[Rd] Choices to remove `srcref` (and its buddies) when serializing objects

[Ivan Krylov]

В Tue, 16 Jan 2024 14:16:19 -0500
Dipterix Wang <dipterix.wang using gmail.com> пишет:

> Could you recommend any packages/functions that compute hash such
> that the source references and sexpinfo_struct are ignored? Basically
> a version of `serialize` that convert R objects to raw without
> storing the ancillary source reference and sexpinfo.

I can show how this can be done, but it's not currently on CRAN or even
a well-defined package API. I have adapted a copy of R's serialize()
[*] with the following changes:

 * Function bytecode and flags are ignored:

f <- function() invisible()
depcache:::hash(f, 2) # This is plain FNV1a-64 of serialize() output
# [1] "9b7a1af5468deba4"
.Call(depcache:::C_hash2, f) # This is the new hash
[1] 91 5f b8 a1 b0 6b cb 40
f() # called once: function gets the MAYBEJIT_MASK flag
depcache:::hash(f, 2)
# [1] "7d30e05546e7a230"
.Call(depcache:::C_hash2, f)
# [1] 91 5f b8 a1 b0 6b cb 40
f() # called twice: function now has bytecode
depcache:::hash(f, 2)
# [1] "2a2cba4150e722b8"
.Call(depcache:::C_hash2, f)
# [1] 91 5f b8 a1 b0 6b cb 40 # new hash stays the same

 * Source references are ignored:

.Call(depcache:::C_hash2, \( ) invisible( ))
# [1] 91 5f b8 a1 b0 6b cb 40 # compare vs. above

# For quoted function definitions, source references have to be handled
# differently 
.Call(depcache:::C_hash2, quote(function(){}))
[1] 58 0d 44 8e d4 fd 37 6f
.Call(depcache:::C_hash2, quote(\( ){      }))
[1] 58 0d 44 8e d4 fd 37 6f

 * ALTREP is ignored:

identical(1:10, 1:10+0L)
# [1] TRUE
identical(serialize(1:10, NULL), serialize(1:10+0L, NULL))
# [1] FALSE
identical(
 .Call(depcache:::C_hash2, 1:10),
 .Call(depcache:::C_hash2, 1:10+0L)
)
# [1] TRUE

 * Strings not marked as bytes are encoded into UTF-8:

identical('\uff', iconv('\uff', 'UTF-8', 'latin1'))
# [1] TRUE
identical(
 serialize('\uff', NULL),
 serialize(iconv('\uff', 'UTF-8', 'latin1'), NULL)
)
# [1] FALSE
identical(
 .Call(depcache:::C_hash2, '\uff'),
 .Call(depcache:::C_hash2, iconv('\uff', 'UTF-8', 'latin1'))
)
# [1] TRUE

 * NaNs with different payloads (except NA_numeric_) are replaced by
   R_NaN.

One of the many downsides to the current approach is that we rely on
the non-API entry point getPRIMNAME() in order to hash builtins.
Looking at the source code for identical() is no help here, because it
uses the private PRIMOFFSET macro.

The bitstream being hashed is also, unfortunately, not exactly
compatible with R serialization format version 2: I had to ignore the
LEVELS of the language objects being hashed both because identical()
seems to ignore those and because I was missing multiple private
definitions (e.g. the MAYBEJIT flag) to handle them properly.

Then there's also the problem of immediate bindings [**]: I've seen bits
of vctrs, rstudio, rlang blow up when calling CAR() on SEXP objects that
are not safe to handle this way, but R_expand_binding_value() (used by
serialize()) is again a private function that is not accessible from
packages. identical() won't help here, because it compares reference
objects (which may or may not contain such immediate bindings) by their
pointer values instead of digging down into them.

Dropping the (already violated) requirement to be compatible with R
serialization bitstream will make it possible to simplify the code
further.

Finally:

a <- new.env()
b <- new.env()
a$x <- b$x <- 42
identical(a, b)
# [1] FALSE
.Call(depcache:::C_hash2, a)
# [1] 44 21 f1 36 5d 92 03 1b
.Call(depcache:::C_hash2, b)
# [1] 44 21 f1 36 5d 92 03 1b

...but that's unavoidable when looking at frozen object contents
instead of their live memory layout.

If you're interested, here's the development version of the package:
install.packages('depcache',contriburl='https://aitap.github.io/Rpackages')

-- 
Best regards,
Ivan

[*]
https://github.com/aitap/depcache/blob/serialize_canonical/src/serialize.c

[**]
https://svn.r-project.org/R/trunk/doc/notes/immbnd.md