[tor-bugs] #23878 [Core Tor/Tor]: Attempt rewriting buffers.c in Rust

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#23878: Attempt rewriting buffers.c in Rust
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 Reporter:  isis                                 |          Owner:  (none)
     Type:  enhancement                          |         Status:  new
 Priority:  Medium                               |      Milestone:  Tor:
                                                 |  unspecified
Component:  Core Tor/Tor                         |        Version:
 Severity:  Normal                               |     Resolution:
 Keywords:  rust-pilot, rust, datatypes,         |  Actual Points:
  034-triage-20180328, 034-removed-20180328      |
Parent ID:                                       |         Points:  3
 Reviewer:                                       |        Sponsor:
                                                 |  Sponsor8-can
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Comment (by cypherpunks):

 Since
 [https://gitweb.torproject.org/tor.git/commit/?id=d8b34e0886c9fd08fb51e0de4fadca7da82056ff
 d8b34e0886c9fd08fb51e0de4fadca7da82056ff], buffers.c has been split and
 refactored four ways now, so there are three callers in the C code that
 still dig into the implementation detail guts of `buf_t` in search of raw
 pointers, presumably with the benefit of zero-copy performance.

 Could `compress_buf.c`, `buffers_net.c`, and `buffers_tls.c` just switch
 to using the single-copy public API, or should a new zero-copy API be
 added that the Rust version can implement too?

 It could be something that synchronously invokes a callback exposing a raw
 pointer, so that the return value tells `buf_t` how many bytes were
 written/read, and the callback serves as a clear sign to users the pointer
 is not guaranteed to remain valid past the end of that function.

 {{{
 typedef size_t (*buf_read_cb)(const char *string, size_t string_len, void
 *data);
 typedef size_t (*buf_write_cb)(char *string, size_t string_len, void
 *data);

 void buf_read_call(buf_t *buf, buf_read_cb *cb, void *data);
 void buf_write_call(buf_t *buf, buf_write_cb *cb, void *data);
 }}}

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/23878#comment:8>
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