Merge #14646: Add expansion cache functions to descriptors (unused for now)

26879509f Add comments to descriptor tests (Pieter Wuille)
82df4c64f Add descriptor expansion cache (Pieter Wuille)
1eda33aab [refactor] Combine the ToString and ToPrivateString implementations (Pieter Wuille)
24d3a7b3a [refactor] Use DescriptorImpl internally, permitting access to new methods (Pieter Wuille)
6be0fb4b3 [refactor] Add a base DescriptorImpl with most common logic (Pieter Wuille)

Pull request description:

  This patch modifies the internal `Descriptor` class to optionally construct and use an "expansion cache". Such a cache is a byte array that encodes all information necessary to expand a `Descriptor` a second time without access to private keys, and without the need to perform expensive BIP32 derivations. For all currently defined descriptors, the cache simply contains a concatenation of all public keys used.

  This is motivated by the goal of importing a descriptor into the wallet and using it as a replacement for the keypool, where it would be impossible to expand descriptors if they use hardened derivation.

Tree-SHA512: f531a0a82ec1eecc30b78ba8a31724d1249826b028cc3543ad32372e1aedd537f137ab03dbffc222c5df444d5865ecd5cec754c1ae1d4989b6e9baeaffade32a

1 files changed, 11 insertions, 1 deletions

diff --git a/src/script/descriptor.h b/src/script/descriptor.h
index 0111972f85..44f0efca03 100644
--- a/src/script/descriptor.h
+++ b/src/script/descriptor.h
@@ -48,8 +48,18 @@ struct Descriptor {
      * provider: the provider to query for private keys in case of hardened derivation.
      * output_script: the expanded scriptPubKeys will be put here.
      * out: scripts and public keys necessary for solving the expanded scriptPubKeys will be put here (may be equal to provider).
+     * cache: vector which will be overwritten with cache data necessary to-evaluate the descriptor at this point without access to private keys.
      */
-    virtual bool Expand(int pos, const SigningProvider& provider, std::vector<CScript>& output_scripts, FlatSigningProvider& out) const = 0;
+    virtual bool Expand(int pos, const SigningProvider& provider, std::vector<CScript>& output_scripts, FlatSigningProvider& out, std::vector<unsigned char>* cache = nullptr) const = 0;
+
+    /** Expand a descriptor at a specified position using cached expansion data.
+     *
+     * pos: the position at which to expand the descriptor. If IsRange() is false, this is ignored.
+     * cache: vector from which cached expansion data will be read.
+     * output_script: the expanded scriptPubKeys will be put here.
+     * out: scripts and public keys necessary for solving the expanded scriptPubKeys will be put here (may be equal to provider).
+     */
+    virtual bool ExpandFromCache(int pos, const std::vector<unsigned char>& cache, std::vector<CScript>& output_scripts, FlatSigningProvider& out) const = 0;
 };
 
 /** Parse a descriptor string. Included private keys are put in out. Returns nullptr if parsing fails. */