Merge #10699: Make all script validation flags backward compatible

01013f5 Simplify tx validation tests (Pieter Wuille)
2dd6f80 Add a test that all flags are softforks (Pieter Wuille)
2851b77 Make all script verification flags softforks (Pieter Wuille)

Pull request description:

  This change makes `SCRIPT_VERIFY_UPGRADABLE_NOPS` not apply to `OP_CHECKLOCKTIMEVERIFY` and `OP_CHECKSEQUENCEVERIFY`. This is a no-op as `UPGRADABLE_NOPS` is only set for mempool transactions, and those always have `SCRIPT_VERIFY_CHECKLOCKTIMEVERIFY` and `SCRIPT_VERIFY_CHECKSEQUENCEVERIFY` set as well. The advantage is that setting more flags now always results in a reduction in acceptable scripts (=softfork).

  This results in a nice and testable property for validation, for which a new test is added.

  This also means that the introduction of a new definition for a NOP or witness version will likely need the following procedure (example OP_NOP8 here)
  * Remove OP_NOP8 from being affected by `SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_NOPS`.
  * Add a `SCRIPT_VERIFY_DISCOURAGE_NOP8`, which only applies to `OP_NOP8`.
  * Add a `SCRIPT_VERIFY_NOP8` which implements the new consensus logic.
  * Before activation, add `SCRIPT_VERIFY_DISCOURAGE_NOP8` to the mempool flags.
  * After activation, add `SCRIPT_VERIFY_NOP8` to both the mempool and consensus flags.

Tree-SHA512: d3b4538986ecf646aac9dba13a8d89318baf9e308e258547ca3b99e7c0509747f323edac6b1fea4e87e7d3c01b71193794b41679ae4f86f6e11ed6be3fd62c72

2 files changed, 14 insertions, 15 deletions

diff --git a/src/script/interpreter.cpp b/src/script/interpreter.cpp
index d6348f17d8..9091af4c0c 100644
--- a/src/script/interpreter.cpp
+++ b/src/script/interpreter.cpp
@@ -349,9 +349,6 @@ bool EvalScript(std::vector<std::vector<unsigned char> >& stack, const CScript&
                 {
                     if (!(flags & SCRIPT_VERIFY_CHECKLOCKTIMEVERIFY)) {
                         // not enabled; treat as a NOP2
-                        if (flags & SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_NOPS) {
-                            return set_error(serror, SCRIPT_ERR_DISCOURAGE_UPGRADABLE_NOPS);
-                        }
                         break;
                     }
 
@@ -391,9 +388,6 @@ bool EvalScript(std::vector<std::vector<unsigned char> >& stack, const CScript&
                 {
                     if (!(flags & SCRIPT_VERIFY_CHECKSEQUENCEVERIFY)) {
                         // not enabled; treat as a NOP3
-                        if (flags & SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_NOPS) {
-                            return set_error(serror, SCRIPT_ERR_DISCOURAGE_UPGRADABLE_NOPS);
-                        }
                         break;
                     }
 
diff --git a/src/script/interpreter.h b/src/script/interpreter.h
index 2eae68179e..83a96739b1 100644
--- a/src/script/interpreter.h
+++ b/src/script/interpreter.h
@@ -27,37 +27,40 @@ enum
     SIGHASH_ANYONECANPAY = 0x80,
 };
 
-/** Script verification flags */
+/** Script verification flags.
+ *
+ *  All flags are intended to be soft forks: the set of acceptable scripts under
+ *  flags (A | B) is a subset of the acceptable scripts under flag (A).
+ */
 enum
 {
     SCRIPT_VERIFY_NONE      = 0,
 
-    // Evaluate P2SH subscripts (softfork safe, BIP16).
+    // Evaluate P2SH subscripts (BIP16).
     SCRIPT_VERIFY_P2SH      = (1U << 0),
 
     // Passing a non-strict-DER signature or one with undefined hashtype to a checksig operation causes script failure.
     // Evaluating a pubkey that is not (0x04 + 64 bytes) or (0x02 or 0x03 + 32 bytes) by checksig causes script failure.
-    // (softfork safe, but not used or intended as a consensus rule).
+    // (not used or intended as a consensus rule).
     SCRIPT_VERIFY_STRICTENC = (1U << 1),
 
-    // Passing a non-strict-DER signature to a checksig operation causes script failure (softfork safe, BIP62 rule 1)
+    // Passing a non-strict-DER signature to a checksig operation causes script failure (BIP62 rule 1)
     SCRIPT_VERIFY_DERSIG    = (1U << 2),
 
     // Passing a non-strict-DER signature or one with S > order/2 to a checksig operation causes script failure
-    // (softfork safe, BIP62 rule 5).
+    // (BIP62 rule 5).
     SCRIPT_VERIFY_LOW_S     = (1U << 3),
 
-    // verify dummy stack item consumed by CHECKMULTISIG is of zero-length (softfork safe, BIP62 rule 7).
+    // verify dummy stack item consumed by CHECKMULTISIG is of zero-length (BIP62 rule 7).
     SCRIPT_VERIFY_NULLDUMMY = (1U << 4),
 
-    // Using a non-push operator in the scriptSig causes script failure (softfork safe, BIP62 rule 2).
+    // Using a non-push operator in the scriptSig causes script failure (BIP62 rule 2).
     SCRIPT_VERIFY_SIGPUSHONLY = (1U << 5),
 
     // Require minimal encodings for all push operations (OP_0... OP_16, OP_1NEGATE where possible, direct
     // pushes up to 75 bytes, OP_PUSHDATA up to 255 bytes, OP_PUSHDATA2 for anything larger). Evaluating
     // any other push causes the script to fail (BIP62 rule 3).
     // In addition, whenever a stack element is interpreted as a number, it must be of minimal length (BIP62 rule 4).
-    // (softfork safe)
     SCRIPT_VERIFY_MINIMALDATA = (1U << 6),
 
     // Discourage use of NOPs reserved for upgrades (NOP1-10)
@@ -68,12 +71,14 @@ enum
     // discouraged NOPs fails the script. This verification flag will never be
     // a mandatory flag applied to scripts in a block. NOPs that are not
     // executed, e.g.  within an unexecuted IF ENDIF block, are *not* rejected.
+    // NOPs that have associated forks to give them new meaning (CLTV, CSV)
+    // are not subject to this rule.
     SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_NOPS  = (1U << 7),
 
     // Require that only a single stack element remains after evaluation. This changes the success criterion from
     // "At least one stack element must remain, and when interpreted as a boolean, it must be true" to
     // "Exactly one stack element must remain, and when interpreted as a boolean, it must be true".
-    // (softfork safe, BIP62 rule 6)
+    // (BIP62 rule 6)
     // Note: CLEANSTACK should never be used without P2SH or WITNESS.
     SCRIPT_VERIFY_CLEANSTACK = (1U << 8),