# Filesystem

Every native session serves 9P2000 on a Unix socket. Pane shells receive
`PARDES_PID` (the editor's process id), `PARDES_9P` (socket path) and
`PARDES_PANE` (pane serial). The socket is
`$XDG_RUNTIME_DIR/pardes-9p-<pid>.sock`, or lives under
`~/.local/state/pardes` when XDG_RUNTIME_DIR is unset. Detached sessions use
their session name; `--9p=<name>` overrides it.

A `pardes <file>` launched from a pane forwards Look to that pane over 9P.
`PARDES_PID` alone says the shell is inside pardes; `PARDES_9P` and
`PARDES_PANE` say how to reach it, and a launch that has the first without the
other two refuses rather than opening a second editor. `--nested` opens a
separate editor and withholds `PARDES_PID` from its pane shells, so a pardes
started in one of them runs a session of its own; its 9P service stays
available.

Look resolves the OS filesystem first, then the editor's virtual filesystem.
Explicit paths bypass that search:

| Editor path | Meaning | 9P server path |
|---|---|---|
| `/n/os/proc/self` | OS filesystem | `/os/proc/self` |
| `/n/self/pane/2/body` | pane 2's text | `/pane/2/body` |
| `/virtual/pane/2/body` | the same, in the editor's own spelling | `/pane/2/body` |
| `/virtual/src/pardes.zig` | source embedded in this build | `/src/pardes.zig` |
| `/n/peer/pane/2/body` | another session's text | peer's `/pane/2/body` |

The mount name `self` is reserved and maps to the server root, so `/n/self/X`
and `/virtual/X` both name the served `/X`.

`--mount=peer=work` mounts the named session `work`; the dial can also be an
absolute socket path, `unix!/path`, `tcp!IP!port`, or `quic!IP!port`.
At runtime, use `Mount peer dial` and
`Unmount peer`. There are eight named mounts; `os` and `self` are reserved.
Unmount refuses mounts still used by a pane, its working directory, or a
pending Save. Mounts are saved in dumps. Save uses the file's original mount.

`pardes --9p-tcp='tcp!127.0.0.1!5640'` adds a TCP listener alongside the Unix
socket. Build with `-Dquic=true` and system OpenSSL 3.6+ to enable QUIC;
`--9p-quic='quic!127.0.0.1!5641'` adds its listener. Both accept numeric
IPv4/IPv6 addresses, not DNS names. Listener port zero chooses a free port;
`/listeners` reports all active dial addresses.

All connections have session access, including `os`. TCP is unencrypted.
QUIC uses an ephemeral TLS identity without peer verification or login.
It carries 9P2000 on one bidirectional stream with ALPN `pardes-9p`.
Unix and TCP connections share four slots served by cloud9's `std.Io`
runner; QUIC has four of its own on the editor's poll loop. OpenSSL's
internal buffers are separate, dynamically allocated memory.

[Plan9port's client](https://9fans.github.io/plan9port/man/man1/9p.html) can
drive Unix or TCP without a kernel mount, and `9ns` mounts the tree in a
private namespace:

```sh
9p -n -a "unix!$PARDES_9P" read index
9p -n -a 'tcp!127.0.0.1!5640' ls pane/1
9ns --unix "$PARDES_9P" -- sh -c 'cat "$NINE_MOUNT/index"'
```

A `9ns --unix` mount lives in the private namespace of the command it runs,
and nothing outside that command sees it. The mount everyone on the machine
shares is the registry one, `9ns --mntgen` (default `/mnt/9p`): every running
editor posts itself there, so `/mnt/9p/pardes/<pid>/` is that editor's tree
for any process. A new pane made through `pane/new` is a scratch named
`<dir>/+New` until it is given a name, and closing a column's last pane
leaves such a `+New` in its place (`Delcol` closes the column).

For [Linux v9fs](https://www.kernel.org/doc/html/latest/filesystems/9p.html),
use `version=9p2000,cache=none,access=any` and `trans=unix`, or `trans=tcp`
with `port=5640`. Set `uname`, `dfltuid`, and `dfltgid` for the local user.
Leave `aname` empty. The opt-in
[Linux v9fs experiment](v9fs.md) tests a kernel mount in a separate subprocess
namespace (`zig build v9fs-test`, requiring explicit mount authorization).
Neither 9P2000.u nor 9P2000.L is implemented.
Existing Plan9port/v9fs clients need a userspace bridge for QUIC.

