descriptors: add HavePrivateKeys()

Previously, to determine if a desc is watchonly, `ToPrivateString()`, was used.
It returns `false` if there is at least one pubkey in the descriptor for which
the provider  does not have a private key.

ToPrivateString() behaviour will change in the following commits to only
return `false` if no priv keys could be found for the pub keys in the descriptor.

HavePrivateKeys() is added here to replace the use of ToPrivateString() for determining
if a descriptor is 'watchonly'.

Co-authored-by: rkrux <rkrux.connect@gmail.com>

src/script/descriptor.cpp

@@ -835,6 +835,25 @@ public:
         return true;
     }
 
+    // NOLINTNEXTLINE(misc-no-recursion)
+    bool HavePrivateKeys(const SigningProvider& arg) const override
+    {
+        if (m_pubkey_args.empty() && m_subdescriptor_args.empty()) return false;
+
+        for (const auto& sub: m_subdescriptor_args) {
+            if (!sub->HavePrivateKeys(arg)) return false;
+        }
+
+        FlatSigningProvider tmp_provider;
+        for (const auto& pubkey : m_pubkey_args) {
+            tmp_provider.keys.clear();
+            pubkey->GetPrivKey(0, arg, tmp_provider);
+            if (tmp_provider.keys.empty()) return false;
+        }
+
+        return true;
+    }
+
     // NOLINTNEXTLINE(misc-no-recursion)
     bool IsRange() const final
     {

src/script/descriptor.h

@@ -111,6 +111,13 @@ struct Descriptor {
     /** Whether this descriptor will return one scriptPubKey or multiple (aka is or is not combo) */
     virtual bool IsSingleType() const = 0;
 
+    /** Whether the given provider has all private keys required by this descriptor.
+     * @return `false` if the descriptor doesn't have any keys or subdescriptors,
+     *         or if the provider does not have all private keys required by
+     *         the descriptor.
+     */
+    virtual bool HavePrivateKeys(const SigningProvider& provider) const = 0;
+
     /** Convert the descriptor to a private string. This fails if the provided provider does not have the relevant private keys. */
     virtual bool ToPrivateString(const SigningProvider& provider, std::string& out) const = 0;
 

src/test/descriptor_tests.cpp

@@ -49,7 +49,7 @@ constexpr int SIGNABLE = 1 << 3; // We can sign with this descriptor (this is no
 constexpr int DERIVE_HARDENED = 1 << 4; // The final derivation is hardened, i.e. ends with *' or *h
 constexpr int MIXED_PUBKEYS = 1 << 5;
 constexpr int XONLY_KEYS = 1 << 6; // X-only pubkeys are in use (and thus inferring/caching may swap parity of pubkeys/keyids)
-constexpr int MISSING_PRIVKEYS = 1 << 7; // Not all private keys are available, so ToPrivateString will fail.
+constexpr int MISSING_PRIVKEYS = 1 << 7; // Not all private keys are available. ToPrivateString() will fail and HavePrivateKeys() will return `false`.
 constexpr int SIGNABLE_FAILS = 1 << 8; // We can sign with this descriptor, but actually trying to sign will fail
 constexpr int MUSIG = 1 << 9; // This is a MuSig so key counts will have an extra key
 constexpr int MUSIG_DERIVATION = 1 << 10; // MuSig with BIP 328 derivation from the aggregate key
@@ -243,6 +243,9 @@ void DoCheck(std::string prv, std::string pub, const std::string& norm_pub, int
         } else {
             BOOST_CHECK_MESSAGE(EqualDescriptor(prv, prv1), "Private ser: " + prv1 + " Private desc: " + prv);
         }
+        BOOST_CHECK(!parse_priv->HavePrivateKeys(keys_pub));
+        BOOST_CHECK(parse_pub->HavePrivateKeys(keys_priv));
+
         BOOST_CHECK(!parse_priv->ToPrivateString(keys_pub, prv1));
         BOOST_CHECK(parse_pub->ToPrivateString(keys_priv, prv1));
         if (expected_prv) {

src/wallet/test/walletload_tests.cpp

@@ -26,6 +26,7 @@ public:
     bool IsRange() const override { return false; }
     bool IsSolvable() const override { return false; }
     bool IsSingleType() const override { return true; }
+    bool HavePrivateKeys(const SigningProvider&) const override { return false; }
     bool ToPrivateString(const SigningProvider& provider, std::string& out) const override { return false; }
     bool ToNormalizedString(const SigningProvider& provider, std::string& out, const DescriptorCache* cache = nullptr) const override { return false; }
     bool Expand(int pos, const SigningProvider& provider, std::vector<CScript>& output_scripts, FlatSigningProvider& out, DescriptorCache* write_cache = nullptr) const override { return false; };