Enable new ScopedReservation to inherit another.

Bug: b:233089187
Bug: b:237811834

Make it possible to create a new scoped reservation for the same
resource interface as another one - useful when the latter is not
available directly (this eliminates the need to propagate the interface
to where it is needed).
Also some clean-ups in the code/declarations.

Change-Id: I0e7f9f4bdbef593eab7c52fbec018715e629839b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3754714
Reviewed-by: Hong Xu <xuhong@google.com>
Commit-Queue: Hong Xu <xuhong@google.com>
Auto-Submit: Leonid Baraz <lbaraz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1023479}

components/reporting/resources/resource_interface.cc

@@ -27,6 +27,17 @@ ScopedReservation::ScopedReservation(
   size_ = size;
 }
 
+ScopedReservation::ScopedReservation(
+    uint64_t size,
+    const ScopedReservation& other_reservation) noexcept
+    : resource_interface_(other_reservation.resource_interface_) {
+  if (size == 0uL || !resource_interface_.get() ||
+      !resource_interface_->Reserve(size)) {
+    return;
+  }
+  size_ = size;
+}
+
 ScopedReservation::ScopedReservation(ScopedReservation&& other) noexcept
     : resource_interface_(other.resource_interface_),
       size_(std::exchange(other.size_, absl::nullopt)) {}

components/reporting/resources/resource_interface.h

@@ -76,6 +76,10 @@ class ScopedReservation {
   ScopedReservation(
       uint64_t size,
       scoped_refptr<ResourceInterface> resource_interface) noexcept;
+  // New reservation on the same resource interface as |other_reservation|.
+  ScopedReservation(uint64_t size,
+                    const ScopedReservation& other_reservation) noexcept;
+  // Move constructor.
   ScopedReservation(ScopedReservation&& other) noexcept;
   ScopedReservation(const ScopedReservation& other) = delete;
   ScopedReservation& operator=(ScopedReservation&& other) = delete;

components/reporting/resources/resource_interface_unittest.cc

@@ -188,6 +188,27 @@ TEST_P(ResourceInterfaceTest, ScopedReservationRepeatingHandOvers) {
               Eq(resource_interface()->GetTotal() - 1));
 }
 
+TEST_P(ResourceInterfaceTest, ScopedReservationRepeatingCopyHandOvers) {
+  uint64_t size = resource_interface()->GetTotal() / 2;
+  ScopedReservation scoped_reservation(size, resource_interface());
+  EXPECT_TRUE(scoped_reservation.reserved());
+
+  for (; size >= 2; size /= 2) {
+    ScopedReservation another_reservation(size / 2, scoped_reservation);
+    EXPECT_TRUE(another_reservation.reserved());
+    scoped_reservation.HandOver(another_reservation);
+  }
+  EXPECT_THAT(resource_interface()->GetUsed(),
+              Eq(resource_interface()->GetTotal() - 1));
+}
+
+TEST_P(ResourceInterfaceTest, ScopedReservationFailureToCopyFromEmpty) {
+  ScopedReservation scoped_reservation;
+  uint64_t size = resource_interface()->GetTotal() / 2;
+  ScopedReservation another_reservation(size, scoped_reservation);
+  EXPECT_FALSE(scoped_reservation.reserved());
+}
+
 TEST_P(ResourceInterfaceTest, ScopedReservationRepeatingHandOversToEmpty) {
   ScopedReservation scoped_reservation;
   EXPECT_FALSE(scoped_reservation.reserved());