add TLS-based fork-inflight guard

Author: SchrodingerZhu

libc/src/__support/OSUtil/linux/pid.cpp

@@ -13,6 +13,7 @@
 namespace LIBC_NAMESPACE {
 
 pid_t ProcessIdentity::cache = -1;
+thread_local bool ProcessIdentity::fork_inflight = false;
 pid_t ProcessIdentity::get_uncached() {
   return syscall_impl<pid_t>(SYS_getpid);
 }

libc/src/__support/OSUtil/pid.h

@@ -14,14 +14,16 @@
 namespace LIBC_NAMESPACE {
 
 class ProcessIdentity {
+  static thread_local bool fork_inflight;
   static pid_t cache;
   static pid_t get_uncached();
 
 public:
-  LIBC_INLINE static void invalidate_cache() { cache = -1; }
+  LIBC_INLINE static void start_fork() { fork_inflight = true; }
+  LIBC_INLINE static void end_fork() { fork_inflight = false; }
   LIBC_INLINE static void refresh_cache() { cache = get_uncached(); }
   LIBC_INLINE static pid_t get() {
-    if (LIBC_UNLIKELY(cache < 0))
+    if (LIBC_UNLIKELY(fork_inflight))
       return get_uncached();
     return cache;
   }

libc/src/unistd/linux/fork.cpp

@@ -30,7 +30,7 @@ LLVM_LIBC_FUNCTION(pid_t, fork, (void)) {
   // getting wrong values. gettid() is not async-signal-safe, but let's provide
   // our best efforts here.
   self.invalidate_tid();
-  ProcessIdentity::invalidate_cache();
+  ProcessIdentity::start_fork();
 
 #ifdef SYS_fork
   pid_t ret = LIBC_NAMESPACE::syscall_impl<pid_t>(SYS_fork);
@@ -46,15 +46,20 @@ LLVM_LIBC_FUNCTION(pid_t, fork, (void)) {
     return -1;
   }
 
+  // Common
   // Refresh tid/pid cache after fork
   self.refresh_tid();
-  ProcessIdentity::refresh_cache();
 
+  // Child process
   if (ret == 0) {
+    ProcessIdentity::refresh_cache();
+    ProcessIdentity::end_fork();
     invoke_child_callbacks();
     return 0;
   }
 
+  // Parent process
+  ProcessIdentity::end_fork();
   invoke_parent_callbacks();
   return ret;
 }