Merge #20082: [bugfix] random: fixes read buffer to use min rather than max

bd5215103eb3985c1622eddea45a040e6173829c random: fixes read buffer resizing in RandAddSeedPerfmon (Ethan Heilman)

Pull request description:

  As shown below when resizing the read buffer `vData` `std::max((vData.size() * 3) / 2, nMaxSize)` is used. This means that the buffer size immediately jumps to `nMaxSize`. I believe the intend of this code is to grow the buffer size through several steps rather than immediately resize it to the max size.

  ```cpp
      std::vector<unsigned char> vData(250000, 0);
      long ret = 0;
      unsigned long nSize = 0;
      const size_t nMaxSize = 10000000; // Bail out at more than 10MB of performance data
      while (true) {
          nSize = vData.size();
          ret = RegQueryValueExA(HKEY_PERFORMANCE_DATA, "Global", nullptr, nullptr, vData.data(), &nSize);
          if (ret != ERROR_MORE_DATA || vData.size() >= nMaxSize)
              break;
          vData.resize(std::max((vData.size() * 3) / 2, nMaxSize)); // Grow size of buffer exponentially
      }
  ```

  vData always starts at size 250,000 and nMaxSize is always 10,000,000 so the first time this line is reached:
  ```cpp
  vData.resize(std::max((vData.size() * 3) / 2, nMaxSize));
  ```
  the effect will always be to resize vData to nMaxSize. Then because the loop terminates when vData.size >= 10,000,000 only one resize operation will take place.

  To fix this issue we replace `std::min` with `std::max`

  This PR also adds a comment clarifying the behavior of this function the first time it is called.

ACKs for top commit:
  fanquake:
    ACK bd5215103eb3985c1622eddea45a040e6173829c - thanks for taking a look at this Ethan. Swapping from `std::max` to `std::min` here certainly seems correct.

Tree-SHA512: 7c65f700e5bbe44bc2f1ffdcdc99ec19c542894c95b5ee9791facd09d02afae88d1f8f35af129719e4860db94bc790856e7adb1d218a395381e7c2913b95f1d0

1 files changed, 3 insertions, 2 deletions

diff --git a/src/randomenv.cpp b/src/randomenv.cpp
index 073d82b491..07122b7f6d 100644
--- a/src/randomenv.cpp
+++ b/src/randomenv.cpp
@@ -67,7 +67,8 @@ void RandAddSeedPerfmon(CSHA512& hasher)
 #ifdef WIN32
     // Seed with the entire set of perfmon data
 
-    // This can take up to 2 seconds, so only do it every 10 minutes
+    // This can take up to 2 seconds, so only do it every 10 minutes.
+    // Initialize last_perfmon to 0 seconds, we don't skip the first call.
     static std::atomic<std::chrono::seconds> last_perfmon{std::chrono::seconds{0}};
     auto last_time = last_perfmon.load();
     auto current_time = GetTime<std::chrono::seconds>();
@@ -83,7 +84,7 @@ void RandAddSeedPerfmon(CSHA512& hasher)
         ret = RegQueryValueExA(HKEY_PERFORMANCE_DATA, "Global", nullptr, nullptr, vData.data(), &nSize);
         if (ret != ERROR_MORE_DATA || vData.size() >= nMaxSize)
             break;
-        vData.resize(std::max((vData.size() * 3) / 2, nMaxSize)); // Grow size of buffer exponentially
+        vData.resize(std::min((vData.size() * 3) / 2, nMaxSize)); // Grow size of buffer exponentially
     }
     RegCloseKey(HKEY_PERFORMANCE_DATA);
     if (ret == ERROR_SUCCESS) {