[scudo][standalone] Enable secondary cache release on Fuchsia

I had left this as a TODO, but it turns out it wasn't complicated.
By specifying `MAP_RESIZABLE`, it allows us to keep the VMO which we
can then use for release purposes.
`releasePagesToOS` also had to be called the "proper" way, as Fuchsia
requires the `Offset` field to be correct. This has no impact on
non-Fuchsia platforms.

Differential Revision: https://reviews.llvm.org/D86800

compiler-rt/lib/scudo/standalone/combined.h

@@ -306,7 +306,7 @@ public:
 
     void *Block = nullptr;
     uptr ClassId = 0;
-    uptr SecondaryBlockEnd;
+    uptr SecondaryBlockEnd = 0;
     if (LIKELY(PrimaryT::canAllocate(NeededSize))) {
       ClassId = SizeClassMap::getClassIdBySize(NeededSize);
       DCHECK_NE(ClassId, 0U);

compiler-rt/lib/scudo/standalone/secondary.h

@@ -75,11 +75,6 @@ template <u32 EntriesArraySize = 32U, u32 DefaultMaxEntriesCount = 32U,
           s32 MaxReleaseToOsIntervalMs = INT32_MAX>
 class MapAllocatorCache {
 public:
-  // Fuchsia doesn't allow releasing Secondary blocks yet. Note that 0 length
-  // arrays are an extension for some compilers.
-  // FIXME(kostyak): support (partially) the cache on Fuchsia.
-  static_assert(!SCUDO_FUCHSIA || EntriesArraySize == 0U, "");
-
   // Ensure the default maximum specified fits the array.
   static_assert(DefaultMaxEntriesCount <= EntriesArraySize, "");
 
@@ -392,9 +387,9 @@ void *MapAllocator<CacheT>::allocate(uptr Size, uptr AlignmentHint,
   }
 
   const uptr CommitSize = MapEnd - PageSize - CommitBase;
-  const uptr Ptr =
-      reinterpret_cast<uptr>(map(reinterpret_cast<void *>(CommitBase),
-                                 CommitSize, "scudo:secondary", 0, &Data));
+  const uptr Ptr = reinterpret_cast<uptr>(
+      map(reinterpret_cast<void *>(CommitBase), CommitSize, "scudo:secondary",
+          MAP_RESIZABLE, &Data));
   LargeBlock::Header *H = reinterpret_cast<LargeBlock::Header *>(Ptr);
   H->MapBase = MapBase;
   H->MapSize = MapEnd - MapBase;

compiler-rt/lib/scudo/standalone/tests/combined_test.cpp

@@ -14,6 +14,7 @@
 #include <condition_variable>
 #include <memory>
 #include <mutex>
+#include <set>
 #include <thread>
 #include <vector>
 
@@ -23,7 +24,8 @@ static bool Ready;
 
 static constexpr scudo::Chunk::Origin Origin = scudo::Chunk::Origin::Malloc;
 
-static void disableDebuggerdMaybe() {
+// Fuchsia complains that the function is not used.
+UNUSED static void disableDebuggerdMaybe() {
 #if SCUDO_ANDROID
   // Disable the debuggerd signal handler on Android, without this we can end
   // up spending a significant amount of time creating tombstones.

compiler-rt/lib/scudo/standalone/tests/secondary_test.cpp

@@ -56,18 +56,12 @@ template <class SecondaryT> static void testSecondaryBasic(void) {
 
 TEST(ScudoSecondaryTest, SecondaryBasic) {
   testSecondaryBasic<scudo::MapAllocator<scudo::MapAllocatorNoCache>>();
-#if !SCUDO_FUCHSIA
   testSecondaryBasic<scudo::MapAllocator<scudo::MapAllocatorCache<>>>();
   testSecondaryBasic<
       scudo::MapAllocator<scudo::MapAllocatorCache<128U, 64U, 1UL << 20>>>();
-#endif
 }
 
-#if SCUDO_FUCHSIA
-using LargeAllocator = scudo::MapAllocator<scudo::MapAllocatorNoCache>;
-#else
 using LargeAllocator = scudo::MapAllocator<scudo::MapAllocatorCache<>>;
-#endif
 
 // This exercises a variety of combinations of size and alignment for the
 // MapAllocator. The size computation done here mimic the ones done by the