runtime: skip TestArenaCollision on failed reservation

If TestArenaCollision cannot reserve the address range it expects to
reserve, it currently fails somewhat mysteriously. Detect this case
and skip the test. This could lead to test rot if we wind up always
skipping this test, but it's not clear that there's a better answer.
If the test does fail, we now also log what it thinks it reserved so
the failure message is more useful in debugging any issues.

Fixes #49415
Fixes #54597

Change-Id: I05cf27258c1c0a7a3ac8d147f36bf8890820d59b
Reviewed-on: https://go-review.googlesource.com/c/go/+/446877
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/runtime/export_test.go

@@ -503,7 +503,10 @@ func KeepNArenaHints(n int) {
 // MapNextArenaHint reserves a page at the next arena growth hint,
 // preventing the arena from growing there, and returns the range of
 // addresses that are no longer viable.
-func MapNextArenaHint() (start, end uintptr) {
+//
+// This may fail to reserve memory. If it fails, it still returns the
+// address range it attempted to reserve.
+func MapNextArenaHint() (start, end uintptr, ok bool) {
 	hint := mheap_.arenaHints
 	addr := hint.addr
 	if hint.down {
@@ -512,7 +515,13 @@ func MapNextArenaHint() (start, end uintptr) {
 	} else {
 		start, end = addr, addr+heapArenaBytes
 	}
-	sysReserve(unsafe.Pointer(addr), physPageSize)
+	got := sysReserve(unsafe.Pointer(addr), physPageSize)
+	ok = (addr == uintptr(got))
+	if !ok {
+		// We were unable to get the requested reservation.
+		// Release what we did get and fail.
+		sysFreeOS(got, physPageSize)
+	}
 	return
 }