](bitcoin/bitcoin#29775 (comment)) in deprecation warnings and hints at the intention to remove support for Testnet3.
  - Adds initial minimum chainwork for Testnet4.
  - Use the `MAX_TIMEWARP` constant as the timewarp defense delta, equal to `MAX_FUTURE_BLOCK_TIME`.

ACKs for top commit:
  Sjors:
    ACK 92c1d7d
  achow101:
    ACK 92c1d7d
  tdb3:
    re ACK 92c1d7d

Tree-SHA512: 7ebdac7809f96231f75ca62706af59cd1ed27f713a4c7be5e2ad69fae95832b146b3ea23c712fb03b412da1deda7e8a5dae55bb2bbd2dcfd9f926e85c2a72666

Author: achow101

doc/release-notes-29775.md

@@ -0,0 +1,10 @@
+Testnet4/BIP94 support
+-----
+
+Support for Testnet4 as specified in [BIP94](https://github.com/bitcoin/bips/blob/master/bip-0094.mediawiki)
+has been added. The network can be selected with the `-testnet4` option and
+the section header is also named `[testnet4]`.
+
+While the intention is to phase out support for Testnet3 in an upcoming
+version, support for it is still available via the known options in this
+release.

src/chainparamsbase.cpp

@@ -17,7 +17,7 @@ void SetupChainParamsBaseOptions(ArgsManager& argsman)
     argsman.AddArg("-regtest", "Enter regression test mode, which uses a special chain in which blocks can be solved instantly. "
                  "This is intended for regression testing tools and app development. Equivalent to -chain=regtest.", ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::CHAINPARAMS);
     argsman.AddArg("-testactivationheight=name@height.", "Set the activation height of 'name' (segwit, bip34, dersig, cltv, csv). (regtest-only)", ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::DEBUG_TEST);
-    argsman.AddArg("-testnet", "Use the testnet3 chain. Equivalent to -chain=test. Support for testnet3 is deprecated and will be removed with the next release. Consider moving to testnet4 now by using -testnet4.", ArgsManager::ALLOW_ANY, OptionsCategory::CHAINPARAMS);
+    argsman.AddArg("-testnet", "Use the testnet3 chain. Equivalent to -chain=test. Support for testnet3 is deprecated and will be removed in an upcoming release. Consider moving to testnet4 now by using -testnet4.", ArgsManager::ALLOW_ANY, OptionsCategory::CHAINPARAMS);
     argsman.AddArg("-testnet4", "Use the testnet4 chain. Equivalent to -chain=testnet4.", ArgsManager::ALLOW_ANY, OptionsCategory::CHAINPARAMS);
     argsman.AddArg("-vbparams=deployment:start:end[:min_activation_height]", "Use given start/end times and min_activation_height for specified version bits deployment (regtest-only)", ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::CHAINPARAMS);
     argsman.AddArg("-signet", "Use the signet chain. Equivalent to -chain=signet. Note that the network is defined by the -signetchallenge parameter", ArgsManager::ALLOW_ANY, OptionsCategory::CHAINPARAMS);

src/kernel/chainparams.cpp

@@ -334,7 +334,7 @@ class CTestNet4Params : public CChainParams {
         consensus.vDeployments[Consensus::DEPLOYMENT_TAPROOT].nTimeout = Consensus::BIP9Deployment::NO_TIMEOUT;
         consensus.vDeployments[Consensus::DEPLOYMENT_TAPROOT].min_activation_height = 0; // No activation delay
 
-        consensus.nMinimumChainWork = uint256{};
+        consensus.nMinimumChainWork = uint256{"000000000000000000000000000000000000000000000056faca98a0cd9bdf5f"};
         consensus.defaultAssumeValid = uint256{};
 
         pchMessageStart[0] = 0x1c;

src/validation.cpp

@@ -107,6 +107,21 @@ const std::vector<std::string> CHECKLEVEL_DOC {
  * */
 static constexpr int PRUNE_LOCK_BUFFER{10};
 
+/**
+ * Maximum number of seconds that the timestamp of the first
+ * block of a difficulty adjustment period is allowed to
+ * be earlier than the last block of the previous period (BIP94).
+ */
+static constexpr int64_t MAX_TIMEWARP{MAX_FUTURE_BLOCK_TIME};
+
+/**
+ * If the timestamp of the last block in a difficulty period is set
+ * MAX_FUTURE_BLOCK_TIME seconds in the future, an honest miner should
+ * be able to mine the first block using the current time. This is not
+ * a consensus rule, but changing MAX_TIMEWARP should be done with caution.
+ */
+static_assert(MAX_FUTURE_BLOCK_TIME <= MAX_TIMEWARP);
+
 GlobalMutex g_best_block_mutex;
 std::condition_variable g_best_block_cv;
 uint256 g_best_block;
@@ -4188,7 +4203,7 @@ static bool ContextualCheckBlockHeader(const CBlockHeader& block, BlockValidatio
         // Check timestamp for the first block of each difficulty adjustment
         // interval, except the genesis block.
         if (nHeight % consensusParams.DifficultyAdjustmentInterval() == 0) {
-            if (block.GetBlockTime() < pindexPrev->GetBlockTime() - 60 * 60 * 2) {
+            if (block.GetBlockTime() < pindexPrev->GetBlockTime() - MAX_TIMEWARP) {
                 return state.Invalid(BlockValidationResult::BLOCK_INVALID_HEADER, "time-timewarp-attack", "block's timestamp is too early on diff adjustment block");
             }
         }