## The served tree

```
/README      this guide, also src/fs-help.txt
/index       one line per pane: serial, kind (text|term|pdf|image), dirty flag, name
/status      pid, version and pane count
/look        write a line: a right click on it at the active pane; read: the serials the last
             look, exec or ctl write touched (made, else acted at)
/exec        write a line: a middle click; read the same serials
/log         recent events, one a line: new|del|rename|save <serial> <name>, msg <serial|-> <text>,
             dump|restore <path>, err <serial|-> <file>: <why>;
             write follow to that open to wait for more
/screen      rendered screen JSON; frozen per open handle
/listeners   the session's dial addresses
/focus       the serial of the pane with the keyboard; write a serial to give it the keyboard
/ctl         the settings, one a line as a write takes them; write a setting or a session builtin
/commands    every builtin: word, `arg` if it takes one, and `root` or `pane`, the ctl that takes it
/pane/new    open it to make a pane; the read answers that pane's serial
/pane/<n>/   name body tag ctl addr dot limit data xdata sel dirty mark scroll
             errors event look exec, plus pty/{ctl,status,data} on terminals
/os/         the host filesystem
/src/        the editor's embedded sources, only when built with -Dembed-sources=true
```

Control messages are split by what they act on, as acme keeps window verbs
on a window's ctl and webfs and upas/fs keep session settings on a root ctl.
Each builtin declares its scope in src/builtins.zig (`scope = .session`;
every setting is one, the rest act on a pane). `/ctl` takes the session's
builtins, one a line, at whichever pane has the keyboard as each runs --
`Newcol`, `Dump`, `Mount name dial`, `Theme ink`, `Verbose off`; `Exit`,
which quits the editor as acme's does (it refuses once, naming each pane with
unsaved text, `<name>: Modified (Exit again to discard)`, and a second
`Exit` with nothing edited since quits, throwing that text away; a scratch
under 100 bytes is not asked about; `Restore`, which replaces every pane,
asks the same first -- `Dump` writes `pardes-<date>-<time>.zon` in
`DumpDir` (default `$XDG_DATA_HOME/pardes`, else `~/.local/share/pardes`)
and logs `dump <path>`, and `Restore` with no path takes the last one; a
Restore puts a new editor under every client, so the write of it is
answered and then every connection is hung up, their fids naming the old
editor's panes: dial again, and the new log names the restored panes and
`restore <path>`. Keeping connections across it would mean carrying serials
and opens into the new editor, which acme, whose Load only adds windows,
never needed); and `Kill`, which
does not quit but stops commands, as acme's does: bare, every command pardes
started, and `Kill make ls`, those whose line begins with one of the words. A
command pardes started is a line it typed into a terminal (a word written to
`exec`, a middle click on one, a `pty/run`), from its shell's start mark (C)
to its end mark (D); Kill sends its foreground job SIGTERM -- acme posts the
"kill" note, which ends a process -- and never signals the shell itself;
in a shell running without job control (`set +m`) the job shares the
shell's group, so there is none to signal: Kill says `Kill: no job to
signal`, and a write of it to `ctl` fails with that; and only the
foreground job, so of `sleep 30; echo done` the `echo` still runs once the
`sleep` is stopped --
and reads every setting there is, one a line, in the words a write of it
takes (`Verbose on`, `WindowOpacity 70`, `PanelSlide off`, `DumpDir` bare
for the default directory, `LocationsConfig ...`), so writing what it reads
back changes nothing; platform and startup facts are `/status`'s and the
Config window's, not settings. A pane's `ctl` takes the builtins that act on
a pane (`Del`, `Save f`, `Collapse`, which folds that pane, `Find pat`)
beside acme's `get`, `lock` and `unlock`. The column words are pane words
too, acting on the column that pane is in: `Delcol`, `DelAbove`, `DelBelow`
and the focus moves `Left`/`Right`/`Up`/`Down` from it. `Joincol` and
`Newcol` are the root's: they act on the column of the pane with the
keyboard, and `Joincol` needs a column to its right (`Joincol: no column to
the right`). The words are case-sensitive and do not alias: acme's verbs
are lowercase and the builtins keep their tag spelling, so `Get` is no word
and `del` none either.

