[Bioc-devel] PROTECT errors in Bioconductor packages
Aaron Lun
alun at wehi.edu.au
Sat Apr 8 18:29:03 CEST 2017
Martin Morgan wrote:
> On 04/08/2017 08:16 AM, Aaron Lun wrote:
>> On 07/04/17 20:46, luke-tierney at uiowa.edu wrote:
>>> On Fri, 7 Apr 2017, Hervé Pagès wrote:
>>>
>>>> On 04/07/2017 05:37 AM, luke-tierney at uiowa.edu wrote:
>>>>> On Fri, 7 Apr 2017, Hervé Pagès wrote:
>>>>>
>>>>>> On 04/06/2017 03:29 AM, Michael Lawrence wrote:
>>>>>>> On Thu, Apr 6, 2017 at 2:59 AM, Martin Morgan
>>>>>>> <martin.morgan at roswellpark.org> wrote:
>>>>>>>> On 04/06/2017 05:33 AM, Aaron Lun wrote:
>>>>>>>>>>>>>>> The tool is not perfect, so assess each report
>>>>> carefully.
>>>>>>> I get a lot of warnings because the tool seems to consider that
>>>>>> extracting an attribute (with getAttrib(x, ...)) or extracting the
>>>>>> slot of an S4 object (with GET_SLOT(x, ...) or R_do_slot(x, ...))
>>>>>> returns an SEXP that needs protection. I always assumed that it
>>>>>> didn't because my understanding is that the returned SEXP is
>>>>>> pointing
>>>>>> to a part of a pre-existing object ('x') and not to a newly created
>>>>>> one. So I decided I could treat it like the SEXP returned by
>>>>>> VECTOR_ELT(), which, AFAIK, doesn't need protection.
>>>>>>> So I suspect these warnings are false positives but I'm not 100%
>>>>> sure.
>>>>>
>>>>> If you are not 100% sure then you should protect :-)
>>>>>
>>>>> There are some cases, in particular related to compact row names on
>>>>> data frames, where getAttrib will allocate.
>>>>
>>>> Seriously? So setAttrib(x, ..., getAttrib) is not going to be a no-op
>>>> anymore? Should I worry that VECTOR_ELT() will also expand some sort
>>>> of compact list element? Why not keep these things low-level
>>>> getters/setters that return whatever the real thing is and use
>>>> higher-level accessors for returning the expanded version of the
>>>> thing?
>>>
>>> Seriously: it's been that way since r37807 in 2006.
>>>
>>> If you want to read about some related future directions you can
>>> look at
>>> https://svn.r-project.org/R/branches/ALTREP/ALTREP.html.
>>>
>>> luke
>>
>> I was curious about this so I checked out what R-exts had to say
>> involving set/getAttrib. Funnily enough, the example it gives in Section
>> 5.9.4 seems to be incorrect in its UNPROTECTing.
>>
>> #include <R.h>
>> #include <Rinternals.h>
>>
>> SEXP out(SEXP x, SEXP y)
>> {
>> int nx = length(x), ny = length(y);
>> SEXP ans = PROTECT(allocMatrix(REALSXP, nx, ny));
>> double *rx = REAL(x), *ry = REAL(y), *rans = REAL(ans);
>>
>> for(int i = 0; i < nx; i++) {
>> double tmp = rx[i];
>> for(int j = 0; j < ny; j++)
>> rans[i + nx*j] = tmp * ry[j];
>> }
>>
>> SEXP dimnames = PROTECT(allocVector(VECSXP, 2));
>> SET_VECTOR_ELT(dimnames, 0, getAttrib(x, R_NamesSymbol));
>> SET_VECTOR_ELT(dimnames, 1, getAttrib(y, R_NamesSymbol));
>> setAttrib(ans, R_DimNamesSymbol, dimnames);
>>
>>
>> UNPROTECT(3);
>> return ans;
>> }
>>
>> There are two PROTECT calls but an UNPROTECT(3), which triggers a stack
>> imbalance warning upon trying to run .Call("out", ...) in R.
>
> Yes, that should be UNPROTECT(2). svn blame says the error was
> introduced when allocMatrix() was introduced; prior to that the code
> had allocVector(), then set dim and dimnames.
>
> As for whether to PROTECT or not, my analysis would be...
>
> SET_VECTOR_ELT does not (currently) allocate (except on error) so
> there is no opportunity for the garbage collector to run, hence no
> need to PROTECT.
>
> Further, getAttrib() (currently) allocates only if (1) the attribute
> is R_RowNamesSymbol and the row names are stored in compact format
> c(NA_integer_, nrow); or (2) the first argument is a classic pairlist
> or language SEXP. None of these conditions apply, so the return value
> of getAttrib() is PROTECTed anyway.
