## devnotes toc

* top
* [future ideas](#future-ideas) - list of dreams which will probably never happen
* [design](#design)
    * [up2k](#up2k) - quick outline of the up2k protocol
        * [why not tus](#why-not-tus) - I didn't know about [tus](https://tus.io/)
        * [why chunk-hashes](#why-chunk-hashes) - a single sha512 would be better, right?
        * [list of chunk-sizes](#list-of-chunk-sizes) - specific chunksizes are enforced
* [hashed passwords](#hashed-passwords) - regarding the curious decisions
* [http api](#http-api)
    * [read](#read)
    * [write](#write)
    * [admin](#admin)
    * [general](#general)
* [event hooks](#event-hooks) - on writing your own [hooks](../README.md#event-hooks)
    * [hook effects](#hook-effects) - hooks can cause intentional side-effects
* [assumptions](#assumptions)
    * [mdns](#mdns)
* [sfx repack](#sfx-repack) - reduce the size of an sfx by removing features
* [building](#building)
    * [dev env setup](#dev-env-setup)
    * [just the sfx](#just-the-sfx)
    * [build from release tarball](#build-from-release-tarball) - uses the included prebuilt webdeps
    * [build from scratch](#build-from-scratch) - how the sausage is made
    * [complete release](#complete-release)
* [debugging](#debugging)
    * [music playback halting on phones](#music-playback-halting-on-phones) - mostly fine on android
    * [discarded ideas](#discarded-ideas)


# future ideas

list of dreams which will probably never happen

* the JS is a mess -- a ~~preact~~ rewrite would be nice
  * preferably without build dependencies like webpack/babel/node.js, maybe a python thing to assemble js files into main.js
  * good excuse to look at using virtual lists (browsers start to struggle when folders contain over 5000 files)
  * maybe preact / vdom isn't the best choice, could just wait for the Next Big Thing
* the UX is a mess -- a proper design would be nice
  * very organic (much like the python/js), everything was an afterthought
  * true for both the layout and the visual flair
  * something like the tron board-room ui (or most other hollywood ones, like ironman) would be :100:
    * would preferably keep the information density, just more organized yet [not too boring](https://blog.rachelbinx.com/2023/02/unbearable-sameness/)
* some of the python files are way too big
  * `up2k.py` ended up doing all the file indexing / db management
  * `httpcli.py` should be separated into modules in general


# design

## up2k

quick outline of the up2k protocol,  see [uploading](https://github.com/9001/copyparty#uploading) for the web-client
* the up2k client splits a file into an "optimal" number of chunks
  * 1 MiB each, unless that becomes more than 256 chunks
  * tries 1.5M, 2M, 3, 4, 6, ... until <= 256 chunks or size >= 32M
* client posts the list of hashes, filename, size, last-modified
* server creates the `wark`, an identifier for this upload
  * `sha512( salt + filesize + chunk_hashes )`
  * and a sparse file is created for the chunks to drop into
* client sends a series of POSTs, with one or more consecutive chunks in each
  * header entries for the chunk-hashes (comma-separated) and wark
  * server writes chunks into place based on the hash
* client does another handshake with the hashlist; server replies with OK or a list of chunks to reupload

up2k has saved a few uploads from becoming corrupted in-transfer already;
* caught an android phone on wifi redhanded in wireshark with a bitflip, however bup with https would *probably* have noticed as well (thanks to tls also functioning as an integrity check)
* also stopped someone from uploading because their ram was bad

regarding the frequent server log message during uploads;  
`6.0M 106M/s 2.77G 102.9M/s n948 thank 4/0/3/1 10042/7198 00:01:09`
* this chunk was `6 MiB`, uploaded at `106 MiB/s`
* on this http connection, `2.77 GiB` transferred, `102.9 MiB/s` average, `948` chunks handled
* client says `4` uploads OK, `0` failed, `3` busy, `1` queued, `10042 MiB` total size, `7198 MiB` and `00:01:09` left

### why not tus

I didn't know about [tus](https://tus.io/)  when I made this, but:
* up2k has the advantage that it supports parallel uploading of non-contiguous chunks straight into the final file -- [tus does a merge at the end](https://tus.io/protocols/resumable-upload.html#concatenation) which is slow and taxing on the server HDD / filesystem (unless i'm misunderstanding)
* up2k has the slight disadvantage of requiring the client to hash the entire file before an upload can begin, but this has the benefit of immediately skipping duplicate files
  * and the hashing happens in a separate thread anyways so it's usually not a bottleneck

