Exclude preview backups when looking for bases (#4593)

Add a comment and test ensuring preview backups are excluded during
base selection as expected. It's still not possible to make a
backup tagged with the preview tag right now

---

#### Does this PR need a docs update or release note?

- [ ] ✅ Yes, it's included
- [ ] 🕐 Yes, but in a later PR
- [x] ⛔ No

#### Type of change

- [x] 🌻 Feature
- [ ] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [ ] 🧹 Tech Debt/Cleanup

#### Test Plan

- [ ] 💪 Manual
- [x] ⚡ Unit test
- [ ] 💚 E2E

src/internal/kopia/base_finder.go

@@ -237,6 +237,10 @@ func (b *baseFinder) findBasesInSet(
 				Reasons:          []identity.Reasoner{reason},
 			}
 
+		case model.PreviewBackup:
+			// Preview backups are listed here for clarity though they use the same
+			// handling as the default case because they can't be used as bases.
+			fallthrough
 		default:
 			logger.Ctx(ictx).Infow(
 				"skipping backup with empty or invalid type for incremental backups",

src/internal/kopia/base_finder_test.go

@@ -698,6 +698,28 @@ func (suite *BaseFinderUnitSuite) TestGetBases() {
 				0: testUser1Mail,
 			},
 		},
+		{
+			name:  "Ignore Preview Base",
+			input: testUser1Mail,
+			// Ordered by time from newest to oldest.
+			data: []baseInfo{
+				newBaseInfoBuilder(3, testT3, testUser1Mail...).
+					setBackupType(model.PreviewBackup).
+					build(),
+				newBaseInfoBuilder(2, testT2, testUser1Mail...).
+					setBackupType(model.AssistBackup).
+					build(),
+				newBaseInfoBuilder(1, testT1, testUser1Mail...).
+					setBackupType(model.MergeBackup).
+					build(),
+			},
+			expectedMergeReasons: map[int][]identity.Reasoner{
+				2: testUser1Mail,
+			},
+			expectedAssistReasons: map[int][]identity.Reasoner{
+				1: testUser1Mail,
+			},
+		},
 	}
 
 	for _, test := range table {

src/internal/model/model.go

@@ -34,25 +34,39 @@ const (
 
 // common tags for filtering
 const (
-	ServiceTag    = "service"
+	ServiceTag = "service"
+	// BackupTypeTag is the key used to store the resulting type of backup from a
+	// backup operation. The type of the backup is determined by a combination of
+	// input options and if errors were encountered during the backup. When making
+	// an incremental backup, previous backups' types are inspected to determine
+	// if they can be used as a base.
+	//
+	// The backup type associated with this key should only be used for
+	// determining if a backup is a valid base. Once the bases for a backup
+	// operation have been found, structs like kopia.BackupBases should be used to
+	// track the type of each base.
 	BackupTypeTag = "backup-type"
 	// AssistBackup denotes that this backup should only be used for kopia
 	// assisted incrementals since it doesn't contain the complete set of data
 	// being backed up.
 	//
-	// This tag should only be used for finding backups during a
+	// See comment on BackupTypeTag for more information.
-	// manifest search. It shouldn't be used to differentiate between backups once
-	// the manifest search completes.
 	AssistBackup = "assist-backup"
 	// MergeBackup denotes that this backup can be used as a merge base during an
 	// incremental backup. It contains a complete snapshot of the data in the
 	// external service. Merge bases can also be used as assist bases during an
 	// incremental backup or demoted to being only an assist base.
 	//
-	// This tag should only be used for finding backups during a
+	// See comment on BackupTypeTag for more information.
-	// manifest search. It shouldn't be used to differentiate between backups once
-	// the manifest search completes.
 	MergeBackup = "merge-backup"
+	// PreviewBackup denotes that this backup contains a subset of information for
+	// the protected resource. PreviewBackups are used to demonstrate value but
+	// are not safe to use as merge bases for incremental backups. It's possible
+	// they could be used as assist bases since the only difference from a regular
+	// backup is the amount of data they contain.
+	//
+	// See comment on BackupTypeTag for more information.
+	PreviewBackup = "preview-backup"
 )
 
 // Valid returns true if the ModelType value fits within the const range.