Bug fix: dolt_commit_diff support for detached head mode

go/libraries/doltcore/sqle/database.go

@@ -357,11 +357,16 @@ func (db Database) getTableInsensitive(ctx *sql.Context, head *doltdb.Commit, ds
 			}
 		}
 
-		ws, err := ds.WorkingSet(ctx, db.RevisionQualifiedName())
-		if err != nil {
+		// Grab the staged root, if we have a valid working set, so we can show the staged changes
+		// in the system table, too. If we're in a detached head mode, just reuse the working root.
+		stagedRoot, err := workingSetStagedRoot(ctx, db.RevisionQualifiedName())
+		if err == doltdb.ErrOperationNotSupportedInDetachedHead {
+			stagedRoot = root
+		} else if err != nil {
 			return nil, false, err
 		}
-		dt, err := dtables.NewCommitDiffTable(ctx, db.Name(), tname, db.ddb, root, ws.StagedRoot())
+
+		dt, err := dtables.NewCommitDiffTable(ctx, db.Name(), tname, db.ddb, root, stagedRoot)
 		if err != nil {
 			return nil, false, err
 		}
@@ -702,6 +707,18 @@ func (db Database) getTableInsensitive(ctx *sql.Context, head *doltdb.Commit, ds
 	return resolveOverriddenNonexistentTable(ctx, tblName, db)
 }
 
+// workingSetStagedRoot returns the staged root for the current session in the database
+// named |dbName|. If a working set is not available (e.g. if a commit or tag is checked
+// out), this function returns an ErrOperationNotSupportedInDetachedHead error.
+func workingSetStagedRoot(ctx *sql.Context, dbName string) (doltdb.RootValue, error) {
+	ds := dsess.DSessFromSess(ctx.Session)
+	ws, err := ds.WorkingSet(ctx, dbName)
+	if err != nil {
+		return nil, err
+	}
+	return ws.StagedRoot(), nil
+}
+
 // resolveAsOf resolves given expression to a commit, if one exists.
 func resolveAsOf(ctx *sql.Context, db Database, asOf interface{}) (*doltdb.Commit, doltdb.RootValue, error) {
 	head, err := db.rsr.CWBHeadRef()

go/libraries/doltcore/sqle/enginetest/dolt_queries_diff.go

@@ -4984,6 +4984,35 @@ var CommitDiffSystemTableScriptTests = []queries.ScriptTest{
 			},
 		},
 	},
+	{
+		// When in a detached head mode, dolt_commit_diff should still work, even though it doesn't have a staged root
+		Name: "detached head",
+		SetUpScript: []string{
+			"CREATE TABLE t (pk int primary key, c1 varchar(100));",
+			"CALL dolt_commit('-Am', 'create table t');",
+			"SET @commit1 = hashof('HEAD');",
+			"INSERT INTO t VALUES (1, 'one');",
+			"CALL dolt_commit('-Am', 'insert 1');",
+			"SET @commit2 = hashof('HEAD');",
+			"CALL dolt_tag('v1', @commit2);",
+		},
+		Assertions: []queries.ScriptTestAssertion{
+			{
+				Query:    "use mydb/v1;",
+				Expected: []sql.Row{},
+			},
+			{
+				// With no working set, this query should still compute the diff between two commits
+				Query:    "SELECT COUNT(*) AS table_diff_num FROM dolt_commit_diff_t WHERE from_commit=@commit1 AND to_commit=@commit2;",
+				Expected: []sql.Row{{1}},
+			},
+			{
+				// With no working set, STAGED should reference the current root of the checked out tag
+				Query:    "SELECT COUNT(*) AS table_diff_num FROM dolt_commit_diff_t WHERE from_commit=@commit1 AND to_commit='STAGED';",
+				Expected: []sql.Row{{1}},
+			},
+		},
+	},
 
 	{
 		// When a column is dropped and recreated with a different type, we expect only the new column