fix(tasks): allocation tracker prevent counter overflow (#13085)

Allocation tracker did not deal with possibility that counters could reach `usize::MAX`. In this case, addition would wrap around back to 0.

This problem was pointed out by @aapoalas in #13043 (review).

Introduce an `AtomicCounter` type which uses saturating addition, to prevent this happening.

Reaching such a high allocation count is highly unlikely on 64-bit systems, but might just about be possible on 32-bit.

Maybe this is overkill, but since `cargo allocs` is just an internal tool, performance is not the highest priority. So I figure on balance probably better to handle this edge case. It doesn't seem to hurt perf much.

```
# Before
% hyperfine -i --warmup 3 'cargo allocs'
Benchmark 1: cargo allocs
  Time (mean ± σ):     299.9 ms ±   2.4 ms    [User: 132.8 ms, System: 31.0 ms]
  Range (min … max):   297.6 ms … 303.9 ms    10 runs

# After
% hyperfine -i --warmup 3 'cargo allocs'
Benchmark 1: cargo allocs
  Time (mean ± σ):     301.3 ms ±   3.4 ms    [User: 133.3 ms, System: 31.5 ms]
  Range (min … max):   297.8 ms … 310.0 ms    10 runs
```

Note: The same problem in arena allocation tracker is addressed in #13043.

Author: overlookmotel

tasks/track_memory_allocations/src/lib.rs

@@ -21,17 +21,46 @@ static GLOBAL: TrackedAllocator = TrackedAllocator;
 
 struct TrackedAllocator;
 
+/// Atomic counter.
+///
+/// Mainly just a wrapper around `AtomicUsize`, but `increment` method ensures that counter value
+/// doesn't wrap around if counter reaches `usize::MAX`.
+/// This is practically infeasible on 64-bit systems, but might just be possible on 32-bit.
+///
+/// Note: `SeqCst` ordering may be stronger than required, but performance is not the primary concern here,
+/// so play it safe.
+struct AtomicCounter(AtomicUsize);
+
+impl AtomicCounter {
+    const fn new() -> Self {
+        Self(AtomicUsize::new(0))
+    }
+
+    fn get(&self) -> usize {
+        self.0.load(SeqCst)
+    }
+
+    fn increment(&self) {
+        // Result of `fetch_update` cannot be `Err` as closure always returns `Some`
+        let _ = self.0.fetch_update(SeqCst, SeqCst, |count| Some(count.saturating_add(1)));
+    }
+
+    fn reset(&self) {
+        self.0.store(0, SeqCst);
+    }
+}
+
 /// Number of system allocations
 // NOTE: We are only tracking the number of system allocations here, and not the number of bytes that are allocated.
 // The original version of this tool did track the number of bytes, but there was some variance between platforms that
 // made it less reliable as a measurement. In general, the number of allocations is closely correlated with the size of
 // allocations, so just tracking the number of allocations is sufficient for our purposes.
-static NUM_ALLOC: AtomicUsize = AtomicUsize::new(0);
-static NUM_REALLOC: AtomicUsize = AtomicUsize::new(0);
+static NUM_ALLOC: AtomicCounter = AtomicCounter::new();
+static NUM_REALLOC: AtomicCounter = AtomicCounter::new();
 
 fn reset_global_allocs() {
-    NUM_ALLOC.store(0, SeqCst);
-    NUM_REALLOC.store(0, SeqCst);
+    NUM_ALLOC.reset();
+    NUM_REALLOC.reset();
 }
 
 // SAFETY: Methods simply delegate to `MiMalloc` allocator to ensure that the allocator
@@ -41,7 +70,7 @@ unsafe impl GlobalAlloc for TrackedAllocator {
     unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
         let ret = unsafe { MiMalloc.alloc(layout) };
         if !ret.is_null() {
-            NUM_ALLOC.fetch_add(1, SeqCst);
+            NUM_ALLOC.increment();
         }
         ret
     }
@@ -53,15 +82,15 @@ unsafe impl GlobalAlloc for TrackedAllocator {
     unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
         let ret = unsafe { MiMalloc.alloc_zeroed(layout) };
         if !ret.is_null() {
-            NUM_ALLOC.fetch_add(1, SeqCst);
+            NUM_ALLOC.increment();
         }
         ret
     }
 
     unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
         let ret = unsafe { MiMalloc.realloc(ptr, layout, new_size) };
         if !ret.is_null() {
-            NUM_REALLOC.fetch_add(1, SeqCst);
+            NUM_REALLOC.increment();
         }
         ret
     }
@@ -116,8 +145,8 @@ pub fn run() -> Result<(), io::Error> {
 
         Parser::new(&allocator, &file.source_text, file.source_type).with_options(options).parse();
 
-        let sys_allocs = NUM_ALLOC.load(SeqCst);
-        let sys_reallocs = NUM_REALLOC.load(SeqCst);
+        let sys_allocs = NUM_ALLOC.get();
+        let sys_reallocs = NUM_REALLOC.get();
         #[cfg(not(feature = "is_all_features"))]
         let (arena_allocs, arena_reallocs) = allocator.get_allocation_stats();
         #[cfg(feature = "is_all_features")]