### why chunk-hashes

a single sha512 would be better, right?

this was due to `crypto.subtle` [not yet](https://github.com/w3c/webcrypto/issues/73) providing a streaming api (or the option to seed the sha512 hasher with a starting hash)

as a result, the hashes are much less useful than they could have been (search the server by sha512, provide the sha512 in the response http headers, ...)

however it allows for hashing multiple chunks in parallel, greatly increasing upload speed from fast storage (NVMe, raid-0 and such)

* both the [browser uploader](https://github.com/9001/copyparty#uploading) and the [commandline one](https://github.com/9001/copyparty/tree/hovudstraum/bin#u2cpy) does this now, allowing for fast uploading even from plaintext http

hashwasm would solve the streaming issue but reduces hashing speed for sha512 (xxh128 does 6 GiB/s), and it would make old browsers and [iphones](https://bugs.webkit.org/show_bug.cgi?id=228552) unsupported

* blake2 might be a better choice since xxh is non-cryptographic, but that gets ~15 MiB/s on slower androids

### list of chunk-sizes

specific chunksizes are enforced  depending on total filesize

each pair of filesize/chunksize is the largest filesize which will use its listed chunksize; a 512 MiB file will use chunksize 2 MiB, but if the file is one byte larger than 512 MiB then it becomes 3 MiB

for the purpose of performance (or dodging arbitrary proxy limitations), it is possible to upload combined and/or partial chunks using stitching and/or subchunks respectively

| filesize           | filesize | chunksize     | chunksz |
| -----------------: | -------: | ------------: | ------: |
|        268 435 456 |  256 MiB |     1 048 576 | 1.0 MiB |
|        402 653 184 |  384 MiB |     1 572 864 | 1.5 MiB |
|        536 870 912 |  512 MiB |     2 097 152 | 2.0 MiB |
|        805 306 368 |  768 MiB |     3 145 728 | 3.0 MiB |
|      1 073 741 824 |  1.0 GiB |     4 194 304 | 4.0 MiB |
|      1 610 612 736 |  1.5 GiB |     6 291 456 | 6.0 MiB |
|      2 147 483 648 |  2.0 GiB |     8 388 608 | 8.0 MiB |
|      3 221 225 472 |  3.0 GiB |    12 582 912 |  12 MiB |
|      4 294 967 296 |  4.0 GiB |    16 777 216 |  16 MiB |
|      6 442 450 944 |  6.0 GiB |    25 165 824 |  24 MiB |
|    137 438 953 472 |  128 GiB |    33 554 432 |  32 MiB |
|    206 158 430 208 |  192 GiB |    50 331 648 |  48 MiB |
|    274 877 906 944 |  256 GiB |    67 108 864 |  64 MiB |
|    412 316 860 416 |  384 GiB |   100 663 296 |  96 MiB |
|    549 755 813 888 |  512 GiB |   134 217 728 | 128 MiB |
|    824 633 720 832 |  768 GiB |   201 326 592 | 192 MiB |
|  1 099 511 627 776 |  1.0 TiB |   268 435 456 | 256 MiB |
|  1 649 267 441 664 |  1.5 TiB |   402 653 184 | 384 MiB |
|  2 199 023 255 552 |  2.0 TiB |   536 870 912 | 512 MiB |
|  3 298 534 883 328 |  3.0 TiB |   805 306 368 | 768 MiB |
|  4 398 046 511 104 |  4.0 TiB | 1 073 741 824 | 1.0 GiB |
|  6 597 069 766 656 |  6.0 TiB | 1 610 612 736 | 1.5 GiB |
|  8 796 093 022 208 |  8.0 TiB | 2 147 483 648 | 2.0 GiB |
| 13 194 139 533 312 | 12.0 TiB | 3 221 225 472 | 3.0 GiB |
| 17 592 186 044 416 | 16.0 TiB | 4 294 967 296 | 4.0 GiB |
| 26 388 279 066 624 | 24.0 TiB | 6 442 450 944 | 6.0 GiB |
| 35 184 372 088 832 | 32.0 TiB | 8 589 934 592 | 8.0 GiB |