A write is checked whole before any line of it runs, and a line is refused
in Plan 9's words for a ctl (kernel/misc/parse.c:82-97), quoting the line:
`unknown control message "X"`; `wrong #args in control message "X"` for an
argument to a builtin that takes none, or none to one that needs it (`Msg`,
`Mount`, `Find`, a setting's value but a switch's, which flips bare);
`bad value in control message "X"` for a setting's value it does not take
(for `Theme`, naming the themes that share the name's first letter, since
all of them, `ThemeSel`'s list, are too many for an error);
and `not a session control message "X"` or `not a window control message
"X"` for a word of the other ctl. 9ns maps them all to EINVAL, and a write
refused here has done nothing. A line that then fails as it runs fails the
write with the error the editor reports for it and the line, e.g. `Mount:
AlreadyMounted "Mount peer /tmp/s"` (EIO), and `control message needs its
argument "Save"` (EINVAL), for a builtin that would have asked at a prompt
(a `Save` on a scratch) rather than open one nobody is there to answer. The
lines before a failing one have taken effect and those after it never run,
which is what acme's ctl loop does (editors/acme/xfid.c:600-790). An error
that only happens as the editor performs what a line asked for -- a `Save`
whose disk write fails -- is reported in the editor and /log, not in the
write's answer. Like any write, a ctl write answers once the editor has
performed what it asked for (a save written, a shell started). A click on
the same word, or the word written to `exec`, still opens its prompt.

`/commands` lists every builtin the registry holds, in registry order, one
a line: its word, `arg` when it takes one, and `root` or `pane` for the ctl
that takes it, e.g. `Newcol root`, `Save arg pane`, `Verbose arg root`. It is
generated from the registry, so it is always this build's own list.

`/focus` reads the serial of the pane with the keyboard, and a serial written
to it gives that pane the keyboard, off any column or workspace tag that had
it -- rio's `current` written to a window's `wctl`, named once for the whole
tree since there is one keyboard. While a column's or the workspace's tag has
the keyboard no pane does: `/focus` reads empty and every pane's `ctl` says
`notcurrent`. A write gives the keyboard and nothing else: a folded pane
stays folded (unfold it with `Collapse` on its ctl), as rio keeps `current`
apart from `unhide`. A serial no pane has fails with `no such
window`; anything but a number, with `ill-formed control message`.

A pane is made by **opening** `/pane/new`, and closed by Tremove on
`/pane/<n>` (`rmdir`), which is the only remove the tree serves; Tcreate is
refused everywhere, as it is in acme. Reading the open fid answers the serial
of the pane that open made, so `n=$(cat /pane/new)` makes one and names it in
a line. Each open makes another pane, and two reads of one fid answer the same
serial: the open acted, the read only observes. Closing the fid leaves the
pane.

This is `/net/tcp/clone`'s mechanism, not acme's `new`, and the difference is
deliberate. acme allocates during the *walk* and lets the walk land inside the
new window, so `/dev/new/body` works in one step (acme(4): "accessing any file
in `new` creates a new window"). acme can also afford to list `new`, because a
Plan 9 directory read carries the stat of every entry and nothing walks. A
kernel or FUSE mount is not so lucky: it walks and stats each name a listing
gave it, so an allocate-on-walk name would make a pane per `ls -l`. Allocating
on open instead keeps `new` listed and `ls` honest — a stat is not an open —
at the cost of acme's one-step `new/body`. Nothing in the tree is created by
list, stat, walk or read; only that one open. Every other name in `/pane` is a
serial.

A session can open its own tree through a mount: a Look at
`/mnt/9p/pardes/<me>/pane/2/body` from inside that very editor opens it, and
a Save of that pane writes back through the mount into pane 2. Requests on
the Unix and TCP listeners are answered on the 9P connection's own task, not
by the editor's loop, so the realpath, the stat and the read the editor makes
out through the mount come back while it waits for them. The one rule is
whose turn it is with the core (`pardes.turn`): the editor has it, and gives
it up while it waits for input and while a step of it is out in a syscall. A
step of a connection task's own -- a Look written to `look` -- goes out the
same way, and the editor waits for it to return before it takes a step of
its own. While any step is out, a request that would change a pane (a write,
a truncation, an rmdir) parks in the engine until none is; everything else,
opening `pane/new` and `screen` included, is answered at once, which is why a
Look at any path in the tree comes back. A write into the tree from the
editor itself only ever happens between steps (a Save), so nothing it waits
on out there is a request that has to park. QUIC is still served on the
editor's loop, so through QUIC the old hang remains. `/n/self/...` names the
same tree without leaving the process.

