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Merge pull request #1 from FRidh/75-intro

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Add introduction for run-time dependencies and wrappers.
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Doron Behar 2020-08-25 07:03:36 +00:00 • committed by GitHub
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rfcs/0075-declarative-wrappers.md
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@ -21,6 +21,37 @@ documented good enough and not easily debug-able shell hooks.
# Motivation
[motivation]: #motivation
Every Nix build produced is stored in a separate path in the store. For a
build to find its runtime dependencies purely, they need to be hardcoded in
the build. Thus, a complete dependency specification is needed.
In case of compiled languages this process is already largely automated;
absolute paths to shared libraries are encoded in the `rpath` of binaries.
Executables that are invoked by binaries need to be dealt with manually,
however. The preferred choice is here to manually patch the source so that
binaries are invoked using absolute paths as well.
This is not always trivial and thus a common work-around is to wrap executables,
setting `$PATH` to include the locations of dependencies. This is a pragmatic
solution that typically works fine, but it comes with a risk: *environment
variables leak*. Executables that shell out may pass along the modified
variables, causing at times unwanted behaviour.
Programs written in interpreted languages tend to import their runtime
dependencies using some kind of search path. E.g., in case of Python there is a
process for building up the `sys.path` variable which is then considered when
importing modules. Like with compiled languages, the preferred choice would also
here be to embed the absolute paths in the code, which is often not done. Note
that in case of Python this *is* done. Like with compiled languages, programs
may shell out and likewise the preferred solution is to patch the invocations to
use absolute paths. Similarly, in case a programs wants to `dlopen` a shared
library this should be patched to include an absolute path, instead of using
`LD_LIBRARY_PATH`.
It is recognized that wrappers setting environment variables are typically not
the preferred choice because of the above mentioned leakage risk, however, often
there is simply not a better or reasonable alternative available.
We have numerous issues regarding wrappers and our wrapper shell hooks. Here's
a list of them, sorted to categories.