ludd

Elixir/Ports and external process wiring: Difference between revisions

← Older editNewer edit →
Adamw (talk | contribs)
201 edits
VisualWikitext
Adamw (talk | contribs)
201 edits
more aside
Adamw (talk | contribs)
201 edits
No edit summary
Line 46: Line 46:
   ]
   ]
)
)
</syntaxhighlight>
Progress lines come in with a fairly self-explanatory format:
<syntaxhighlight lang="text">
      3,342,336  33%    3.14MB/s    0:00:02
</syntaxhighlight>
</syntaxhighlight>


Line 67: Line 62:
}}
}}


Each rsync output line is sent to the library's <code>handle_info</code> callback as <code>{:data, line}</code> and after the transfer is finished we receive a conclusive <code>{:exit_status, status_code}</code>.
Rsync outputs progress lines in a fairly self-explanatory format:<syntaxhighlight lang="text">
      3,342,336  33%    3.14MB/s    0:00:02
</syntaxhighlight>


We extract the percent_done column and strictly reject any other output:
Our Port captures output and each line is sent to the library's <code>handle_info</code> callback as <code>{:data, line}</code>.  After the transfer is finished we receive a conclusive <code>{:exit_status, status_code}</code> message.
 
As a first step, we extract the percent_done column and log any unrecognized output:
<syntaxhighlight lang="elixir">
<syntaxhighlight lang="elixir">
with terms when terms != [] <- String.split(line, ~r"\s", trim: true),
with terms when terms != [] <- String.split(line, ~r"\s", trim: true),
Line 79: Line 78:
         {:unknown, line}
         {:unknown, line}
     end
     end
</syntaxhighlight>The <code>trim</code> lets us ignore spacing and newline trickery—or even a leading carriage return as you can see in the rsync source code,
</syntaxhighlight>The <code>trim</code> is lifting more than its weight here: it lets us completely ignore spacing and newline trickery—and even a leading carriage return that we can see in the rsync source code,
<syntaxhighlight lang="c">
<syntaxhighlight lang="c">
rprintf(FCLIENT, "\r%15s %3d%% %7.2f%s %s%s", ...);
rprintf(FCLIENT, "\r%15s %3d%% %7.2f%s %s%s", ...);
</syntaxhighlight>The carriage return <code>\r</code> deserves a special mention: this "control" character is just a byte in the binary data coming over the pipe from rsync, but it plays a control function because of how the tty interprets it.  On the terminal the effect is to overwrite the current line!
</syntaxhighlight>Carriage return <code>\r</code> deserves a special mention: this "control" character is just a byte in the binary data coming over the pipe from rsync, but its normal role is playing a control function because of how the terminal emulator responds to it.  On a terminal the effect is to rewind the cursor and overwrite the current line!
 
A repeated theme in inter-process communication is that data and control are leaky categories.  We come to the more formal control side channels later.


A repeated theme is that data and control are leaky categories.  We come to the more formal control side channels later.
{{Aside|text=
{{Aside|text=
[[File:Chinese typewriter 03.jpg|right|200x200px]]
[[File:Chinese typewriter 03.jpg|right|200x200px]]
Line 90: Line 90:
On the terminal, rsync progress lines are updated in place by emitting a [[w:Carriage return|carriage return]] control character, <code>\r</code>, <code>0x0d</code> sometimes rendered as <code>^M</code>.  The character seems to be named after pushing the physical paper carriage of a typewriter back to the beginning of the line without feeding the roller.
On the terminal, rsync progress lines are updated in place by emitting a [[w:Carriage return|carriage return]] control character, <code>\r</code>, <code>0x0d</code> sometimes rendered as <code>^M</code>.  The character seems to be named after pushing the physical paper carriage of a typewriter back to the beginning of the line without feeding the roller.


[[w:https://en.wikipedia.org/wiki/Newline#Issues_with_different_newline_formats|Disagreement about carriage return]] vs. newline has caused eye-rolling since the dawn of personal computing.
[[w:https://en.wikipedia.org/wiki/Newline#Issues_with_different_newline_formats|Disagreement about carriage return]] vs. line feed has caused eye-rolling since the dawn of personal computing.


[[File:Nilgais fighting, Lakeshwari, Gwalior district, India.jpg|left|200x200px]]
[[File:Nilgais fighting, Lakeshwari, Gwalior district, India.jpg|left|200x200px]]
Retrieved from "https://mw.ludd.net/wiki/Elixir/Ports_and_external_process_wiring"