Increase LevelDB max_open_files unless on 32-bit Unix.

This change significantly increases IBD performance by increasing the
amount of the UTXO index that can remain in memory. To ensure this
doesn't cause problems in the future, a static_assert on the LevelDB
version has been added, which must be updated by anyone upgrading
LevelDB.

src/dbwrapper.cpp

@@ -71,6 +71,31 @@ public:
     }
 };
 
+static void SetMaxOpenFiles(leveldb::Options *options) {
+    // On most platforms the default setting of max_open_files (which is 1000)
+    // is optimal. On Windows using a large file count is OK because the handles
+    // do not interfere with select() loops. On 64-bit Unix hosts this value is
+    // also OK, because up to that amount LevelDB will use an mmap
+    // implementation that does not use extra file descriptors (the fds are
+    // closed after being mmaped).
+    //
+    // Increasing the value beyond the default is dangerous because LevelDB will
+    // fall back to a non-mmap implementation when the file count is too large.
+    // On 32-bit Unix host we should decrease the value because the handles use
+    // up real fds, and we want to avoid fd exhaustion issues.
+    //
+    // See PR #12495 for further discussion.
+
+    int default_open_files = options->max_open_files;
+#ifndef WIN32
+    if (sizeof(void*) < 8) {
+        options->max_open_files = 64;
+    }
+#endif
+    LogPrint(BCLog::LEVELDB, "LevelDB using max_open_files=%d (default=%d)\n",
+             options->max_open_files, default_open_files);
+}
+
 static leveldb::Options GetOptions(size_t nCacheSize)
 {
     leveldb::Options options;
@@ -78,13 +103,13 @@ static leveldb::Options GetOptions(size_t nCacheSize)
     options.write_buffer_size = nCacheSize / 4; // up to two write buffers may be held in memory simultaneously
     options.filter_policy = leveldb::NewBloomFilterPolicy(10);
     options.compression = leveldb::kNoCompression;
-    options.max_open_files = 64;
     options.info_log = new CBitcoinLevelDBLogger();
     if (leveldb::kMajorVersion > 1 || (leveldb::kMajorVersion == 1 && leveldb::kMinorVersion >= 16)) {
         // LevelDB versions before 1.16 consider short writes to be corruption. Only trigger error
         // on corruption in later versions.
         options.paranoid_checks = true;
     }
+    SetMaxOpenFiles(&options);
     return options;
 }