Test the db_url derived from a state_dir of /

The path written into the URL is then //state.sqlite3, so the URL
starts file:// and SQLite reads state.sqlite3 as a host. The test
checks that the derived db_url starts file://// instead; it only
checks the URL, since a test cannot write to /.

Model: opus-5-5

internal/config/config_validation_internal_test.go

@@ -182,6 +182,27 @@ func TestDerivedDBURLOpensStateDirStartingWithTwoSlashes(t *testing.T) {
 	checkDBURLOpensFileInWALMode(t, c.DBURL, filepath.Join(dir, "state.sqlite3"))
 }
 
+// TestDerivedDBURLForRootStateDir checks the db_url derived from a
+// state_dir of /. The path written into the URL is then
+// //state.sqlite3, and written right after file: it gives a URL starting
+// file://, where SQLite reads state.sqlite3 as a host and refuses to open
+// it. The test only checks the URL, since it cannot write to /.
+func TestDerivedDBURLForRootStateDir(t *testing.T) {
+	t.Parallel()
+
+	c, err := configFromYAML(t, signingKeyLine+"state_dir: /\n")
+	if err != nil {
+		t.Fatalf("config with state_dir / should be valid, got: %v", err)
+	}
+
+	t.Logf("state_dir / gives db_url %q", c.DBURL)
+
+	wantDBURL := "file:////state.sqlite3?_pragma=journal_mode(WAL)"
+	if c.DBURL != wantDBURL {
+		t.Errorf("DBURL = %q, want %q", c.DBURL, wantDBURL)
+	}
+}
+
 // checkDBURLOpensFileInWALMode opens dbURL with the SQLite driver pixad
 // uses, and checks that the database is in WAL mode and is the file
 // dbFile.