zet.tlv: switch file writes to new API

The interface for apps looks much nicer now, see 'main' in zet.tlv.
However there are some open issues:

- It can still be confusing to the computer owner that an app tries to
  write to some temporary file that isn't mentioned anywhere.

- File renames can fail if /tmp is on a different volume.

- What happens if an app overrides start_writing()? The computer owner
  may think they've audited the caller of start_writing and give it
  blanket file permissions. Teliva tunnels through start_writing when
  computing the caller. If the app can control what start_writing does,
  the app could be performing arbitrary malicious file operations.

  Right now things actually seem perfectly secure. Overriding
  start_writing has no effect. Our approach for loading .tlv files (in
  reverse chronological order, preventing older versions from overriding
  newer ones) has the accidentally _great_ property that Teliva apps can
  never override system definitions.

  So we have a new reason to put standard libraries in a .lua file: if
  we need to prevent apps from overriding it.

  This feels like something that needs an automated test, both to make
  sure I'm running the right experiment and to ensure I don't
  accidentally cause a regression in the future. I can totally imagine a
  future rewrite that tried a different approach than
  reverse-chronological.

1 files changed, 37 insertions, 0 deletions

diff --git a/zet.tlv b/zet.tlv
index e9e08cc..1963796 100644
--- a/zet.tlv
+++ b/zet.tlv
@@ -3593,3 +3593,40 @@
     >function read_zettels(infile)
     >  zettels = jsonf.decode(infile)
     >end
+- __teliva_timestamp:
+    >Mon Mar  7 10:31:27 2022
+  main:
+    >function main()
+    >  init_colors()
+    >  curses.curs_set(0)  -- hide cursor except when editing
+    >
+    >  -- load any saved zettels
+    >  local infile = start_reading(nil, 'zet')
+    >  if infile then
+    >    read_zettels(infile)
+    >  end
+    >  current_zettel_id = zettels.root  -- cursor
+    >  view_settings.first_zettel = zettels.root  -- start rendering here
+    >
+    >  while true do
+    >    render(Window)
+    >    update(Window)
+    >
+    >    -- save zettels
+    >    local outfile = start_writing(nil, 'zet')
+    >    if outfile then
+    >      write_zettels(outfile)
+    >    end
+    >    -- TODO: what if io.open failed for a non-sandboxing related reason?!
+    >    -- We could silently fail to save.
+    >  end
+    >end
+- __teliva_timestamp:
+    >Mon Mar  7 10:32:08 2022
+  __teliva_note:
+    >switch to new file API for writing
+  write_zettels:
+    >function write_zettels(outfile)
+    >  outfile:send(json.encode(zettels))
+    >  outfile:close()
+    >end