Merge bitcoin/bitcoin#24371: util: Fix `ReadBinaryFile` reading beyond maxsize

a84650ebd5ac2cbb49f14eb7c98736a3f8215bf1 util: Fix ReadBinaryFile reading beyond maxsize (klementtan)

Pull request description:

  Currently `ReadBinaryFile` will read beyond `maxsize` if `maxsize` is not a multiple of `128` (size of buffer)

  This is due to `fread` being called with `count = 128` instead of `count = min(128, maxsize - retval.size()` at every iteration

  The following unit test will fail:
  ```cpp
  BOOST_AUTO_TEST_CASE(util_ReadWriteFile)
  {
    fs::path tmpfolder = m_args.GetDataDirBase();
    fs::path tmpfile = tmpfolder / "read_binary.dat";
    std::string expected_text(300,'c');
    {
        std::ofstream file{tmpfile};
        file << expected_text;
    }
    {
        // read half the contents in file
        auto [valid, text] = ReadBinaryFile(tmpfile, expected_text.size() / 2);
        BOOST_CHECK_EQUAL(text.size(), 150);
    }
  }
  ```
  Error:
  ```
  test/util_tests.cpp:2593: error: in "util_tests/util_ReadWriteFile": check text.size() == 150 has failed [256 != 150]
  ```

ACKs for top commit:
  laanwj:
    Code review ACK a84650ebd5ac2cbb49f14eb7c98736a3f8215bf1
  theStack:
    Code-review ACK a84650ebd5ac2cbb49f14eb7c98736a3f8215bf1

Tree-SHA512: 752eebe58bc2102dec199b6775f8c3304d899f0ce36d6a022a58e27b076ba945ccd572858b19137b769effd8c6de73a9277f641be24dfb17657fb7173ea0eda0

2 files changed, 50 insertions, 3 deletions

diff --git a/src/test/util_tests.cpp b/src/test/util_tests.cpp
index 9f78215de2..1881573e7a 100644
--- a/src/test/util_tests.cpp
+++ b/src/test/util_tests.cpp
@@ -17,6 +17,7 @@
 #include <util/message.h> // For MessageSign(), MessageVerify(), MESSAGE_MAGIC
 #include <util/moneystr.h>
 #include <util/overflow.h>
+#include <util/readwritefile.h>
 #include <util/spanparsing.h>
 #include <util/strencodings.h>
 #include <util/string.h>
@@ -24,9 +25,10 @@
 #include <util/vector.h>
 
 #include <array>
-#include <optional>
+#include <fstream>
 #include <limits>
 #include <map>
+#include <optional>
 #include <stdint.h>
 #include <string.h>
 #include <thread>
@@ -2592,4 +2594,49 @@ BOOST_AUTO_TEST_CASE(util_ParseByteUnits)
     BOOST_CHECK(!ParseByteUnits("1x", noop));
 }
 
+BOOST_AUTO_TEST_CASE(util_ReadBinaryFile)
+{
+    fs::path tmpfolder = m_args.GetDataDirBase();
+    fs::path tmpfile = tmpfolder / "read_binary.dat";
+    std::string expected_text;
+    for (int i = 0; i < 30; i++) {
+        expected_text += "0123456789";
+    }
+    {
+        std::ofstream file{tmpfile};
+        file << expected_text;
+    }
+    {
+        // read all contents in file
+        auto [valid, text] = ReadBinaryFile(tmpfile);
+        BOOST_CHECK(valid);
+        BOOST_CHECK_EQUAL(text, expected_text);
+    }
+    {
+        // read half contents in file
+        auto [valid, text] = ReadBinaryFile(tmpfile, expected_text.size() / 2);
+        BOOST_CHECK(valid);
+        BOOST_CHECK_EQUAL(text, expected_text.substr(0, expected_text.size() / 2));
+    }
+    {
+        // read from non-existent file
+        fs::path invalid_file = tmpfolder / "invalid_binary.dat";
+        auto [valid, text] = ReadBinaryFile(invalid_file);
+        BOOST_CHECK(!valid);
+        BOOST_CHECK(text.empty());
+    }
+}
+
+BOOST_AUTO_TEST_CASE(util_WriteBinaryFile)
+{
+    fs::path tmpfolder = m_args.GetDataDirBase();
+    fs::path tmpfile = tmpfolder / "write_binary.dat";
+    std::string expected_text = "bitcoin";
+    auto valid = WriteBinaryFile(tmpfile, expected_text);
+    std::string actual_text;
+    std::ifstream file{tmpfile};
+    file >> actual_text;
+    BOOST_CHECK(valid);
+    BOOST_CHECK_EQUAL(actual_text, expected_text);
+}
 BOOST_AUTO_TEST_SUITE_END()
diff --git a/src/util/readwritefile.cpp b/src/util/readwritefile.cpp
index a45c41d367..628e6a3980 100644
--- a/src/util/readwritefile.cpp
+++ b/src/util/readwritefile.cpp
@@ -18,7 +18,7 @@ std::pair<bool,std::string> ReadBinaryFile(const fs::path &filename, size_t maxs
     std::string retval;
     char buffer[128];
     do {
-        const size_t n = fread(buffer, 1, sizeof(buffer), f);
+        const size_t n = fread(buffer, 1, std::min(sizeof(buffer), maxsize - retval.size()), f);
         // Check for reading errors so we don't return any data if we couldn't
         // read the entire file (or up to maxsize)
         if (ferror(f)) {
@@ -26,7 +26,7 @@ std::pair<bool,std::string> ReadBinaryFile(const fs::path &filename, size_t maxs
             return std::make_pair(false,"");
         }
         retval.append(buffer, buffer+n);
-    } while (!feof(f) && retval.size() <= maxsize);
+    } while (!feof(f) && retval.size() < maxsize);
     fclose(f);
     return std::make_pair(true,retval);
 }