`/look` and `/exec` are the editor's two clicks, one per line of a write:

- a line written to `look` is a right click on it: a path opens a file,
  `file:12` jumps to a line, a directory opens a shell there, a URL opens in
  the browser.
- a line written to `exec` is a middle click: a command word from
  `src/builtins.zig` (`Save`, `Del`, `New`, `Newcol`, `Mount NAME DIAL`,
  `Unmount NAME`, `Dump`, `Restore`, `Msg TEXT`, `Find`, `Grep`, `Tty`, ...),
  or anything else, which is typed into a terminal: the pane itself when it
  is a terminal at its prompt, otherwise a terminal in the pane's directory
  that is at its prompt, and failing both a new one made below the last
  column. That is not an error, whatever the shell makes of the line, and a
  misspelled builtin word ends up there too (whether it should is an open
  question, docs/open-questions.md). `echo Tty > pane/<n>/ctl` makes a
  terminal in that pane's directory outright.

The root's pair clicks at the active pane and `/pane/<n>/look` and
`/pane/<n>/exec` at that pane. Blank lines are skipped, and every other line
is checked before any of them runs, so a control character fails the whole
write with EINVAL; a command that fails inside the editor is reported on the
message row, not as a write error. Reading any of these files answers the
serials of the panes the last command created, or, when it created none, the
pane a look focused or the pane an exec acted on (even one it closed), one
per line.

`/pane/<n>/name` reads the pane's file name (a terminal's directory) and
writing it renames the buffer; a relative name resolves against the pane's
directory. `body` appends on write and replaces on truncating open. `sel`
reads the selected text and writing it replaces the selection. `errors`
appends to the directory's `+Errors` pane. Holding `event` open redirects the
pane's Look and Exec clicks to that client, and so does a line written to the
pane's own `look` or `exec`, a click with no place in the text: an `F` record
at `0 0` carrying the line (the root's `look` and `exec` still act at once);
writing a record back performs the action. acme takes back only `<origin>
<action><q0> <q1>`, the text of that range; pardes takes the record whole as
it was read too, and for an empty range acts on its text, which is how such
a line is done. A click in a file's body carries the offsets of the text it
took; one in a terminal's body cannot, since that body is a history
snapshot, and is also at `0 0` with its text. A click that takes no text
sends nothing, as in acme. `ctl` reads acme's window status line — serial, tag length, body
length, a reserved zero, the dirty flag, the width in cells, the font and the
tab width — followed by rio's `current` or `notcurrent` (rio(4), `wctl`):
whether the pane has the keyboard. It takes the pane's builtins (below),
`get`, which reloads the buffer from the name it
carries, and acme's `lock` and `unlock` (editors/acme/xfid.c:603-611), for an
edit of several writes to `addr` and `data` that another client must not
land in the middle of. As in acme the lock binds only the clients that take
it: a `lock` while another open holds it fails at once with `file in use`
(EBUSY), to be tried again, until that open writes `unlock` or closes (or
the pane does) -- where acme's blocks, because through a kernel or FUSE
mount a blocked write would hold up the holder's own `unlock` and close on
that file -- and nothing else is refused for it --
not a write to any other file, not the person at the keyboard. It belongs to
the open that wrote it, so only that open's `unlock` is taken; a write on an
open that cannot write (or the editor's own, on none) cannot lock. From a
shell the lock needs an open held across the edit, since `echo lock > ctl`
closes, and so unlocks, at once: `exec 3>ctl; echo lock >&3; ...edits...;
exec 3>&-`.