# hashed passwords

regarding the curious decisions

there is a static salt for all passwords;
* because most copyparty APIs allow users to authenticate using only their password, making the username unknown, so impossible to do per-account salts
* the drawback of this is that an attacker can bruteforce all accounts in parallel, however most copyparty instances only have a handful of accounts in the first place, and it can be compensated by increasing the hashing cost anyways


# http api

* table-column `params` = URL parameters; `?foo=bar&qux=...`
* table-column `body` = POST payload
* method `jPOST` = json post
* method `mPOST` = multipart post
* method `uPOST` = url-encoded post
* `FILE` = conventional HTTP file upload entry (rfc1867 et al, filename in `Content-Disposition`)

authenticate using header `Cookie: cppwd=foo` or url param `&pw=foo`

## read

| method | params | result |
|--|--|--|
| GET | `?dl` | download file (don't show in-browser) |
| GET | `?ls` | list files/folders at URL as JSON |
| GET | `?ls&dots` | list files/folders at URL as JSON, including dotfiles |
| GET | `?ls=t` | list files/folders at URL as plaintext |
| GET | `?ls=v` | list files/folders at URL, terminal-formatted |
| GET | `?lt` | in listings, use symlink timestamps rather than targets |
| GET | `?b` | list files/folders at URL as simplified HTML |
| GET | `?tree=.` | list one level of subdirectories inside URL |
| GET | `?tree` | list one level of subdirectories for each level until URL |
| GET | `?tar` | download everything below URL as a gnu-tar file |
| GET | `?tar=gz:9` | ...as a gzip-level-9 gnu-tar file |
| GET | `?tar=xz:9` | ...as an xz-level-9 gnu-tar file |
| GET | `?tar=pax` | ...as a pax-tar file |
| GET | `?tar=pax,xz` | ...as an xz-level-1 pax-tar file |
| GET | `?zip` | ...as a zip file |
| GET | `?zip=dos` | ...as a WinXP-compatible zip file |
| GET | `?zip=crc` | ...as an MSDOS-compatible zip file |
| GET | `?tar&w` | pregenerate webp thumbnails |
| GET | `?tar&j` | pregenerate jpg thumbnails |
| GET | `?tar&p` | pregenerate audio waveforms |
| GET | `?shares` | list your shared files/folders |
| GET | `?dls` | show active downloads (do this as admin) |
| GET | `?ups` | show recent uploads from your IP |
| GET | `?ups&filter=f` | ...where URL contains `f` |
| GET | `?ru` | show all recent uploads |
| GET | `?ru&filter=f` | ...where URL contains `f` |
| GET | `?ru&j` | ...as json |
| GET | `?mime=foo` | specify return mimetype `foo` |
| GET | `?v` | render markdown file at URL |
| GET | `?v` | open image/video/audio in mediaplayer |
| GET | `?txt` | get file at URL as plaintext |
| GET | `?txt=iso-8859-1` | ...with specific charset |
| GET | `?tail` | continuously stream a growing file |
| GET | `?tail=1024` | ...starting from byte 1024 |
| GET | `?tail=-128` | ...starting 128 bytes from the end |
| GET | `?th` | get image/video at URL as thumbnail |
| GET | `?th=opus` | convert audio file to 128kbps opus |
| GET | `?th=caf` | ...in the iOS-proprietary container |

| method | body | result |
|--|--|--|
| jPOST | `{"q":"foo"}` | do a server-wide search; see the `[🔎]` search tab `raw` field for syntax |

| method | params | body | result |
|--|--|--|--|
| jPOST | `?tar` | `["foo","bar"]` | download folders `foo` and `bar` inside URL as a tar file |

## write

| method | params | result |
|--|--|--|
| POST | `?copy=/foo/bar` | copy the file/folder at URL to /foo/bar |
| POST | `?move=/foo/bar` | move/rename the file/folder at URL to /foo/bar |

