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Restore ambiguous value printer for nix-instantiate

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The Nix team has requested that this output format remain unchanged.
I've added a warning to the man page explaining that `nix-instantiate
--eval` output will not parse correctly in many situations.
This commit is contained in:
Rebecca Turner 2024-01-09 11:13:45 -08:00
parent 0fa08b4516
commit df84dd4d8d
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6 changed files with 189 additions and 26 deletions
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80
doc/manual/src/command-ref/nix-instantiate.md
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@ -35,13 +35,50 @@ standard input.
- `--parse`\
Just parse the input files, and print their abstract syntax trees on
standard output in ATerm format.
standard output as a Nix expression.
- `--eval`\
Just parse and evaluate the input files, and print the resulting
values on standard output. No instantiation of store derivations
takes place.
> **Warning**
>
> This option produces ambiguous output which is not suitable for machine
> consumption. For example, these two Nix expressions print the same result
> despite having different types:
>
> ```console
> $ nix-instantiate --eval --expr '{ a = {}; }'
> { a = <CODE>; }
> $ nix-instantiate --eval --expr '{ a = <CODE>; }'
> { a = <CODE>; }
> ```
>
> For human-readable output, `nix eval` (experimental) is more informative:
>
> ```console
> $ nix-instantiate --eval --expr 'a: a'
> <LAMBDA>
> $ nix eval --expr 'a: a'
> «lambda @ «string»:1:1»
> ```
>
> For machine-readable output, the `--xml` option produces unambiguous
> output:
>
> ```console
> $ nix-instantiate --eval --xml --expr '{ foo = <CODE>; }'
> <?xml version='1.0' encoding='utf-8'?>
> <expr>
> <attrs>
> <attr column="3" line="1" name="foo">
> <unevaluated />
> </attr>
> </attrs>
> </expr>
> ```
- `--find-file`\
Look up the given files in Nix’s search path (as specified by the
`NIX_PATH` environment variable). If found, print the corresponding
@ -61,11 +98,11 @@ standard input.
- `--json`\
When used with `--eval`, print the resulting value as an JSON
representation of the abstract syntax tree rather than as an ATerm.
representation of the abstract syntax tree rather than as a Nix expression.
- `--xml`\
When used with `--eval`, print the resulting value as an XML
representation of the abstract syntax tree rather than as an ATerm.
representation of the abstract syntax tree rather than as a Nix expression.
The schema is the same as that used by the [`toXML`
built-in](../language/builtins.md).
@ -133,28 +170,29 @@ $ nix-instantiate --eval --xml --expr '1 + 2'
The difference between non-strict and strict evaluation:
```console
$ nix-instantiate --eval --xml --expr 'rec { x = "foo"; y = x; }'
...
<attr name="x">
<string value="foo" />
</attr>
<attr name="y">
<unevaluated />
</attr>
...
$ nix-instantiate --eval --xml --expr '{ x = {}; }'
<?xml version='1.0' encoding='utf-8'?>
<expr>
<attrs>
<attr column="3" line="1" name="x">
<unevaluated />
</attr>
</attrs>
</expr>
```
Note that `y` is left unevaluated (the XML representation doesn’t
attempt to show non-normal forms).
```console
$ nix-instantiate --eval --xml --strict --expr 'rec { x = "foo"; y = x; }'
...
<attr name="x">
<string value="foo" />
</attr>
<attr name="y">
<string value="foo" />
</attr>
...
$ nix-instantiate --eval --xml --strict --expr '{ x = {}; }'
<?xml version='1.0' encoding='utf-8'?>
<expr>
<attrs>
<attr column="3" line="1" name="x">
<attrs>
</attrs>
</attr>
</attrs>
</expr>
```

