[Darwin][compiler-rt] kern.osproductversion unavailable pre macOS 10.13

[yln]

Summary

Add a fallback for sysctl kern.osproductversion for XNU 17 (macOS
10.13) and below, which do not provide this property.

Unfortunately, this means we have to take the detour via Darwin kernel
version again (at least for the fallback).

[yln]

Fix linter warning.

[delcypher]

LGTM

compiler-rt/lib/sanitizer_common/sanitizer_mac.cpp

635

If for some reason in the future we need the minor OS version too then we could use the kern.osversion sysctl. Right now we don't need it for those older OSs AFAIK so let's go with your approach.

compiler-rt/lib/sanitizer_common/sanitizer_mac.cpp

HandleSignalMode GetHandleSignalMode(int signum) {
   if ((SANITIZER_WATCHOS || SANITIZER_TVOS) && !(SANITIZER_IOSSIM))
     return kHandleSignalNo;
   HandleSignalMode result = GetHandleSignalModeImpl(signum);
   if (result == kHandleSignalYes && !common_flags()->allow_user_segv_handler)
     return kHandleSignalExclusive;
   return result;
 }
 
+// Offset example:
+// XNU 17 -- macOS 10.13 -- iOS 11 -- tvOS 11 -- watchOS 4
+constexpr u16 GetOSMajorKernelOffset() {
+  if (TARGET_OS_OSX) return 4;
+  if (SANITIZER_IOS || SANITIZER_TVOS) return 6;
+  if (SANITIZER_WATCHOS) return 13;
+}
+
 using VersStr = char[64];
 
 static void GetOSVersion(VersStr vers) {
   uptr len = sizeof(VersStr);
   if (SANITIZER_IOSSIM) {
     const char *vers_env = GetEnv("SIMULATOR_RUNTIME_VERSION");
     if (!vers_env) {
       Report("ERROR: Running in simulator but SIMULATOR_RUNTIME_VERSION env "
           "var is not set.\n");
       Die();
     }
     len = internal_strlcpy(vers, vers_env, len);
   } else {
     int res =
         internal_sysctlbyname("kern.osproductversion", vers, &len, nullptr, 0);
-    CHECK_EQ(res, 0);
+    if (res) {
+      // Fallback for XNU 17 (macOS 10.13) and below that do not provide the
+      // `kern.osproductversion` property.
+      u16 kernel_major = GetDarwinKernelVersion().major;
+      u16 offset = GetOSMajorKernelOffset();
+      CHECK_LE(kernel_major, 17);
+      CHECK_GE(kernel_major, offset);
+      u16 os_major = kernel_major - offset;
+
+      auto format = TARGET_OS_OSX ? "10.%d" : "%d.0";
+      len = internal_snprintf(vers, len, format, os_major);
+    }
   }
   CHECK_LT(len, sizeof(VersStr));
 }
 
 void ParseVersion(const char *vers, u16 *major, u16 *minor) {
   // Format: <major>.<minor>[.<patch>]\0
   CHECK_GE(internal_strlen(vers), 3);
   const char *p = vers;