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Merge pull request #9747 from awakesecurity/mz/fix-quadratic-splitString

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Fix performance of builtins.substring for empty substrings

(cherry picked from commit b2deff1947c2fe57fdbf1a472eb9003eb407f8d3)
Change-Id: I4ddfc8d26a4781c9520fff9807849a073ee7bed8
This commit is contained in:
eldritch horrors 2024-03-04 08:06:36 +01:00
parent 64b077cdaa
commit 2c85fcce87
3 changed files with 28 additions and 3 deletions
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19
src/libexpr/primops.cc
View file

@ -3700,9 +3700,6 @@ static RegisterPrimOp primop_toString({
static void prim_substring(EvalState & state, const PosIdx pos, Value * * args, Value & v)
{
int start = state.forceInt(*args[0], pos, "while evaluating the first argument (the start offset) passed to builtins.substring");
int len = state.forceInt(*args[1], pos, "while evaluating the second argument (the substring length) passed to builtins.substring");
NixStringContext context;
auto s = state.coerceToString(pos, *args[2], context, "while evaluating the third argument (the string) passed to builtins.substring");
if (start < 0)
state.debugThrowLastTrace(EvalError({
@ -3710,6 +3707,22 @@ static void prim_substring(EvalState & state, const PosIdx pos, Value * * args,
.errPos = state.positions[pos]
}));
int len = state.forceInt(*args[1], pos, "while evaluating the second argument (the substring length) passed to builtins.substring");
// Special-case on empty substring to avoid O(n) strlen
// This allows for the use of empty substrings to efficently capture string context
if (len == 0) {
state.forceValue(*args[2], pos);
if (args[2]->type() == nString) {
v.mkString("", args[2]->string.context);
return;
}
}
NixStringContext context;
auto s = state.coerceToString(pos, *args[2], context, "while evaluating the third argument (the string) passed to builtins.substring");
v.mkString((unsigned int) start >= s->size() ? "" : s->substr(start, len), context);
}

1
tests/functional/lang/eval-okay-substring-context.exp Normal file
View file

@ -0,0 +1 @@
"okay"

11
tests/functional/lang/eval-okay-substring-context.nix Normal file
View file

@ -0,0 +1,11 @@
with builtins;
let
s = "${builtins.derivation { name = "test"; builder = "/bin/sh"; system = "x86_64-linux"; }}";
in
if getContext s == getContext "${substring 0 0 s + unsafeDiscardStringContext s}"
then "okay"
else throw "empty substring should preserve context"