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

1 files changed, 2 insertions, 2 deletions

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);
 }