The three range files `addr`, `dot` and `limit` each read the pair of offsets
they also accept, so copying one onto another is all that acme's `addr=dot`,
`dot=addr` and `limit=addr` ever were. A write is either that pair or an
address expression (`#0,#5`, `/pattern/`, `2+1`); `addr` selects what `data`
and `xdata` read or replace, `dot` is the editor's own selection and moving it
scrolls the pane into view, and `limit` bounds a search and reads empty until
it is set. Truncating a range file empties it; truncating `limit` lifts it.
Truncating `data` or `xdata` deletes the range `addr` names and nothing
else, so a shell's `echo NEW > data` replaces that range, `: > data`
deletes it, and `>>` inserts at it; only truncating `body` empties the
whole buffer. Truncating `data` is pardes's own: acme ignores OTRUNC there
(editors/acme/fsys.c:543) and every write inserts. And as in acme a write
leaves `addr` just past what it wrote, so a second `echo x > data` deletes
the empty range there and inserts after the first rather than replacing it
again; write `addr` before each replacement.
`addr` belongs to the pane rather than to a client and keeps what was written
until someone writes or truncates it, so writing an address and reading it
back evaluates it, which is what acme(4) promises of its own `addr`.

The regular expressions are mvzr's (sets, `\d`/`\w`/`\s`, `{m,n}` and
lazy `*?` included), searched the way sam searches (editors/acme/regx.c): as
lines, so `^` and `$` match at the start and end of any line, `.` and a
negated class never match a newline, and `$` also matches at the end of a
text with no final newline. A pattern that names a newline (`\n`) runs over
the whole text instead, its `.` kept to one line. An expression is
evaluated from the current address, the range last written to `addr` (or
left by the last `data` write, just past it), as acme evaluates it from
`w->addr` (xfid.c:446): `.` is that address, not the selection (`dot` is
the selection's own file), and `#9/re/` searches from `#9`. `/re/` searches
forward from the end of the current range to `limit` if one is set, and
otherwise wraps to the start of the text; `?re?` and `-/re/` find the last
match ending before the range, wrapping to the text's last. The match is the leftmost, but of
the alternatives at that place mvzr takes the first that matches where sam
takes the longest (`/gam|gamma/` finds `gam`); in a search begun in the
middle of a line, `^` inside an alternation can match there; and in a
pattern that spans lines, `^`, `$` and `[^...]` keep mvzr's own meaning.
mvzr backtracks without bound of its own (`a?` twenty times then twenty
`a`s is 2^20 steps from each place it tries), and a search holds the editor, so pardes patches a
step budget into mvzr's matcher (build.zig): a search that spends it,
about 300 ms, fails with `regular expression search gave up, backtracking
past its step budget` rather
than answer a match it is not sure of. Ordinary patterns spend a few
thousand steps; what runs out is exponential backtracking, and a quadratic
pattern over a very long line (`\s*(\w+)\s*=` over 20 KB of letters).
pardes has no regex engine of its own on purpose; these are its limits.
Normal mode's `s` and `S` search a selection the same way (src/regexp.zig
is the one place both call), so `^` there also means a line's start.

An address that does not evaluate fails the write with why: `bad address
syntax`, `no match for regexp`, `address out of range`, `bad regular
expression`, `regular expression search gave up, ...`, or sam's
`addresses out of order` for a range that ends before it starts
(`#100,#50`), which acme lets through. A failed write to `addr` leaves no
address at all, where acme
keeps the old one: until an address is written or `addr` is truncated,
reading `addr`, and reading, writing or truncating `data` and `xdata`, fail
with `no address: the last one written to addr failed`, so a script that
missed its target cannot then write at the last one.

The three flag files `dirty`, `mark` and `scroll` read `0` or `1` and take
`0` or `1`: whether the buffer differs from its file, whether a write pushes
an undo point (writing `1` pushes one now), and whether a write scrolls the
pane.

`tag` reads the whole tag as the pane shows it: the computed path, dirty
marker or PDF page, then the text you may edit. A write appends to that text,
newlines included, and a tag with more than one line takes a row per line on
screen; truncating `tag` clears it, as acme's `cleartag` does -- the default
words (`Del`, `Put` and the rest) with it, since they are that text until
you edit it, so `echo Make > tag` leaves only `Make`; append with `>>` to
keep them. The clearing is an edit of the tag like a typed one and its undo
history is kept: `u` in the tag brings back the text it cleared, words
included.