>
> Luke's analysis would be more straight-forward: if in doubt, PROTECT.
>
> I think Herve, Gabe, and perhaps Michael would take Luke's advice, and
> maybe also note that my advice, in addition to being an analysis of
> some surprisingly complicated code by a practitioner of dubious
> credibility, involves the current state of affairs, and you never know
> (and apparently ALTREP makes it less certain) what the future will
> hold. So they'd probably say PROTECT.
>
> One might be tempted to write
>
> SET_VECTOR_ELT(dimnames, 0, PROTECT(getAttrib(x, R_NamesSymbol)));
>
> but I'm not sure whether C guarantees that function arguments are
> fully evaluated, maybe it's legal for a compiler to evaluate
> getAttrib(), then do some more work with other arguments, then
> evaluate PROTECT(), so long as the overall logic is not disrupted. So
> the 'if in doubt' argument would make me write
>
> SEXP nms = PROTECT(getAttrib(x, R_NamesSymbol));
> SET_VECTOR_ELT(dimnames, 0, nms);
>
> I think , in C is called a 'sequence point'. Google takes me to
>
> https://msdn.microsoft.com/en-us/library/azk8zbxd.aspx
>
> where it seems like the left operand of ',' are fully evaluated before
> proceeding, and furthermore that arguments to functions are evaluated
> before entering the function, implying that it is safe to use the
> one-liner
>
> SET_VECTOR_ELT(dimnames, 0, PROTECT(getAttrib(x, R_NamesSymbol)));
>
> At any rate I changed the example in R-exts to UNPROTECT(2), leaving
> the nuances for further discussion.
>
> Martin
I wonder if the following is a sensible idea:
int Rf_num_protected; // global variable
void Rf_start_protection() {
Rf_num_protected=0;
return;
}
SEXP Rf_add_protection(SEXP x) {
++Rf_num_protected;
return PROTECT(x);
}
void Rf_end_protection() {
UNPROTECT(Rf_num_protected);
return;
}
The idea would be to:
1. call Rf_start_protection() at the top of the native routine
2. replace all uses of PROTECT with Rf_add_protection
3. call Rf_end_protection() just before returning to R
This would avoid having to keep track of the number of PROTECTs
performed, which may not be trivial if the routine can return at
multiple points.
It might also useful for C++ native routine creating class instances
that need to do internal PROTECTs for the lifetime of the instance. As
long as those PROTECTs are done via Rf_add_protection(), a single
Rf_end_protection() call at the bottom of the top-level routine would be
sufficient to handle them all. In contrast, putting a matching UNPROTECT
in the class destructor is not safe, as it is possible to trigger the
destructor to UNPROTECT an unrelated SEXP:
SEXP blah(SEXP x) {
my_class* ptr=new my_class(x); // say this does an internal PROTECT
SEXP output=PROTECT(allocVector(INTSXP, 1));
// ... do something with output here...
delete ptr; // if UNPROTECT is in the destructor, it UNPROTECTs
output instead
// ... do some more stuff, possibly involving allocations ...
UNPROTECT(1); // this actually UNPROTECTs whatever was in my_class
return output;
}
>>
>> Anyway, getting back to the topic of this thread; if we were to pretend
>> that getAttrib() allocates in the above example, would that mean that
>> both getAttrib() calls now need to be PROTECTed by the developer? Or is
>> this handled somewhere internally?
>>
>>>>
>>>> Thanks,
>>>> H.
>>>>
>>>>>
>>>>> Best,
>>>>>
>>>>> luke
>>>>>
>>>>>>>>>>>>>>>>>> I also get a warning on almost every C++
>>>>> function I've written,
>>>>>>>>> because
>>>>>>>>> I use the following code to handle exceptions:
>>>>>>>>>>>>> SEXP output=PROTECT(allocVector(...));
>>>>>>>>> try {
>>>>>>>>> // do something that might raise an exception
>>>>>>>>> } catch (std::exception& e) {
>>>>>>>>> UNPROTECT(1);
>>>>>>>>> throw; // break out of this part of the function
>>>>>>>>> }
>>>>>>>>> UNPROTECT(1);
>>>>>>>>> return output;
>>>>>>>>>>>>> Presumably the check doesn't account for transfer of
>>>>> control to > > > > the
>>>>>>>>> catch block. I find that R itself is pretty good at
>>>>> complaining > > > > about
>>>>>>>>> stack imbalances during execution of tests, examples, etc.
