udpscope/docs/claude/invariants-session-and-export.md

Invariants: the session file, the export and the source chooser

What a session persists and in what order, when a save is refused, and when an export is collected.

Part of CLAUDE.md. These are rules that cost a defect to learn; read the ones touching the area you are editing before you edit it.

• The session is the layout. LayoutState persists what makes a reopened scope look like the one that was closed: the window, the rows and their slot signals, the calibration, the enabled signals, the chosen trace colours ([[color]], name + rgb = "#RRGGBB" — only the overrides, so a stream's new signals still take the positional palette and a saved session does not freeze a colour that is meant to follow the signal list), the math channels with their graphs, the measurements pane, and measure_stats — the measurements statistics toggle; written as a bare top-level boolean (measure_stats = true/false), defaults to false when absent. Signal identity is the name throughout (a CONFIG renumbers channels), 'enabled' is read off the channels the model built -- an enabled signal is exactly one that produced a channel -- and apply_layout() installs the graphs before assigning the rows, since a row may hold a math channel's name and that name is only a signal once its graph is in. Two format rules, both learned the hard way: write an op or a measure type as its index through math_op_at()/measure_type_at(), never as the raw enum value -- those accessors' orders are the dialog's, not the declaration order, so writing one and reading the other silently renames operations. And a top-level key written after a [[table]] belongs to that table in TOML: enabled = [...] emitted after [[calibration]] was parsed into the last calibration record and the enable state was lost without a word. mkdirs() used to create only components with a trailing slash, so ~/.config/udpscope was never made and the first save failed silently -- worth knowing when a layout change "does not stick".
• Measurement options persist under named keys -- noise and hf -- so a file written before them loads at the defaults, which are exactly the behaviour it was written against: noise = 10 is the hysteresis band the app has always counted with, and hf = 0 is no filter.
• The math node string is thirteen pipe fields, and the reader accepts twelve or thirteen. med_n is the thirteenth, after y; a twelve-field node is a file written before the de-noise node existed, and its med_n stays at the struct's default -- which is what such a file meant. Field 0 is the op as a math_op_at() index, which is why MathOp may only be appended to.
• The export is collected at click time, not in the file-dialog callback. The live view keeps scrolling while the chooser is up, so a late collection would write a window the user never saw. The writer is picked by the chosen path's extension.
• The chooser runs before ScopeModel exists, and --server is what decides. Config carries server_specified, set only by the --server branch, and main() asks only when it is false — so declining costs no buffer and no socket, and ScopeModel, Acquisition, RowSet and run_ui() need no notion of "no source yet". It is --server alone and not "no arguments": --multicast still needs a server to fetch CONFIG from. Every documented probe passes --server, which is what makes the feature invisible to the regression net — a probe added without it would reach the chooser and hang. gtk_init() plus a nested GMainLoop, and then run_ui()'s GtkApplication afterwards in the same process, is a verified combination (gtk_init() is idempotent; running your own loop is the documented GTK4 replacement for the removed gtk_dialog_run()).
• A session file's name cannot say how its source was reached, so the file says it. <host>_<port>.toml carries neither the multicast group nor the interface, so a chooser row could not reconnect to a multicast source at all. LayoutState::multicast/interface_addr are written in the top block, before the first [[table]] — a bare key after a table header is that table's key in TOML, which is exactly how enabled was parsed into the last [[calibration]] record and lost without a word. A file from an older build has neither key and loads as unicast on the default route. The chooser's list filters to sessions that name at least one signal, because a probe run leaves a file per throwaway port and all of them hold signals = ["", "", "", ""]; known_sources() splits the filename at the last _, since sanitize() turns anything unsafe in a host into one.
• save_layout_now() refuses to write while signals() is empty. A run against a source whose streamer was down receives no CONFIG, so every slot name stays empty and the save would overwrite a configured session with signals = ["", ...]. The chooser then filters that file out — a source whose streamer was merely down during one run would silently vanish from the only list it can be picked from, for ever. Keeping the old file loses nothing: a run that configured nothing had nothing worth persisting.

A configuration is a named session, and a source may hold several

One source, several arrangements: the same streamer serves different experiments, and retyping a layout to look at the second one is how a session gets lost. Storage is therefore a directory per source — ~/.config/udpscope/<host>_<port>/ — holding one <name>.toml per configuration plus an active file naming the one the source is on. configuration_path() is the only expression of the scheme, and the name is sanitize()d exactly as the host is, so a name typed with a slash cannot escape the directory.

A configuration carries the whole session and the settings that can only be decided before the first socket: buffer_mb, relative_time, capture_packets. The startup chooser applies them (apply_source()), which works because choose_source() runs before ScopeModel is constructed — the one moment a buffer size can be applied at all. Switching configurations inside a running app applies what can change live and says in the status line that the rest takes effect on the next start; pretending otherwise would be a configuration that half-applies.

A configuration's startup settings reach the model from two directions, and both are needed: apply_source() for a source picked in the chooser, and apply_stored_settings() for a source named with --server, which never reaches the chooser at all. Without the second, a saved host's buffer size, clock and capture ring were silently ignored — the layout still applied, because the app loads that separately once a CONFIG names the signals — so an edit to any of the three read as one that never took effect. A flag the user typed wins over the file (--buffer-mb, --relative-time, --capture-packets): the file fills in what the command line did not say, and Config::buffer_mb_specified is what tells "not asked" from "asked for the default".

The active file is written on every save and by the chooser's pick, so "which configuration was I on" survives a restart. A session written before configurations existed is imported once as the configuration named default, and the old file is renamed to <host>_<port>.toml.imported rather than deleted: an import that cannot be undone by hand is a migration nobody can back out of.

Nothing is lost by switching: the dialog saves the current session before loading the picked one, which is the only ordering in which a switch cannot lose what it switches from.