[lldb] Fix 'ptsname_r' is only available on macOS 10.13.4 or newer

A deployment target less than 10.13.4 causes an error saying that
'ptsname_r' is only available on macOS 10.13.4 or newer. The current
logic only checks if the symbol is available and doesn't account for the
deployment target. This patch fixes that by adding an availability
check.

Differential revision: https://reviews.llvm.org/D125995

lldb/source/Host/common/PseudoTerminal.cpp

@@ -22,6 +22,10 @@
 
 #include "lldb/Host/PosixApi.h"
 
+#if defined(__APPLE__)
+#include <Availability.h>
+#endif
+
 #if defined(__ANDROID__)
 int posix_openpt(int flags);
 #endif
@@ -99,21 +103,33 @@ llvm::Error PseudoTerminal::OpenSecondary(int oflag) {
       std::error_code(errno, std::generic_category()));
 }
 
+static std::string use_ptsname(int fd) {
+  static std::mutex mutex;
+  std::lock_guard<std::mutex> guard(mutex);
+  const char *r = ptsname(fd);
+  assert(r != nullptr);
+  return r;
+}
+
 std::string PseudoTerminal::GetSecondaryName() const {
   assert(m_primary_fd >= 0);
 #if HAVE_PTSNAME_R
-  char buf[PATH_MAX];
-  buf[0] = '\0';
-  int r = ptsname_r(m_primary_fd, buf, sizeof(buf));
-  (void)r;
-  assert(r == 0);
-  return buf;
+#if defined(__APPLE__)
+  if (__builtin_available(macos 10.13.4, iOS 11.3, tvOS 11.3, watchOS 4.4, *)) {
+#endif
+    char buf[PATH_MAX];
+    buf[0] = '\0';
+    int r = ptsname_r(m_primary_fd, buf, sizeof(buf));
+    (void)r;
+    assert(r == 0);
+    return buf;
+#if defined(__APPLE__)
+  } else {
+    return use_ptsname(m_primary_fd);
+  }
+#endif
 #else
-  static std::mutex mutex;
-  std::lock_guard<std::mutex> guard(mutex);
-  const char *r = ptsname(m_primary_fd);
-  assert(r != nullptr);
-  return r;
+  return use_ptsname(m_primary_fd);
 #endif
 }