miniscript: introduce a MsContext() helper to contexts

We are going to introduce Tapscript support in Miniscript, for which
some of Miniscript rules and properties change (new or modified
fragments, different typing rules, different resources consumption, ..).

src/script/descriptor.cpp

@@ -1426,8 +1426,11 @@ struct KeyParser {
     mutable std::vector<std::unique_ptr<PubkeyProvider>> m_keys;
     //! Used to detect key parsing errors within a Miniscript.
     mutable std::string m_key_parsing_error;
+    //! The script context we're operating within (Tapscript or P2WSH).
+    const miniscript::MiniscriptContext m_script_ctx;
 
-    KeyParser(FlatSigningProvider* out LIFETIMEBOUND, const SigningProvider* in LIFETIMEBOUND) : m_out(out), m_in(in) {}
+    KeyParser(FlatSigningProvider* out LIFETIMEBOUND, const SigningProvider* in LIFETIMEBOUND, miniscript::MiniscriptContext ctx)
+        : m_out(out), m_in(in), m_script_ctx(ctx) {}
 
     bool KeyCompare(const Key& a, const Key& b) const {
         return *m_keys.at(a) < *m_keys.at(b);
@@ -1475,6 +1478,10 @@ struct KeyParser {
         }
         return {};
     }
+
+    miniscript::MiniscriptContext MsContext() const {
+        return m_script_ctx;
+    }
 };
 
 /** Parse a script in a particular context. */
@@ -1714,7 +1721,7 @@ std::unique_ptr<DescriptorImpl> ParseScript(uint32_t& key_exp_index, Span<const
     }
     // Process miniscript expressions.
     {
-        KeyParser parser(&out, nullptr);
+        KeyParser parser(/*out = */&out, /* in = */nullptr, /* ctx = */miniscript::MiniscriptContext::P2WSH);
         auto node = miniscript::FromString(std::string(expr.begin(), expr.end()), parser);
         if (node) {
             if (ctx != ParseScriptContext::P2WSH) {
@@ -1887,7 +1894,7 @@ std::unique_ptr<DescriptorImpl> InferScript(const CScript& script, ParseScriptCo
     }
 
     if (ctx == ParseScriptContext::P2WSH) {
-        KeyParser parser(nullptr, &provider);
+        KeyParser parser(/* out = */nullptr, /* in = */&provider, /* ctx = */miniscript::MiniscriptContext::P2WSH);
         auto node = miniscript::FromScript(script, parser);
         if (node && node->IsSane()) {
             return std::make_unique<MiniscriptDescriptor>(std::move(parser.m_keys), std::move(node));

src/script/miniscript.h

@@ -229,6 +229,11 @@ enum class Availability {
     MAYBE,
 };
 
+enum class MiniscriptContext {
+    P2WSH,
+    TAPSCRIPT,
+};
+
 namespace internal {
 
 //! Helper function for Node::CalcType.

src/script/sign.cpp

@@ -404,6 +404,8 @@ struct Satisfier {
     SignatureData& m_sig_data;
     const BaseSignatureCreator& m_creator;
     const CScript& m_witness_script;
+    //! For now Miniscript is only available under P2WSH.
+    const miniscript::MiniscriptContext m_script_ctx{miniscript::MiniscriptContext::P2WSH};
 
     explicit Satisfier(const SigningProvider& provider LIFETIMEBOUND, SignatureData& sig_data LIFETIMEBOUND,
                        const BaseSignatureCreator& creator LIFETIMEBOUND,
@@ -466,6 +468,10 @@ struct Satisfier {
     miniscript::Availability SatHASH160(const std::vector<unsigned char>& hash, std::vector<unsigned char>& preimage) const {
         return MsLookupHelper(m_sig_data.hash160_preimages, hash, preimage);
     }
+
+    miniscript::MiniscriptContext MsContext() const {
+        return m_script_ctx;
+    }
 };
 
 bool ProduceSignature(const SigningProvider& provider, const BaseSignatureCreator& creator, const CScript& fromPubKey, SignatureData& sigdata)

src/test/fuzz/miniscript.cpp

@@ -128,6 +128,10 @@ struct ParserContext {
         if (it == TEST_DATA.dummy_keys_map.end()) return {};
         return it->second;
     }
+
+    miniscript::MiniscriptContext MsContext() const {
+        return miniscript::MiniscriptContext::P2WSH;
+    }
 } PARSER_CTX;
 
 //! Context that implements naive conversion from/to script only, for roundtrip testing.
@@ -172,6 +176,10 @@ struct ScriptParserContext {
         key.is_hash = true;
         return key;
     }
+
+    miniscript::MiniscriptContext MsContext() const {
+        return miniscript::MiniscriptContext::P2WSH;
+    }
 } SCRIPT_PARSER_CONTEXT;
 
 //! Context to produce a satisfaction for a Miniscript node using the pre-computed data.

src/test/miniscript_tests.cpp

@@ -114,6 +114,8 @@ typedef std::pair<ChallengeType, uint32_t> Challenge;
 struct KeyConverter {
     typedef CPubKey Key;
 
+    const miniscript::MiniscriptContext m_script_ctx{miniscript::MiniscriptContext::P2WSH};
+
     bool KeyCompare(const Key& a, const Key& b) const {
         return a < b;
     }
@@ -158,6 +160,10 @@ struct KeyConverter {
     std::optional<std::string> ToString(const Key& key) const {
         return HexStr(ToPKBytes(key));
     }
+
+    miniscript::MiniscriptContext MsContext() const {
+        return m_script_ctx;
+    }
 };
 
 /** A class that encapsulates all signing/hash revealing operations. */