musl: allocator: Avoid deadlock in pthread_atfork

Musl 1.2.3 and before call malloc() in pthread_atfork(),
which may result in a deadlock:

    PartitionRoot::EnableThreadCacheIfSupported()
      ::partition_alloc::internal::ScopedGuard guard{lock_};
      ThreadCache::Create(this);
        ThreadCache::ThreadCache()
          PlatformThread::CurrentId()
            InitAtFork::InitAtFork()
              pthread_atfork()
                malloc()
                  ShimMalloc()
                    PartitionAllocFunctionsInternal::Malloc()
                      PartitionRoot::AllocInternal()
                        PartitionRoot::AllocInternalNoHooks()
                          PartitionRoot::RawAlloc()
                            ::partition_alloc::internal::ScopedGuard guard{internal::PartitionRootLock(this)};

src/base/allocator/partition_allocator/src/partition_alloc/partition_root.cc

@@ -54,6 +54,10 @@
 #include <pthread.h>
 #endif  // PA_BUILDFLAG(IS_LINUX) || PA_BUILDFLAG(IS_CHROMEOS)
 
+#if defined(__MUSL__)
+#include "partition_alloc/shim/allocator_shim.h"
+#endif
+
 namespace partition_alloc::internal {
 
 #if PA_BUILDFLAG(RECORD_ALLOC_INFO)
@@ -305,9 +309,27 @@ void PartitionAllocMallocInitOnce() {
   // However, no perfect solution really exists to make threads + fork()
   // cooperate, but deadlocks are real (and fork() is used in DEATH_TEST()s),
   // and other malloc() implementations use the same techniques.
+
+#if defined(__MUSL__)
+  allocator_shim::AllocatorDispatch d =
+      *allocator_shim::GetAllocatorDispatchChainHeadForTesting();
+  d.alloc_function =
+      +[](size_t size, allocator_shim::AllocToken, void*) -> void* {
+    // The size of the scratch fits struct atfork_funcs in Musl
+    // pthread_atfork.c.
+    static char scratch[5 * sizeof(void*)];
+    return size != sizeof(scratch) ? nullptr : scratch;
+  };
+  allocator_shim::InsertAllocatorDispatch(&d);
+#endif
+
   int err =
       pthread_atfork(BeforeForkInParent, AfterForkInParent, AfterForkInChild);
   PA_CHECK(err == 0);
+
+#if defined(__MUSL__)
+  allocator_shim::RemoveAllocatorDispatchForTesting(&d);
+#endif
 #endif  // PA_BUILDFLAG(IS_LINUX) || PA_BUILDFLAG(IS_CHROMEOS)
 }