Stats report real lengths for `index`, `status`, `look`, `exec`, `listeners`,
`name`, `body`, `tag`, `sel`, `ctl`, the range files and the flag files, and
for `event` and `pty/data` the length of the record a read would
answer, which is zero when nothing is waiting; for `log`, the text an open
would freeze now. Modes are 0644/0666 (0444 for
read-only files, 0222 for write-only); mtime is the pane's last edit or the
process start. The qid version of `body`, `data` and `xdata` is the pane's
revision, so a stat sees an edit land without reading the text; every other
file leaves it zero rather than promise a version it cannot keep. Directory
entries carry no sizes, and neither does `/screen`, which has no length until
an open renders its frame; stat the entry.

`/log` is one ring (64 KiB) that records whether or not anyone reads it:
`new`, `del`, `rename` (a terminal's too, as its shell changes directory,
since a terminal is named by its directory) and `save <serial> <name>`,
`dump <path>` when a Dump is written and `restore <path>` in a Restore's
new log after its panes' `new`s, and `msg <serial|-> <text>`
for every line the editor says, repeats included (with `verbose` on, that
includes each builtin announcing itself as it runs), and `err <serial|->
<file>: <why>` for every write or truncation the tree refused or that
failed -- through a mount a shell sees only the errno its kernel mapped the
reply to, usually `Invalid argument`, and this is the reason (`err 3 addr:
no match for regexp`). The same err again, before any follower has read
the first, is that record counted (`err 3 addr: no match for regexp (x4)`),
so a client retrying a failing write does not push the rest out of the
ring; a follower that read it gets each repeat. Through a kernel mount a client sees only an errno, which 9ns reads from the
error's words (cloud9's 9ns/src/nine.zig, `enameToErrno`): a malformed write
-- an unknown or ill-formed control message, `bad address syntax`, `bad
regular expression` -- is EINVAL; a lock another open holds, EBUSY; a pane
gone, ENOENT; a well-formed write that fails -- `no match for regexp`,
`address out of range`, `addresses out of order`, a search that gave up,
`<name>: Modified (Exit again to discard)` -- EIO. The err record has the
words. There is no per-pane error file to read instead:
acme's `errors` only takes text, and one record stream is simpler to watch
than a file per pane. A `msg` said while a
pane is being made can precede that pane's `new`; panes present at boot are
recorded before anything else.
Control characters in a record become spaces, so a record is one line.
(An `event` record is not: acme's `<origin><action><q0> <q1> <flag> <n>
<text>\n`, whose text may hold newlines; read `n` bytes of it rather than
up to a newline -- bytes here, where acme counts runes.) An
open freezes the ring's text the way `/screen` freezes a frame: reads walk it
and end. Writing `follow` to that same open makes reads past it wait for the
next record, one per read; a follower the ring outran reads `lost N` first.
Closing the open is the only way back, as with rio's `consctl`.

A read with nothing to give yet -- a following `log`, `event`, `pty/data`, a
`pty/run` before its answer -- is held, the way factotum holds its log's reads
(security/auth/factotum/log.c) and acme an event read: the core keeps it, and
whoever next has news for it (a record, output, a run's answer, the pane
closing, which answers acme's "window shut down") answers it on the
connection it came on as the turn is given up. Nothing else parked is
retried for it. The core keeps the ticket cloud9 gave the park
(`Conn.hold`) and answers only while that very park waits, so a read the
client flushed or whose fid it clunked meanwhile is dropped unanswered, no
record is spent on it, and a tag the client reuses is never answered with
what was meant for the old one. An open waits with one read at a time, as
acme's window keeps one `eventx`; a second read on it meanwhile fails with
"file in use". QUIC connections still retry their parked reads each tick.

