[libc] Lock the output stream for the 'puts' call (#76513)

Summary:
The `puts` function consists of an initial write and then another write
to append the newline. When executing code in parallel, it is possible
for these writes to becomes disjointed. This code adds an explicit lock
call to ensure that the string is always appended by the newline as the
users expects.

Wasn't sure if this required a test as it would be difficult since
reproducing it would be flaky.

Author: jhuber6

libc/src/stdio/generic/puts.cpp

@@ -15,17 +15,34 @@
 
 namespace LIBC_NAMESPACE {
 
+namespace {
+
+// Simple helper to unlock the file once destroyed.
+struct ScopedLock {
+  ScopedLock(LIBC_NAMESPACE::File *stream) : stream(stream) { stream->lock(); }
+  ~ScopedLock() { stream->unlock(); }
+
+private:
+  LIBC_NAMESPACE::File *stream;
+};
+
+} // namespace
+
 LLVM_LIBC_FUNCTION(int, puts, (const char *__restrict str)) {
   cpp::string_view str_view(str);
-  auto result = LIBC_NAMESPACE::stdout->write(str, str_view.size());
+
+  // We need to lock the stream to ensure the newline is always appended.
+  ScopedLock lock(LIBC_NAMESPACE::stdout);
+
+  auto result = LIBC_NAMESPACE::stdout->write_unlocked(str, str_view.size());
   if (result.has_error())
     libc_errno = result.error;
   size_t written = result.value;
   if (str_view.size() != written) {
     // The stream should be in an error state in this case.
     return EOF;
   }
-  result = LIBC_NAMESPACE::stdout->write("\n", 1);
+  result = LIBC_NAMESPACE::stdout->write_unlocked("\n", 1);
   if (result.has_error())
     libc_errno = result.error;
   written = result.value;