Merge #18034: Get the OutputType for a descriptor

7e80f646b24a2abf3c031a649bcc706a695f80da Get the OutputType for a descriptor (Andrew Chow)

Pull request description:

  Adds a `GetOutputType()` method to get the OutputType of a descriptor. Some descriptors don't have a determinate OutputType, so we actually use an `Optional<OutputType>`. For descriptors with indeterminate OutputType, we return `nullopt`.

  `addr()` and `raw()` use OutputTypes as determined by the CTxDestination they have. For simplicity, `ScriptHash` destinations are `LEGACY` even though they could be `P2SH_SEGWIT`.
  `combo()`, `pk()`, and `multi()` are `nullopt` as they either don't have an OutputType or they have multiple. `DescriptorImpl` defaults to `nullopt`.
  `pkh()` is `LEGACY` as expected
  `wpkh()` and `wsh()` are `BECH32` as expected.
  `sh()` checks whether the sub-descriptor is `BECH32`. If so, it is `P2SH_SEGWIT`. Otherwise it is `LEGACY`.

  The descriptor tests are updated to check the OutputType too.

ACKs for top commit:
  fjahr:
    ACK 7e80f646b24a2abf3c031a649bcc706a695f80da
  meshcollider:
    utACK 7e80f646b24a2abf3c031a649bcc706a695f80da
  instagibbs:
    cursory ACK https://github.com/bitcoin/bitcoin/pull/18034/commits/7e80f646b24a2abf3c031a649bcc706a695f80da
  Sjors:
    Code review ACK 7e80f646b24a2abf3c031a649bcc706a695f80da
  jonatack:
    ACK 7e80f64 code review/build/tests

Tree-SHA512: c5a813447b62e982435e1c948066f8d6c148c9ebffb0a5eb5a9028b173b01d5ead2f076a5ca3f7f37698538baa346f82a977ee48f583d89cb4e5ebd9111b2341

2 files changed, 50 insertions, 4 deletions

diff --git a/src/script/descriptor.cpp b/src/script/descriptor.cpp
index 773d6a55c4..b1d9a5bda7 100644
--- a/src/script/descriptor.cpp
+++ b/src/script/descriptor.cpp
@@ -339,14 +339,15 @@ class DescriptorImpl : public Descriptor
 {
     //! Public key arguments for this descriptor (size 1 for PK, PKH, WPKH; any size for Multisig).
     const std::vector<std::unique_ptr<PubkeyProvider>> m_pubkey_args;
+    //! The string name of the descriptor function.
+    const std::string m_name;
+
+protected:
     //! The sub-descriptor argument (nullptr for everything but SH and WSH).
     //! In doc/descriptors.m this is referred to as SCRIPT expressions sh(SCRIPT)
     //! and wsh(SCRIPT), and distinct from KEY expressions and ADDR expressions.
     const std::unique_ptr<DescriptorImpl> m_subdescriptor_arg;
-    //! The string name of the descriptor function.
-    const std::string m_name;
 
-protected:
     //! Return a serialization of anything except pubkey and script arguments, to be prepended to those.
     virtual std::string ToStringExtra() const { return ""; }
 
@@ -364,7 +365,7 @@ protected:
     virtual std::vector<CScript> MakeScripts(const std::vector<CPubKey>& pubkeys, const CScript* script, FlatSigningProvider& out) const = 0;
 
 public:
-    DescriptorImpl(std::vector<std::unique_ptr<PubkeyProvider>> pubkeys, std::unique_ptr<DescriptorImpl> script, const std::string& name) : m_pubkey_args(std::move(pubkeys)), m_subdescriptor_arg(std::move(script)), m_name(name) {}
+    DescriptorImpl(std::vector<std::unique_ptr<PubkeyProvider>> pubkeys, std::unique_ptr<DescriptorImpl> script, const std::string& name) : m_pubkey_args(std::move(pubkeys)), m_name(name), m_subdescriptor_arg(std::move(script)) {}
 
     bool IsSolvable() const override
     {
@@ -500,6 +501,8 @@ public:
             out = Merge(out, subprovider);
         }
     }
+
+    Optional<OutputType> GetOutputType() const override { return nullopt; }
 };
 