`pty/run` runs one line at a terminal's prompt and answers how it ended, on
the same open (factotum's `rpc` shape): write the line, then read `exit N`
once the command has ended and the shell is back at a prompt, followed by
what it printed. The output is what the screen showed between the command's
start and end marks: stderr interleaved, `\r` progress collapsed to its last
state, no colour, tabs as the spaces they drew, trailing spaces trimmed and
trailing blank lines dropped (`printf 'a\n\n\n'` answers `a`), leading
whitespace kept; a program on the alternate screen (vim, less, htop) leaves
none, and rows a program redrew above its start are missed. A bash job notice
printed before its PROMPT_COMMAND lands in the next run's output. Only its
last 64 KiB are kept, from a line start, and the header then reads `exit N
cut M` (M bytes left out). `exit N cut`, with no count, says the output's
start is not there to read: it scrolled out of the history, the command
erased the screen (`clear`, `watch`, a full-screen program's redraw, a
reset -- so such a command's output may read as cut), a start or end mark
came on the alternate
screen, or the command printed more than 8192 rows, of which only the last
are read so that the answer costs the editor a bounded amount. `exit ?` is a
command whose end mark carried no status, which is not a success; `error out
of memory` is an answer that could not be made. The header is always the
whole first line. It reads
`busy` at once when a command is running or text is typed at the prompt,
which is also when the third field of `pty/status` reads 1. A line written
before a new terminal's shell has drawn its first prompt is not busy: it
waits for that prompt (a respawn meanwhile keeps it waiting for the new
shell's) and is sent then, so the first command a script gives a fresh
terminal is not lost. A shell that never draws a tagged prompt (one pardes
could not instrument, or a startup that hangs) leaves such a line waiting
for ever: cancel the read (interrupt it, or close the open) to give up;
`error not run` when the shell refused
the line without running it (a fish syntax error; the line is taken back off
the prompt); `error shell gone` when the pane closed or its shell was
replaced; `error no prompt marks` for a shell pardes could not instrument.
It relies on the OSC 133 marks pardes injects into bash and fish, tagged
`aid=pardes` so fish's own marks and a nested shell's are ignored. A second
line on an open whose command still runs fails the write. `exec zsh` or a
continuation prompt never reports an end; cancel the read. A read of `run`
waits in pardes, so through 9ns it needs 9ns's concurrent requests or it
holds up the rest of the mount. A record
longer than a read comes in pieces, so a shell's `read` loop works. `tail -f`
never writes `follow`, so it sees nothing new: use the follow open instead. `/screen` returns JSON with
`cols`, `rows`, `cursor`, a `styles` table, and row-major `cells` of
`[grapheme, style_index]`. Each open freezes one frame until close. A
terminal `body` freezes its history on the first read of each open handle;
`pty/data` streams live output. An open that holds something between open
and close -- a frozen screen, terminal body or log, a run, an `event` or
`pty/data` open -- takes one of 64 records (lib9p's per-fid aux, acme's
Fid), released on close or disconnect; past that such an open fails with
`ENFILE`. Other opens hold nothing and are not counted.

`-Dembed-sources=true` embeds the editor's sources and serves them under
`/src` (and `/shaders` on GUI builds). `EffectCode <effect>` lists the current
backend's implementation files under `/virtual`, which Look opens; without the
option the command reports the sources as unavailable. The esp32p4 build
enables the option by default, so the device can serve its own source.

This is a control filesystem, not a complete POSIX export. Native filenames
may contain up to 255 bytes. Existing regular OS files support read, write,
and truncation to zero; under `/os` protocol create, remove, rename and other
metadata changes are refused, as is every create in the control tree and every
remove in it but a pane directory's. Ownership and permissions under `/os` are
synthetic.
Zero-length truncation accepts the accompanying `mtime` hint sent by Linux
v9fs; the hint is not stored. Standalone timestamp changes remain refused.

The tree lives in `src/ninep/`: `tree.zig` (nodes, lookup, readdir, dispatch,
and the editor's reply payload over cloud9's backend contract), `pane.zig`
(pane files), `ctl.zig`, `addr.zig`, `pty.zig`, `events.zig` (event and log
streams), `screen.zig` and `sources.zig`. The protocol engine is cloud9's
`fs.Server`, configured in `src/9p.zig` (the editor's and the board's
capacities); the transports are `src/9p_io.zig` (cloud9's `serve.Runner`
for Unix and TCP, a poll loop for QUIC, and the 9P client for mounts);
`src/fs.zig` keeps host access, mounts, resolution, find and grep.

`zig build fs-test` drives real sessions using the independent Python client
in `test/ninep.py`; `zig build fs-discovery-test` checks that browsing
creates nothing, that an open of `/pane/new` and a remove work, and that
`look`, `exec`, `name`, `sel` and `log` behave. `zig build
9p-test` checks the two engine configurations' budgets (the engine's own
tests are cloud9's `zig build test`); `zig build fs-bench` measures
filesystem transactions in the core.
`zig build fs-test quic-test -Dquic=true` also exercises QUIC mounts and I/O.