| method | params | body | result |
|--|--|--|--|
| PUT | | (binary data) | upload into file at URL |
| PUT | `?j` | (binary data) | ...and reply with json |
| PUT | `?ck` | (binary data) | upload without checksum gen (faster) |
| PUT | `?ck=md5` | (binary data) | return md5 instead of sha512 |
| PUT | `?gz` | (binary data) | compress with gzip and write into file at URL |
| PUT | `?xz` | (binary data) | compress with xz and write into file at URL |
| mPOST | | `f=FILE` | upload `FILE` into the folder at URL |
| mPOST | `?j` | `f=FILE` | ...and reply with json |
| mPOST | `?ck` | `f=FILE` | ...and disable checksum gen (faster) |
| mPOST | `?ck=md5` | `f=FILE` | ...and return md5 instead of sha512 |
| mPOST | `?replace` | `f=FILE` | ...and overwrite existing files |
| mPOST | `?media` | `f=FILE` | ...and return medialink (not hotlink) |
| mPOST | | `act=mkdir`, `name=foo` | create directory `foo` at URL |
| POST | `?delete` | | delete URL recursively |
| POST | `?eshare=rm` | | stop sharing a file/folder |
| POST | `?eshare=3` | | set expiration to 3 minutes |
| jPOST | `?share` | (complicated) | create temp URL for file/folder |
| jPOST | `?delete` | `["/foo","/bar"]` | delete `/foo` and `/bar` recursively |
| uPOST | | `msg=foo` | send message `foo` into server log |
| mPOST | | `act=tput`, `body=TEXT` | overwrite markdown document at URL |

upload modifiers:

| http-header | url-param | effect |
|--|--|--|
| `Accept: url` | `want=url` | return just the file URL |
| `Accept: json` | `want=json` | return upload info as json; same as `?j` |
| `Rand: 4` | `rand=4` | generate random filename with 4 characters |
| `Life: 30` | `life=30` | delete file after 30 seconds |
| `CK: no` | `ck` | disable serverside checksum (maybe faster) |
| `CK: md5` | `ck=md5` | return md5 checksum instead of sha512 |
| `CK: sha1` | `ck=sha1` | return sha1 checksum |
| `CK: sha256` | `ck=sha256` | return sha256 checksum |
| `CK: b2` | `ck=b2` | return blake2b checksum |
| `CK: b2s` | `ck=b2s` | return blake2s checksum |

* `life` only has an effect if the volume has a lifetime, and the volume lifetime must be greater than the file's

* server behavior of `msg` can be reconfigured with `--urlform`

## admin

| method | params | result |
|--|--|--|
| GET | `?reload=cfg` | reload config files and rescan volumes |
| GET | `?scan` | initiate a rescan of the volume which provides URL |
| GET | `?scan=/a,/b` | initiate a rescan of volumes `/a` and `/b` |
| GET | `?stack` | show a stacktrace of all threads |

## general

| method | params | result |
|--|--|--|
| GET | `?pw=x` | logout |
| GET | `?grid` | ui: show grid-view |
| GET | `?imgs` | ui: show grid-view with thumbnails |
| GET | `?grid=0` | ui: show list-view |
| GET | `?imgs=0` | ui: show list-view |
| GET | `?thumb` | ui, grid-mode: show thumbnails |
| GET | `?thumb=0` | ui, grid-mode: show icons |


# event hooks

on writing your own [hooks](../README.md#event-hooks)

## hook effects

hooks can cause intentional side-effects,  such as redirecting an upload into another location, or creating+indexing additional files, or deleting existing files, by returning json on stdout

* `reloc` can redirect uploads before/after uploading has finished, based on filename, extension, file contents, uploader ip/name etc.
  * example: [reloc-by-ext](https://github.com/9001/copyparty/blob/hovudstraum/bin/hooks/reloc-by-ext.py)
* `idx` informs copyparty about a new file to index as a consequence of this upload
  * example: [podcast-normalizer.py](https://github.com/9001/copyparty/blob/hovudstraum/bin/hooks/podcast-normalizer.py)
* `del` tells copyparty to delete an unrelated file by vpath
  * example: (　´・ω・) nyoro~n

for these to take effect, the hook must be defined with the `c1` flag; see example [reloc-by-ext](https://github.com/9001/copyparty/blob/hovudstraum/bin/hooks/reloc-by-ext.py)

a subset of effect types are available for a subset of hook types,

* most hook types (xbu/xau/xbr/xar/xbd/xad/xm) support `idx` and `del` for all http protocols (up2k / basic-uploader / webdav), but not ftp/tftp/smb
* most hook types will abort/reject the action if the hook returns nonzero, assuming flag `c` is given, see examples [reject-extension](https://github.com/9001/copyparty/blob/hovudstraum/bin/hooks/reject-extension.py) and [reject-mimetype](https://github.com/9001/copyparty/blob/hovudstraum/bin/hooks/reject-mimetype.py)
* `xbu` supports `reloc` for all http protocols (up2k / basic-uploader / webdav), but not ftp/tftp/smb
* `xau` supports `reloc` for basic-uploader / webdav only, not up2k or ftp/tftp/smb
  * so clients like sharex are supported, but not dragdrop into browser

