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Warn about the usage of filterSource with Nix store paths #5286

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Oct 6, 2021
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Warn about the usage of filterSource with Nix store paths #5286

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17 changes: 15 additions & 2 deletions src/libexpr/primops.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -413,7 +413,7 @@ static RegisterPrimOp primop_isNull({
Return `true` if *e* evaluates to `null`, and `false` otherwise.

> **Warning**
>
>
> This function is *deprecated*; just write `e == null` instead.
)",
.fun = prim_isNull,
Expand Down Expand Up @@ -1919,6 +1919,19 @@ static RegisterPrimOp primop_filterSource({
.name = "__filterSource",
.args = {"e1", "e2"},
.doc = R"(
> **Warning**
>
> `filterSource` should not be used to filter store paths. Since
> `filterSource` uses the name of the input directory while naming
> the output directory, doing so will produce a directory name in
> the form of `<hash2>-<hash>-<name>`, where `<hash>-<name>` is
> the name of the input directory. Since `<hash>` depends on the
> unfiltered directory, the name of the output directory will
> indirectly depend on files that are filtered out by the
> function. This will trigger a rebuild even when a filtered out
> file is changed. Use `builtins.path` instead, which allows
> specifying the name of the output directory.

This function allows you to copy sources into the Nix store while
filtering certain files. For instance, suppose that you want to use
the directory `source-dir` as an input to a Nix expression, e.g.
Expand Down Expand Up @@ -2519,7 +2532,7 @@ static RegisterPrimOp primop_tail({
the argument isn’t a list or is an empty list.

> **Warning**
>
>
> This function should generally be avoided since it's inefficient:
> unlike Haskell's `tail`, it takes O(n) time, so recursing over a
> list by repeatedly calling `tail` takes O(n^2) time.
Expand Down