main: open block and undo files read-only when they're opened for reading

OpenDiskFile opened every block or undo file "rb+", even for its
fReadOnly callers, so every reader held a writable fd. That's what let
the wolfSSL CloseSocket bug corrupt blocks/*.dat. A P2P double close
took the wallet's BDB fd away; the number was then handed to a block
file opened for a getblock/getdata read; BDB's next wallet page write
landed in it, at offset pgno*4096. On production that destroyed blocks
2,728,132-2,728,161 in blk00011.dat and block 3,356,880 in blk00020.dat.

Every fReadOnly caller only reads through the handle:
- ReadBlockFromDisk, UndoReadFromDisk, both GetTransaction paths and
  hush_blockload read via CAutoFile >>;
- the reindex loop hands it to LoadExternalBlockFile, which reads via
  CBufferedFile;
- the existence check at startup only opens it;
- hush_prefetch only freads.
Open those "rb". A stray write through a reused fd number then fails
(EBADF) instead of silently corrupting a block file. Missing files
still fail to open as before; writers keep "rb+"/"wb+".

FdDefense.ReadOnlyBlockFileIsNotWritable: pwrite through a read-only
block file's fd fails with EBADF, while the writable open still writes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/gtest/test_fd_defense.cpp

@@ -0,0 +1,55 @@
+// Copyright (c) 2016-2024 The Hush developers
+// Distributed under the GPLv3 software license, see the accompanying
+// file COPYING or https://www.gnu.org/licenses/gpl-3.0.en.html
+
+#include <gtest/gtest.h>
+
+#include <errno.h>
+#include <stdio.h>
+#include <unistd.h>
+
+#include <boost/filesystem.hpp>
+
+#include "chainparams.h"
+#include "main.h"
+#include "util.h"
+
+// A block file opened for reading must not be writable through its fd: a stray write through a reused fd number
+// (the wolfSSL CloseSocket macro let the wallet's BDB fd be closed behind its back) used to land in it.
+TEST(FdDefense, ReadOnlyBlockFileIsNotWritable) {
+    SelectParams(CBaseChainParams::MAIN);
+    const boost::filesystem::path dir = boost::filesystem::temp_directory_path() / boost::filesystem::unique_path("fddefense-%%%%%%%%");
+    const bool hadDatadir = mapArgs.count("-datadir") > 0;
+    const std::string oldDatadir = hadDatadir ? mapArgs["-datadir"] : "";
+    boost::filesystem::create_directories(dir);
+    mapArgs["-datadir"] = dir.string();
+    ClearDatadirCache();
+
+    const CDiskBlockPos pos(0, 0);
+    const boost::filesystem::path blk = GetBlockPosFilename(pos, "blk");
+    boost::filesystem::create_directories(blk.parent_path());
+    {
+        FILE* f = fopen(blk.string().c_str(), "wb");
+        ASSERT_TRUE(f != nullptr);
+        fputs("block bytes", f);
+        fclose(f);
+    }
+
+    FILE* ro = OpenBlockFile(pos, true);
+    ASSERT_TRUE(ro != nullptr);
+    char c = 0;
+    EXPECT_EQ(1u, fread(&c, 1, 1, ro)) << "still readable";
+    errno = 0;
+    EXPECT_EQ(-1, pwrite(fileno(ro), "X", 1, 0)) << "a write through a read-only block file's fd must fail";
+    EXPECT_EQ(EBADF, errno);
+    fclose(ro);
+
+    FILE* rw = OpenBlockFile(pos, false);
+    ASSERT_TRUE(rw != nullptr);
+    EXPECT_EQ(1, pwrite(fileno(rw), "b", 1, 0)) << "the writable open still writes";
+    fclose(rw);
+
+    if (hadDatadir) mapArgs["-datadir"] = oldDatadir; else mapArgs.erase("-datadir");
+    ClearDatadirCache();
+    boost::filesystem::remove_all(dir);
+}