to trigger indexing of files `/foo/1.txt` and `/foo/bar/2.txt`, a hook can `print(json.dumps({"idx":{"vp":["/foo/1.txt","/foo/bar/2.txt"]}}))` (and replace "idx" with "del" to delete instead)
* note: paths starting with `/` are absolute URLs, but you can also do `../3.txt` relative to the destination folder of each uploaded file


# assumptions

## mdns

* outgoing replies will always fit in one packet
* if a client mentions any of our services, assume it's not missing any
* always answer with all services, even if the client only asked for a few
* not-impl: probe tiebreaking (too complicated)
* not-impl: unicast listen (assume avahi took it)


# sfx repack

reduce the size of an sfx by removing features

if you don't need all the features, you can repack the sfx and save a bunch of space; all you need is an sfx and a copy of this repo (nothing else to download or build, except if you're on windows then you need msys2 or WSL)
* `393k` size of original sfx.py as of v1.1.3
* `310k` after `./scripts/make-sfx.sh re no-cm`
* `269k` after `./scripts/make-sfx.sh re no-cm no-hl`

the features you can opt to drop are
* `cm`/easymde, the "fancy" markdown editor, saves ~89k
* `hl`, prism, the syntax highlighter, saves ~41k
* `fnt`, source-code-pro, the monospace font, saves ~9k

for the `re`pack to work, first run one of the sfx'es once to unpack it

**note:** you can also just download and run [/scripts/copyparty-repack.sh](https://github.com/9001/copyparty/blob/hovudstraum/scripts/copyparty-repack.sh) -- this will grab the latest copyparty release from github and do a few repacks; works on linux/macos (and windows with msys2 or WSL)


# building

## dev env setup

you need python 3.9 or newer due to type hints

setting up a venv with the below packages is only necessary if you want it for vscode or similar

```sh
python3 -m venv .venv
. .venv/bin/activate
pip install jinja2 strip_hints  # MANDATORY
pip install argon2-cffi  # password hashing
pip install pyzmq  # send 0mq from hooks
pip install mutagen  # audio metadata
pip install pyftpdlib  # ftp server
pip install partftpy  # tftp server
pip install impacket  # smb server -- disable Windows Defender if you REALLY need this on windows
pip install Pillow pillow-heif  # thumbnails
pip install pyvips  # faster thumbnails
pip install psutil  # better cleanup of stuck metadata parsers on windows 
pip install black==21.12b0 click==8.0.2 bandit pylint flake8 isort mypy  # vscode tooling
```


## just the sfx

if you just want to modify the copyparty source code (py/html/css/js) then this is the easiest approach

build the sfx using any of the following examples:

```sh
./scripts/make-sfx.sh           # regular edition
./scripts/make-sfx.sh fast      # build faster (worse js/css compression)
./scripts/make-sfx.sh gz no-cm  # gzip-compressed + no fancy markdown editor
```


## build from release tarball

uses the included prebuilt webdeps

if you downloaded a [release](https://github.com/9001/copyparty/releases) source tarball from github (for example [copyparty-1.6.15.tar.gz](https://github.com/9001/copyparty/releases/download/v1.6.15/copyparty-1.6.15.tar.gz) so not the autogenerated one) you can build it like so,

```bash
python3 -m pip install --user -U build setuptools wheel jinja2 strip_hints
bash scripts/run-tests.sh python3  # optional
python3 -m build
```

if you are unable to use `build`, you can use the old setuptools approach instead,

```bash
python3 setup.py install --user setuptools wheel jinja2
python3 setup.py build
# you now have a wheel which you can install. or extract and repackage:
python3 setup.py install --skip-build --prefix=/usr --root=$HOME/pe/copyparty
```


