From 8229562a28b2f13ead76976ebda941c62d556887 Mon Sep 17 00:00:00 2001 From: Rebecca Turner Date: Tue, 9 Jan 2024 11:13:45 -0800 Subject: [PATCH] Restore ambiguous value printer for `nix-instantiate` 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. --- doc/manual/src/command-ref/nix-instantiate.md | 80 ++++++++++---- src/libexpr/print-ambiguous.cc | 100 ++++++++++++++++++ src/libexpr/print-ambiguous.hh | 24 +++++ src/nix-env/user-env.cc | 3 +- src/nix-instantiate/nix-instantiate.cc | 6 +- tests/functional/lang/eval-okay-print.exp | 2 +- 6 files changed, 189 insertions(+), 26 deletions(-) create mode 100644 src/libexpr/print-ambiguous.cc create mode 100644 src/libexpr/print-ambiguous.hh diff --git a/doc/manual/src/command-ref/nix-instantiate.md b/doc/manual/src/command-ref/nix-instantiate.md index e1b4a3e80143..483150aa88a6 100644 --- a/doc/manual/src/command-ref/nix-instantiate.md +++ b/doc/manual/src/command-ref/nix-instantiate.md @@ -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 = ; } + > $ nix-instantiate --eval --expr '{ a = ; }' + > { a = ; } + > ``` + > + > For human-readable output, `nix eval` (experimental) is more informative: + > + > ```console + > $ nix-instantiate --eval --expr 'a: a' + > + > $ 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 = ; }' + > + > + > + > + > + > + > + > + > ``` + - `--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; }' -... - - - - - - -... +$ nix-instantiate --eval --xml --expr '{ x = {}; }' + + + + + + + + ``` 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; }' -... - - - - - - -... +$ nix-instantiate --eval --xml --strict --expr '{ x = {}; }' + + + + + + + + + ``` diff --git a/src/libexpr/print-ambiguous.cc b/src/libexpr/print-ambiguous.cc new file mode 100644 index 000000000000..07c398dd25a6 --- /dev/null +++ b/src/libexpr/print-ambiguous.cc @@ -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 *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 << ""; + } 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 << ""; + } else if (v.isPrimOp()) { + str << ""; + } else if (v.isPrimOpApp()) { + str << ""; + } + break; + case nExternal: + str << *v.external; + break; + case nFloat: + str << v.fpoint; + break; + default: + printError("Nix evaluator internal error: printAmbiguous: invalid value type"); + abort(); + } +} + +} diff --git a/src/libexpr/print-ambiguous.hh b/src/libexpr/print-ambiguous.hh new file mode 100644 index 000000000000..50c260a9b849 --- /dev/null +++ b/src/libexpr/print-ambiguous.hh @@ -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 ``. + * + * See: https://github.com/NixOS/nix/issues/9730 + */ +void printAmbiguous( + Value &v, + const SymbolTable &symbols, + std::ostream &str, + std::set *seen, + int depth); + +} diff --git a/src/nix-env/user-env.cc b/src/nix-env/user-env.cc index 3d07cab7aa60..973b6ee2b777 100644 --- a/src/nix-env/user-env.cc +++ b/src/nix-env/user-env.cc @@ -108,8 +108,7 @@ bool createUserEnv(EvalState & state, DrvInfos & elems, environment. */ auto manifestFile = ({ std::ostringstream str; - std::set seen; - printAmbiguous(manifest, state.symbols, str, &seen, std::numeric_limits::max()); + printAmbiguous(manifest, state.symbols, str, nullptr, std::numeric_limits::max()); // TODO with C++20 we can use str.view() instead and avoid copy. std::string str2 = str.str(); StringSource source { str2 }; diff --git a/src/nix-instantiate/nix-instantiate.cc b/src/nix-instantiate/nix-instantiate.cc index 9b36dccc69d9..87bc986e898b 100644 --- a/src/nix-instantiate/nix-instantiate.cc +++ b/src/nix-instantiate/nix-instantiate.cc @@ -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 seen; + printAmbiguous(vRes, state.symbols, std::cout, &seen, std::numeric_limits::max()); std::cout << std::endl; } } else { diff --git a/tests/functional/lang/eval-okay-print.exp b/tests/functional/lang/eval-okay-print.exp index aa1b2379ecec..0d960fb705b4 100644 --- a/tests/functional/lang/eval-okay-print.exp +++ b/tests/functional/lang/eval-okay-print.exp @@ -1 +1 @@ -[ null «primop toString» «partially applied primop deepSeq» «lambda @ /pwd/lang/eval-okay-print.nix:1:61» [ [ «repeated» ] ] ] +[ null [ [ «repeated» ] ] ]