Merge bitcoin/bitcoin#18847: compressor: use a prevector in CompressScript serialization [ZAP1]

83a425d25af033086744c1c8c892015014ed46bd compressor: use a prevector in compressed script serialization (William Casarin)

Pull request description:

  This function was doing millions of unnecessary heap allocations during IBD.

  I'm start to catalog unnecessary heap allocations as a pet project of mine: as-zero-as-possible-alloc IBD. This is one small step.

  before:
  ![May01-174536](https://user-images.githubusercontent.com/45598/80850964-9a38de80-8bd3-11ea-8eec-08cd38ee1fa1.png)

  after:
  ![May01-174610](https://user-images.githubusercontent.com/45598/80850974-a91f9100-8bd3-11ea-94a1-e2077391f6f4.png)

  ~should I type alias this?~ *I type aliased it*

  This is a part of the Zero Allocations Project #18849 (ZAP1). This code came up as a place where many allocations occur.

ACKs for top commit:
  Empact:
    ACK https://github.com/bitcoin/bitcoin/pull/18847/commits/83a425d25af033086744c1c8c892015014ed46bd
  elichai:
    tACK 83a425d25af033086744c1c8c892015014ed46bd
  sipa:
    utACK 83a425d25af033086744c1c8c892015014ed46bd

Tree-SHA512: f0ffa6ab0ea1632715b0b76362753f9f6935f05cdcc80d85566774401155a3c57ad45a687942a1806d3503858f0bb698da9243746c8e2edb8fdf13611235b0e0

4 files changed, 27 insertions, 13 deletions

diff --git a/src/compressor.cpp b/src/compressor.cpp
index a70306d320..ef3135e7a5 100644
--- a/src/compressor.cpp
+++ b/src/compressor.cpp
@@ -52,7 +52,7 @@ static bool IsToPubKey(const CScript& script, CPubKey &pubkey)
     return false;
 }
 
-bool CompressScript(const CScript& script, std::vector<unsigned char> &out)
+bool CompressScript(const CScript& script, CompressedScript& out)
 {
     CKeyID keyID;
     if (IsToKeyID(script, keyID)) {
@@ -92,7 +92,7 @@ unsigned int GetSpecialScriptSize(unsigned int nSize)
     return 0;
 }
 
-bool DecompressScript(CScript& script, unsigned int nSize, const std::vector<unsigned char> &in)
+bool DecompressScript(CScript& script, unsigned int nSize, const CompressedScript& in)
 {
     switch(nSize) {
     case 0x00:
diff --git a/src/compressor.h b/src/compressor.h
index 478bfff0b6..40b2496f06 100644
--- a/src/compressor.h
+++ b/src/compressor.h
@@ -6,14 +6,26 @@
 #ifndef BITCOIN_COMPRESSOR_H
 #define BITCOIN_COMPRESSOR_H
 
+#include <prevector.h>
 #include <primitives/transaction.h>
 #include <script/script.h>
 #include <serialize.h>
 #include <span.h>
 
-bool CompressScript(const CScript& script, std::vector<unsigned char> &out);
+/**
+ * This saves us from making many heap allocations when serializing
+ * and deserializing compressed scripts.
+ *
+ * This prevector size is determined by the largest .resize() in the
+ * CompressScript function. The largest compressed script format is a
+ * compressed public key, which is 33 bytes.
+ */
+using CompressedScript = prevector<33, unsigned char>;
+
+
+bool CompressScript(const CScript& script, CompressedScript& out);
 unsigned int GetSpecialScriptSize(unsigned int nSize);
-bool DecompressScript(CScript& script, unsigned int nSize, const std::vector<unsigned char> &out);
+bool DecompressScript(CScript& script, unsigned int nSize, const CompressedScript& in);
 
