[libc] Fix off by one error in strftime (llvm#165711)

This patch fixes a bug in strftime's return value when the formatted
output exactly fills the buffer, not including the null terminator. The
previous check failed to account for the null terminator in this case,
incorrectly returning the written count instead of 0.

Author: mleleszi

libc/src/time/strftime.cpp

@@ -26,7 +26,7 @@ LLVM_LIBC_FUNCTION(size_t, strftime,
   int ret = strftime_core::strftime_main(&writer, format, timeptr);
   if (buffsz > 0) // if the buffsz is 0 the buffer may be a null pointer.
     wb.buff[wb.buff_cur] = '\0';
-  return (ret < 0 || static_cast<size_t>(ret) > buffsz) ? 0 : ret;
+  return (ret < 0 || static_cast<size_t>(ret) >= buffsz) ? 0 : ret;
 }
 
 } // namespace LIBC_NAMESPACE_DECL

libc/src/time/strftime_l.cpp

@@ -29,7 +29,7 @@ LLVM_LIBC_FUNCTION(size_t, strftime_l,
   int ret = strftime_core::strftime_main(&writer, format, timeptr);
   if (buffsz > 0) // if the buffsz is 0 the buffer may be a null pointer.
     wb.buff[wb.buff_cur] = '\0';
-  return (ret < 0 || static_cast<size_t>(ret) > buffsz) ? 0 : ret;
+  return (ret < 0 || static_cast<size_t>(ret) >= buffsz) ? 0 : ret;
 }
 
 } // namespace LIBC_NAMESPACE_DECL

libc/test/src/time/strftime_test.cpp

@@ -2326,3 +2326,23 @@ TEST(LlvmLibcStrftimeTest, TimeFormatFullDateTime) {
 //    size_t written = 0;
 //    SimplePaddedNum spn;
 //  }
+
+TEST(LlvmLibcStrftimeTest, BufferTooSmall) {
+  struct tm time;
+  char buffer[1];
+
+  time.tm_year = get_adjusted_year(2025);
+  time.tm_mon = 10;
+  time.tm_mday = 24;
+
+  size_t written =
+      LIBC_NAMESPACE::strftime(buffer, sizeof(buffer), "%F", &time);
+  EXPECT_EQ(written, size_t{0});
+
+  char buffer2[10];
+
+  // The string "2025-11-24" is 10 chars,
+  // so strftime needs 10 + 1 bytes to write the string and the null terminator.
+  written = LIBC_NAMESPACE::strftime(buffer, sizeof(buffer2), "%F", &time);
+  EXPECT_EQ(written, size_t{0});
+}