## build from scratch

how the sausage is made:

to get started, first `cd` into the `scripts` folder

* the first step is the webdeps; they end up in `../copyparty/web/deps/` for example `../copyparty/web/deps/marked.js.gz` -- if you need to build the webdeps, run `make -C deps-docker`
  * this needs rootless podman and the `podman-docker` compat-layer to pretend it's docker, although it *should* be possible to use rootful/rootless docker too
  * if you don't have rootless podman/docker then `sudo make -C deps-docker` is fine too
  * alternatively, you can entirely skip building the webdeps and instead extract the compiled webdeps from the latest github release with `./make-sfx.sh fast dl-wd`

* next, build `copyparty-sfx.py` by running `./make-sfx.sh gz fast`
  * this is a dependency for most of the remaining steps, since they take the sfx as input
  * removing `fast` makes it compress better
  * removing `gz` too compresses even better, but startup gets slower

* if you want to build the `.pyz` standalone "binary", now run `./make-pyz.sh`

* if you want to build the `tar.gz` for use in a linux-distro package, now run `./make-tgz-release.sh theVersionNumber`

* if you want to build a pypi package, now run `./make-pypi-release.sh d`

* if you want to build a docker-image, you have two options:
  * if you want to use podman to build all docker-images for all supported architectures, now run `(cd docker; ./make.sh hclean; ./make.sh hclean pull img)`
  * if you want to use docker to build all docker-images for your native architecture, now run `sudo make -C docker`
  * if you want to do something else, please take a look at `docker/make.sh` or `docker/Makefile` for inspiration

* if you want to build the windows exe, first grab some snacks and a beer, [you'll need it](https://github.com/9001/copyparty/tree/hovudstraum/scripts/pyinstaller)

the complete list of buildtime dependencies to do a build from scratch is as follows:

* on ubuntu-server, install podman or [docker](https://get.docker.com/), and then `sudo apt install make zip bzip2`
  * because ubuntu is specifically what someone asked about :-p


## complete release

also builds the sfx so skip the sfx section above

*WARNING: `rls.sh` has not yet been updated with the docker-images and arch/nix packaging*

does everything completely from scratch, straight from your local repo

in the `scripts` folder:

* run `make -C deps-docker` to build all dependencies
* run `./rls.sh 1.2.3` which uploads to pypi + creates github release + sfx


# debugging

## music playback halting on phones

mostly fine on android,  but still haven't find a way to massage iphones into behaving well

* conditionally starting/stopping mp.fau according to mp.au.readyState <3 or <4 doesn't help
* loop=true doesn't work, and manually looping mp.fau from an onended also doesn't work (it does nothing)
* assigning fau.currentTime in a timer doesn't work, as safari merely pretends to assign it
* on ios 16.7.7, mp.fau can sometimes make everything visibly work correctly, but no audio is actually hitting the speakers

can be reproduced with `--no-sendfile --s-wr-sz 8192 --s-wr-slp 0.3 --rsp-slp 6` and then play a collection of small audio files with the screen off, `ffmpeg -i track01.cdda.flac -c:a libopus -b:a 128k -segment_time 12 -f segment smol-%02d.opus`


## discarded ideas

* optimization attempts which didn't improve performance
  * remove brokers / multiprocessing stuff; https://github.com/9001/copyparty/tree/no-broker
  * reduce the nesting / indirections in `HttpCli` / `httpcli.py`
    * nearly zero benefit from stuff like replacing all the `self.conn.hsrv` with a local `hsrv` variable
* single sha512 across all up2k chunks?
  * crypto.subtle cannot into streaming, would have to use hashwasm, expensive
* separate sqlite table per tag
  * performance fixed by skipping some indexes (`+mt.k`)
* audio fingerprinting
  * only makes sense if there can be a wasm client and that doesn't exist yet (except for olaf which is agpl hence counts as not existing)
* `os.copy_file_range` for up2k cloning
  * almost never hit this path anyways
* up2k partials ui
  * feels like there isn't much point
* cache sha512 chunks on client
  * too dangerous -- overtaken by turbo mode
* comment field
  * nah
* look into android thumbnail cache file format
  * absolutely not
* indexedDB for hashes, cfg enable/clear/sz, 2gb avail, ~9k for 1g, ~4k for 100m, 500k items before autoeviction
  * blank hashlist when up-ok to skip handshake
    * too many confusing side-effects
* hls framework for Someone Else to drop code into :^)
  * probably not, too much stuff to consider -- seeking, start at offset, task stitching (probably np-hard), conditional passthru, rate-control (especially multi-consumer), session keepalive, cache mgmt...
