util: Restore GetIntArg saturating behavior

The new locale-independent atoi64 method introduced in #20452 parses
large integer values higher than maximum representable value as 0
instead of the maximum value, which breaks backwards compatibility.
This commit restores compatibility and adds test coverage for this case
in terms of the related GetIntArg and strtoll functions.

Co-authored-by: Ryan Ofsky <ryan@ofsky.org>

1 files changed, 16 insertions, 3 deletions

diff --git a/src/util/strencodings.h b/src/util/strencodings.h
index 0945556b5e..1f83fa3ffa 100644
--- a/src/util/strencodings.h
+++ b/src/util/strencodings.h
@@ -16,6 +16,7 @@
 #include <charconv>
 #include <cstdint>
 #include <iterator>
+#include <limits>
 #include <optional>
 #include <string>
 #include <vector>
@@ -93,8 +94,12 @@ void SplitHostPort(std::string in, uint16_t& portOut, std::string& hostOut);
 // New code should use ToIntegral or the ParseInt* functions
 // which provide parse error feedback.
 //
-// The goal of LocaleIndependentAtoi is to replicate the exact defined behaviour
-// of atoi and atoi64 as they behave under the "C" locale.
+// The goal of LocaleIndependentAtoi is to replicate the defined behaviour of
+// std::atoi as it behaves under the "C" locale, and remove some undefined
+// behavior. If the parsed value is bigger than the integer type's maximum
+// value, or smaller than the integer type's minimum value, std::atoi has
+// undefined behavior, while this function returns the maximum or minimum
+// values, respectively.
 template <typename T>
 T LocaleIndependentAtoi(const std::string& str)
 {
@@ -109,7 +114,15 @@ T LocaleIndependentAtoi(const std::string& str)
         s = s.substr(1);
     }
     auto [_, error_condition] = std::from_chars(s.data(), s.data() + s.size(), result);
-    if (error_condition != std::errc{}) {
+    if (error_condition == std::errc::result_out_of_range) {
+        if (s.length() >= 1 && s[0] == '-') {
+            // Saturate underflow, per strtoll's behavior.
+            return std::numeric_limits<T>::min();
+        } else {
+            // Saturate overflow, per strtoll's behavior.
+            return std::numeric_limits<T>::max();
+        }
+    } else if (error_condition != std::errc{}) {
         return 0;
     }
     return result;