 /**
  * Compress amount.
@@ -51,7 +63,7 @@ struct ScriptCompression
 
     template<typename Stream>
     void Ser(Stream &s, const CScript& script) {
-        std::vector<unsigned char> compr;
+        CompressedScript compr;
         if (CompressScript(script, compr)) {
             s << MakeSpan(compr);
             return;
@@ -66,7 +78,7 @@ struct ScriptCompression
         unsigned int nSize = 0;
         s >> VARINT(nSize);
         if (nSize < nSpecialScripts) {
-            std::vector<unsigned char> vch(GetSpecialScriptSize(nSize), 0x00);
+            CompressedScript vch(GetSpecialScriptSize(nSize), 0x00);
             s >> MakeSpan(vch);
             DecompressScript(script, nSize, vch);
             return;
diff --git a/src/test/compress_tests.cpp b/src/test/compress_tests.cpp
index 4ddbc8338e..7b661a0d1d 100644
--- a/src/test/compress_tests.cpp
+++ b/src/test/compress_tests.cpp
@@ -72,7 +72,7 @@ BOOST_AUTO_TEST_CASE(compress_script_to_ckey_id)
     CScript script = CScript() << OP_DUP << OP_HASH160 << ToByteVector(pubkey.GetID()) << OP_EQUALVERIFY << OP_CHECKSIG;
     BOOST_CHECK_EQUAL(script.size(), 25U);
 
-    std::vector<unsigned char> out;
+    CompressedScript out;
     bool done = CompressScript(script, out);
     BOOST_CHECK_EQUAL(done, true);
 
@@ -89,7 +89,7 @@ BOOST_AUTO_TEST_CASE(compress_script_to_cscript_id)
     script << OP_HASH160 << ToByteVector(CScriptID(redeemScript)) << OP_EQUAL;
     BOOST_CHECK_EQUAL(script.size(), 23U);
 
-    std::vector<unsigned char> out;
+    CompressedScript out;
     bool done = CompressScript(script, out);
     BOOST_CHECK_EQUAL(done, true);
 
@@ -107,7 +107,7 @@ BOOST_AUTO_TEST_CASE(compress_script_to_compressed_pubkey_id)
     CScript script = CScript() << ToByteVector(key.GetPubKey()) << OP_CHECKSIG; // COMPRESSED_PUBLIC_KEY_SIZE (33)
     BOOST_CHECK_EQUAL(script.size(), 35U);
 
-    std::vector<unsigned char> out;
+    CompressedScript out;
     bool done = CompressScript(script, out);
     BOOST_CHECK_EQUAL(done, true);
 
@@ -124,7 +124,7 @@ BOOST_AUTO_TEST_CASE(compress_script_to_uncompressed_pubkey_id)
     CScript script =  CScript() << ToByteVector(key.GetPubKey()) << OP_CHECKSIG; // PUBLIC_KEY_SIZE (65)
     BOOST_CHECK_EQUAL(script.size(), 67U);                   // 1 char code + 65 char pubkey + OP_CHECKSIG
 
-    std::vector<unsigned char> out;
+    CompressedScript out;
     bool done = CompressScript(script, out);
     BOOST_CHECK_EQUAL(done, true);
 
diff --git a/src/test/fuzz/script.cpp b/src/test/fuzz/script.cpp
index c8f95c2980..b87bcf2ef5 100644
--- a/src/test/fuzz/script.cpp
+++ b/src/test/fuzz/script.cpp
@@ -43,7 +43,7 @@ FUZZ_TARGET_INIT(script, initialize_script)
     if (!script_opt) return;
     const CScript script{*script_opt};
 
-    std::vector<unsigned char> compressed;
+    CompressedScript compressed;
     if (CompressScript(script, compressed)) {
         const unsigned int size = compressed[0];
         compressed.erase(compressed.begin());
@@ -143,10 +143,12 @@ FUZZ_TARGET_INIT(script, initialize_script)
 
     {
         const std::vector<uint8_t> bytes = ConsumeRandomLengthByteVector(fuzzed_data_provider);
+        CompressedScript compressed_script;
+        compressed_script.assign(bytes.begin(), bytes.end());
         // DecompressScript(..., ..., bytes) is not guaranteed to be defined if the bytes vector is too short
-        if (bytes.size() >= 32) {
+        if (compressed_script.size() >= 32) {
             CScript decompressed_script;
-            DecompressScript(decompressed_script, fuzzed_data_provider.ConsumeIntegral<unsigned int>(), bytes);
+            DecompressScript(decompressed_script, fuzzed_data_provider.ConsumeIntegral<unsigned int>(), compressed_script);
         }
     }