100
src/libexpr/print-ambiguous.cc Normal file
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@ -0,0 +1,100 @@
#include "print-ambiguous.hh"
#include "print.hh"
#include "signals.hh"
namespace nix {
// See: https://github.com/NixOS/nix/issues/9730
void printAmbiguous(
Value &v,
const SymbolTable &symbols,
std::ostream &str,
std::set<const void *> *seen,
int depth)
{
checkInterrupt();
if (depth <= 0) {
str << "«too deep»";
return;
}
switch (v.type()) {
case nInt:
str << v.integer;
break;
case nBool:
printLiteralBool(str, v.boolean);
break;
case nString:
printLiteralString(str, v.string_view());
break;
case nPath:
str << v.path().to_string(); // !!! escaping?
break;
case nNull:
str << "null";
break;
case nAttrs: {
if (seen && !v.attrs->empty() && !seen->insert(v.attrs).second)
str << "«repeated»";
else {
str << "{ ";
for (auto & i : v.attrs->lexicographicOrder(symbols)) {
str << symbols[i->name] << " = ";
printAmbiguous(*i->value, symbols, str, seen, depth - 1);
str << "; ";
}
str << "}";
}
break;
}
case nList:
if (seen && v.listSize() && !seen->insert(v.listElems()).second)
str << "«repeated»";
else {
str << "[ ";
for (auto v2 : v.listItems()) {
if (v2)
printAmbiguous(*v2, symbols, str, seen, depth - 1);
else
str << "(nullptr)";
str << " ";
}
str << "]";
}
break;
case nThunk:
if (!v.isBlackhole()) {
str << "<CODE>";
} else {
// Although we know for sure that it's going to be an infinite recursion
// when this value is accessed _in the current context_, it's likely
// that the user will misinterpret a simpler «infinite recursion» output
// as a definitive statement about the value, while in fact it may be
// a valid value after `builtins.trace` and perhaps some other steps
// have completed.
str << "«potential infinite recursion»";
}
break;
case nFunction:
if (v.isLambda()) {
str << "<LAMBDA>";
} else if (v.isPrimOp()) {
str << "<PRIMOP>";
} else if (v.isPrimOpApp()) {
str << "<PRIMOP-APP>";
}
break;
case nExternal:
str << *v.external;
break;
case nFloat:
str << v.fpoint;
break;
default:
printError("Nix evaluator internal error: printAmbiguous: invalid value type");
abort();
}
}
}

24
src/libexpr/print-ambiguous.hh Normal file
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@ -0,0 +1,24 @@
#pragma once
#include "value.hh"
namespace nix {
/**
* Print a value in the deprecated format used by `nix-instantiate --eval` and
* `nix-env` (for manifests).
*
* This output can't be changed because it's part of the `nix-instantiate` API,
* but it produces ambiguous output; unevaluated thunks and lambdas (and a few
* other types) are printed as Nix path syntax like `<CODE>`.
*
* See: https://github.com/NixOS/nix/issues/9730
*/
void printAmbiguous(
Value &v,
const SymbolTable &symbols,
std::ostream &str,
std::set<const void *> *seen,
int depth);
}

3
src/nix-env/user-env.cc
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@ -108,8 +108,7 @@ bool createUserEnv(EvalState & state, DrvInfos & elems,
environment. */
auto manifestFile = ({
std::ostringstream str;
std::set<const void *> seen;
printAmbiguous(manifest, state.symbols, str, &seen, std::numeric_limits<int>::max());
printAmbiguous(manifest, state.symbols, str, nullptr, std::numeric_limits<int>::max());
// TODO with C++20 we can use str.view() instead and avoid copy.
std::string str2 = str.str();
StringSource source { str2 };

6
src/nix-instantiate/nix-instantiate.cc
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@ -1,9 +1,11 @@
#include "globals.hh"
#include "print-ambiguous.hh"
#include "shared.hh"
#include "eval.hh"
#include "eval-inline.hh"
#include "get-drvs.hh"
#include "attr-path.hh"
#include "signals.hh"
#include "value-to-xml.hh"
#include "value-to-json.hh"
#include "store-api.hh"
@ -24,7 +26,6 @@ static int rootNr = 0;
enum OutputKind { okPlain, okXML, okJSON };
void processExpr(EvalState & state, const Strings & attrPaths,
bool parseOnly, bool strict, Bindings & autoArgs,
bool evalOnly, OutputKind output, bool location, Expr * e)
@ -56,7 +57,8 @@ void processExpr(EvalState & state, const Strings & attrPaths,
std::cout << std::endl;
} else {
if (strict) state.forceValueDeep(vRes);
vRes.print(state, std::cout);
std::set<const void *> seen;
printAmbiguous(vRes, state.symbols, std::cout, &seen, std::numeric_limits<int>::max());
std::cout << std::endl;
}
} else {

2
tests/functional/lang/eval-okay-print.exp
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@ -1 +1 @@
[ null «primop toString» «partially applied primop deepSeq» «lambda @ /pwd/lang/eval-okay-print.nix:1:61» [ [ «repeated» ] ] ]
[ null <PRIMOP> <PRIMOP-APP> <LAMBDA> [ [ «repeated» ] ] ]