libsnapshot: Improve low space tests.

These tests have historically been pretty flaky, and have gone through a
few rounds of improvement. This is yet another: remove the
parameterization, so we can test that at least one variant of the test
succeeds, rather than all of them.

Filesystems (especially F2FS) have a lot going on underneath the hood,
and there's no guarantee that precise free space can be allocated,
measured, or relied upon in the ways this test expects. And all that we
really need to test is that some kind of out-of-space error can be
triggered.

Bug: 266645706
Test: vts_libsnapshot_test
Change-Id: I9620e5b496d5020b21cc37074e87dd21fc419ed2
diff --git a/fs_mgr/libsnapshot/snapshot_test.cpp b/fs_mgr/libsnapshot/snapshot_test.cpp
index 4014380..13314da 100644
--- a/fs_mgr/libsnapshot/snapshot_test.cpp
+++ b/fs_mgr/libsnapshot/snapshot_test.cpp
@@ -2669,44 +2669,46 @@
                                     "Merge"s;
                          });
 
-// Test behavior of ImageManager::Create on low space scenario. These tests assumes image manager
-// uses /data as backup device.
-class ImageManagerTest : public SnapshotTest, public WithParamInterface<uint64_t> {
+class ImageManagerTest : public SnapshotTest {
   protected:
     void SetUp() override {
         SKIP_IF_NON_VIRTUAL_AB();
         SnapshotTest::SetUp();
-        userdata_ = std::make_unique<LowSpaceUserdata>();
-        ASSERT_TRUE(userdata_->Init(GetParam()));
     }
     void TearDown() override {
         RETURN_IF_NON_VIRTUAL_AB();
-
+        CleanUp();
+    }
+    void CleanUp() {
         EXPECT_TRUE(!image_manager_->BackingImageExists(kImageName) ||
                     image_manager_->DeleteBackingImage(kImageName));
     }
+
     static constexpr const char* kImageName = "my_image";
-    std::unique_ptr<LowSpaceUserdata> userdata_;
 };
 
-TEST_P(ImageManagerTest, CreateImageNoSpace) {
-    uint64_t to_allocate = userdata_->free_space() + userdata_->bsize();
-    auto res = image_manager_->CreateBackingImage(kImageName, to_allocate,
-                                                  IImageManager::CREATE_IMAGE_DEFAULT);
-    ASSERT_FALSE(res) << "Should not be able to create image with size = " << to_allocate
-                      << " bytes because only " << userdata_->free_space() << " bytes are free";
-    ASSERT_EQ(FiemapStatus::ErrorCode::NO_SPACE, res.error_code()) << res.string();
-}
-
-std::vector<uint64_t> ImageManagerTestParams() {
-    std::vector<uint64_t> ret;
+TEST_F(ImageManagerTest, CreateImageNoSpace) {
+    bool at_least_one_failure = false;
     for (uint64_t size = 1_MiB; size <= 512_MiB; size *= 2) {
-        ret.push_back(size);
-    }
-    return ret;
-}
+        auto userdata = std::make_unique<LowSpaceUserdata>();
+        ASSERT_TRUE(userdata->Init(size));
 
-INSTANTIATE_TEST_SUITE_P(ImageManagerTest, ImageManagerTest, ValuesIn(ImageManagerTestParams()));
+        uint64_t to_allocate = userdata->free_space() + userdata->bsize();
+
+        auto res = image_manager_->CreateBackingImage(kImageName, to_allocate,
+                                                      IImageManager::CREATE_IMAGE_DEFAULT);
+        if (!res) {
+            at_least_one_failure = true;
+        } else {
+            ASSERT_EQ(res.error_code(), FiemapStatus::ErrorCode::NO_SPACE) << res.string();
+        }
+
+        CleanUp();
+    }
+
+    ASSERT_TRUE(at_least_one_failure)
+            << "We should have failed to allocate at least one over-sized image";
+}
 
 bool Mkdir(const std::string& path) {
     if (mkdir(path.c_str(), 0700) && errno != EEXIST) {
diff --git a/fs_mgr/libsnapshot/test_helpers.cpp b/fs_mgr/libsnapshot/test_helpers.cpp
index b05123a..9f1d676 100644
--- a/fs_mgr/libsnapshot/test_helpers.cpp
+++ b/fs_mgr/libsnapshot/test_helpers.cpp
@@ -227,7 +227,7 @@
         return AssertionFailure() << "Temp file allocated to " << big_file_->path << ", not in "
                                   << kUserDataDevice;
     }
-    uint64_t next_consume = std::min(available_space_ - max_free_space,
+    uint64_t next_consume = std::min(std::max(available_space_, max_free_space) - max_free_space,
                                      (uint64_t)std::numeric_limits<off_t>::max());
     off_t allocated = 0;
     while (next_consume > 0 && free_space_ > max_free_space) {