`xloctime`: `do_put` shouldn't modify errno on success (#2049)

Co-authored-by: Stephan T. Lavavej <stl@microsoft.com>

stl/inc/xloctime

@@ -735,7 +735,8 @@ protected:
             _Fmt[3] = _Specifier;
         }
 
-        const int& _Errno_ref = errno; // Nonzero cost, pay it once
+        int& _Errno_ref      = errno; // Nonzero cost, pay it once
+        const int _Old_errno = _Errno_ref;
 
         for (_Num = 16;; _Num *= 2) { // convert into ever larger string buffer until success
             _Str.append(_Num, '\0');
@@ -748,6 +749,7 @@ protected:
             }
         }
 
+        _Errno_ref = _Old_errno;
         return _STD copy(&_Str[1], &_Str[_Count], _Dest);
     }
 
@@ -864,7 +866,8 @@ protected:
             _Fmt[3] = static_cast<_Elem>(_Specifier);
         }
 
-        const int& _Errno_ref = errno; // Nonzero cost, pay it once
+        int& _Errno_ref      = errno; // Nonzero cost, pay it once
+        const int _Old_errno = _Errno_ref;
 
         for (_Num = 16;; _Num *= 2) { // convert into ever larger string buffer until success
             _Str.append(_Num, '\0');
@@ -876,6 +879,8 @@ protected:
                 return _Dest;
             }
         }
+
+        _Errno_ref = _Old_errno;
         return _STD copy(&_Str[1], &_Str[_Count], _Dest);
     }
 
@@ -993,7 +998,8 @@ protected:
             _Fmt[3] = static_cast<_Elem>(_Specifier);
         }
 
-        const int& _Errno_ref = errno; // Nonzero cost, pay it once
+        int& _Errno_ref      = errno; // Nonzero cost, pay it once
+        const int _Old_errno = _Errno_ref;
 
         for (_Num = 16;; _Num *= 2) { // convert into ever larger string buffer until success
             _Str.append(_Num, '\0');
@@ -1006,6 +1012,7 @@ protected:
             }
         }
 
+        _Errno_ref = _Old_errno;
         return _STD copy(&_Str[1], &_Str[_Count], _Dest);
     }
 

tests/std/test.lst

@@ -193,6 +193,7 @@ tests\GH_001964_constexpr_generate_canonical
 tests\GH_002030_asan_annotate_string
 tests\GH_002030_asan_annotate_vector
 tests\GH_002039_byte_is_not_trivially_swappable
+tests\GH_002045_put_time_changes_errno
 tests\GH_002058_debug_iterator_race
 tests\GH_002120_streambuf_seekpos_and_seekoff
 tests\GH_002299_implicit_sfinae_constraints

tests/std/tests/GH_002045_put_time_changes_errno/env.lst

@@ -0,0 +1,4 @@
+# Copyright (c) Microsoft Corporation.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+RUNALL_INCLUDE ..\usual_matrix.lst

tests/std/tests/GH_002045_put_time_changes_errno/test.cpp

@@ -0,0 +1,48 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <cassert>
+#include <cerrno>
+#include <chrono>
+#include <ctime>
+#include <iomanip>
+#include <sstream>
+
+using namespace std;
+using namespace chrono;
+
+tm tm_now() {
+    const time_t t(system_clock::to_time_t(system_clock::now()));
+
+    tm tm_val;
+
+    const errno_t err = localtime_s(&tm_val, &t);
+    assert(err == 0);
+
+    return tm_val;
+}
+
+int main() {
+    const tm tm_val = tm_now();
+
+    {
+        ostringstream os;
+
+        assert(errno == 0);
+
+        os << put_time(&tm_val, "%Y-%m-%d %H:%M:%S %Z");
+
+        assert(os);
+        assert(errno == 0);
+    }
+    {
+        wostringstream os;
+
+        assert(errno == 0);
+
+        os << put_time(&tm_val, L"%Y-%m-%d %H:%M:%S %Z");
+
+        assert(os);
+        assert(errno == 0);
+    }
+}