>>>>>>>>>>>>>> 'My' packages
>>>>>>>>>> (Rsamtools, DirichletMultinomial) had several false
>>>>> positives > > > > > (all
>>>>>>>>>> associated with use of an attribute of a protected SEXP),
>>>>> one > > > > > subtle
>>>>>>>>>> problem (a symbol from a PROTECT'ed package name space;
>>>>> the > > > > > symbol
>>>>>>>>>> could
>>>>>>>>>> in theory be an active binding and the value obtained not
>>>>>>>>>> PROTECTed by
>>>>>>>>>> the name space), and a genuine bug
>>>>>>>>>>>>>>> tag = NEW_CHARACTER(n);
>>>>>>>>>> for (int j = 0; j < n; ++j)
>>>>>>>>>> SET_STRING_ELT(tag, j, NA_STRING);
>>>>>>>>>> if ('A' == aux[0]) {
>>>>>>>>>> buf_A = R_alloc(2, sizeof(char)); #
>>>>> <<- bug
>>>>>>>>>> buf_A[1] = '\0';
>>>>>>>>>> }
>>>>>>>>>> ...
>>>>>>>>>> SET_VECTOR_ELT(tags, i, tag); # PROTECT
>>>>> tag, too
>>>>>>>>>> late!
>>>>>>>>>>>>>>>>> I assume the bug refers to the un-PROTECT'd
>>>>> NEW_CHARACTER here - > > > > the
>>>>>>>>> R_alloc call looks okay to me...
>>>>>>>>>>>>>> yes, tag needs protection.
>>>>>>>>>>> I attributed the bug to R_alloc because I failed to
>>>>> reason that > > > R_alloc
>>>>>>>> (obviously) allocates and hence can trigger a garbage collection.
>>>>>>>>>>> Somehow it reflects my approach to PROTECTion, probably
>>>>> not shared > > > by
>>>>>>>> everyone. I like to PROTECT only when necessary, rather than
>>>>>>>> indiscriminately. Probably this has no practical consequence in
>>>>>>>> terms of
>>>>>>>> performance, making the code a little easier to read at the
>>>>> expense > > > of
>>>>>>>> exposing me to bugs like this.
>>>>>>>>>>>> I guess it's a tradeoff between syntactic complexity
>>>>> and logical
>>>>>>> complexity. You have to think pretty hard to minimize use of the
>>>>>>> protect stack.
>>>>>>> I prefer to call it logical obscurity ;-)
>>>>>>> The hard thinking consists in assessing whether or not the code
>>>>> between
>>>>>> the line where a new SEXP is allocated (line X) and the line where
>>>>>> it's put in a safe place (line Y) can trigger garbage collection.
>>>>>> Hard to figure out in general indeed, but not impossible! Problem
>>>>>> is that the result of this assessment is valid at a certain point
>>>>>> in time but might change in the future, even if your code has not
>>>>>> changed.
>>>>>>> So a dangerous game for virtually zero benefits.
>>>>>>>>>> One recommendation might be to UNPROTECT() as soon as the
>>>>> pointer on
>>>>>>> the top is unneeded, rather than trying to figure out the
>>>>>>> number to
>>>>>>> pop just before returning to R.
>>>>>>> If you PROTECT() in a loop, you definitely want to do that.
>>>>> Otherwise,
>>>>>> does it make a big difference?
>>>>>>>>>> One thing that got me is that the order in which C
>>>>> evaluates function
>>>>>>> call arguments is undefined. I did a lot of R_setAttrib(x,
>>>>>>> install("foo"), allocBar()), thinking that the symbol would be
>>>>>>> automatically protected, and allocBar() would not need protection,
>>>>>>> since it happened last. Unfortunately, it might be evaluated
>>>>>>> first.
>>>>>>> I got hit by this too long time ago but with defineVar()
>>>>>>> instead of
>>>>>> R_setAttrib():
>>>>>>>
>>>>> https://urldefense.proofpoint.com/v2/url?u=https-3A__stat.ethz.ch_pipermail_r-2Ddevel_2008-2DJanuary_048040.html&d=DwID-g&c=eRAMFD45gAfqt84VtBcfhQ&r=BK7q3XeAvimeWdGbWY_wJYbW0WYiZvSXAJJKaaPhzWA&m=FscW1HcPCwUqtMwKVFDfd1NyW0oHh0tJOPdFb3C1IWk&s=O3CcB-Z_OkVKaC1aV0aIc5SCDNqGQrkvGSmPf0TRAsw&e=
>>>>>
>>>>>
>>>>>>> H.
>>>>>>>>>> Btw, I think my package RGtk2 got the record: 1952 errors.
>>>>> Luckily
>>>>>>> almost all of them happened inside a few macros and autogenerated
>>>>>>> code.
>>>>>>>>>> Martin
>>>>>>>>>>>>>>>> Cheers,
>>>>>>>>>>>>> Aaron
>>>>>>>>> _______________________________________________
>>>>>>>>> Bioc-devel at r-project.org mailing list
>>>>>>>>>
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>>>>>
>>>>>
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>>>>>
>>>>>>>>>>>
>>>>
>>>>
>>>
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