 /** A parsed addr(A) descriptor. */
@@ -512,6 +515,19 @@ protected:
 public:
     AddressDescriptor(CTxDestination destination) : DescriptorImpl({}, {}, "addr"), m_destination(std::move(destination)) {}
     bool IsSolvable() const final { return false; }
+
+    Optional<OutputType> GetOutputType() const override
+    {
+        switch (m_destination.which()) {
+            case 1 /* PKHash */:
+            case 2 /* ScriptHash */: return OutputType::LEGACY;
+            case 3 /* WitnessV0ScriptHash */:
+            case 4 /* WitnessV0KeyHash */:
+            case 5 /* WitnessUnknown */: return OutputType::BECH32;
+            case 0 /* CNoDestination */:
+            default: return nullopt;
+        }
+    }
 };
 
 /** A parsed raw(H) descriptor. */
@@ -524,6 +540,21 @@ protected:
 public:
     RawDescriptor(CScript script) : DescriptorImpl({}, {}, "raw"), m_script(std::move(script)) {}
     bool IsSolvable() const final { return false; }
+
+    Optional<OutputType> GetOutputType() const override
+    {
+        CTxDestination dest;
+        ExtractDestination(m_script, dest);
+        switch (dest.which()) {
+            case 1 /* PKHash */:
+            case 2 /* ScriptHash */: return OutputType::LEGACY;
+            case 3 /* WitnessV0ScriptHash */:
+            case 4 /* WitnessV0KeyHash */:
+            case 5 /* WitnessUnknown */: return OutputType::BECH32;
+            case 0 /* CNoDestination */:
+            default: return nullopt;
+        }
+    }
 };
 
 /** A parsed pk(P) descriptor. */
@@ -547,6 +578,7 @@ protected:
     }
 public:
     PKHDescriptor(std::unique_ptr<PubkeyProvider> prov) : DescriptorImpl(Vector(std::move(prov)), {}, "pkh") {}
+    Optional<OutputType> GetOutputType() const override { return OutputType::LEGACY; }
 };
 
 /** A parsed wpkh(P) descriptor. */
@@ -561,6 +593,7 @@ protected:
     }
 public:
     WPKHDescriptor(std::unique_ptr<PubkeyProvider> prov) : DescriptorImpl(Vector(std::move(prov)), {}, "wpkh") {}
+    Optional<OutputType> GetOutputType() const override { return OutputType::BECH32; }
 };
 
 /** A parsed combo(P) descriptor. */
@@ -612,6 +645,13 @@ protected:
     std::vector<CScript> MakeScripts(const std::vector<CPubKey>&, const CScript* script, FlatSigningProvider&) const override { return Vector(GetScriptForDestination(ScriptHash(*script))); }
 public:
     SHDescriptor(std::unique_ptr<DescriptorImpl> desc) : DescriptorImpl({}, std::move(desc), "sh") {}
+
+    Optional<OutputType> GetOutputType() const override
+    {
+        assert(m_subdescriptor_arg);
+        if (m_subdescriptor_arg->GetOutputType() == OutputType::BECH32) return OutputType::P2SH_SEGWIT;
+        return OutputType::LEGACY;
+    }
 };
 
 /** A parsed wsh(...) descriptor. */
@@ -621,6 +661,7 @@ protected:
     std::vector<CScript> MakeScripts(const std::vector<CPubKey>&, const CScript* script, FlatSigningProvider&) const override { return Vector(GetScriptForDestination(WitnessV0ScriptHash(*script))); }
 public:
     WSHDescriptor(std::unique_ptr<DescriptorImpl> desc) : DescriptorImpl({}, std::move(desc), "wsh") {}
+    Optional<OutputType> GetOutputType() const override { return OutputType::BECH32; }
 };
 
 ////////////////////////////////////////////////////////////////////////////
diff --git a/src/script/descriptor.h b/src/script/descriptor.h
index a5a41d78dd..58b920c681 100644
--- a/src/script/descriptor.h
+++ b/src/script/descriptor.h
@@ -5,6 +5,8 @@
 #ifndef BITCOIN_SCRIPT_DESCRIPTOR_H
 #define BITCOIN_SCRIPT_DESCRIPTOR_H
 
+#include <optional.h>
+#include <outputtype.h>
 #include <script/script.h>
 #include <script/sign.h>
 #include <script/signingprovider.h>
@@ -71,6 +73,9 @@ struct Descriptor {
      * @param[out] out Any private keys available for the specified `pos`.
      */
     virtual void ExpandPrivate(int pos, const SigningProvider& provider, FlatSigningProvider& out) const = 0;
+
+    /** @return The OutputType of the scriptPubKey(s) produced by this descriptor. Or nullopt if indeterminate (multiple or none) */
+    virtual Optional<OutputType> GetOutputType() const = 0;
 };
 
 /** Parse a `descriptor` string. Included private keys are put in `out`.