From c50a3a426fc5686e480bf75bc3417d3459e3f2ea Mon Sep 17 00:00:00 2001
From: eldritch horrors <pennae@lix.systems>
Date: Sun, 25 Jan 2026 17:29:32 +0100
Subject: [PATCH] libutil: add simple NUL escaping/unescaping functions

Change-Id: Ia4cc7b8f1058439f312066422eebfaff1c2c0c6c
---
 lix/libutil/strings.cc      | 23 +++++++++++++++++++++++
 lix/libutil/strings.hh      | 10 +++++++++-
 tests/unit/libutil/tests.cc | 17 +++++++++++++++++
 3 files changed, 49 insertions(+), 1 deletion(-)

diff --git a/lix/libutil/strings.cc b/lix/libutil/strings.cc
index 8ea3e5132..9eb2842b0 100644
--- a/lix/libutil/strings.cc
+++ b/lix/libutil/strings.cc
@@ -366,4 +366,27 @@ std::string showBytes(uint64_t bytes)
     return fmt("%.2f MiB", bytes / (1024.0 * 1024.0));
 }
 
+std::string escapeNul(const std::string & in)
+{
+    using std::operator""sv;
+    return replaceStrings(replaceStrings(std::move(in), R"(\)", R"(\\)"), "\0"sv, R"(\0)");
+}
+
+std::string unescapeNul(const std::string & in)
+{
+    std::string result;
+    result.reserve(in.size());
+    bool inSequence = false;
+    for (auto c : in) {
+        if (inSequence) {
+            result += c == '0' ? '\0' : c;
+            inSequence = false;
+        } else if (c == '\\') {
+            inSequence = true;
+        } else {
+            result += c;
+        }
+    }
+    return result;
+}
 }
diff --git a/lix/libutil/strings.hh b/lix/libutil/strings.hh
index bc3d9285b..03f32ce14 100644
--- a/lix/libutil/strings.hh
+++ b/lix/libutil/strings.hh
@@ -141,7 +141,15 @@ inline std::string rewriteStrings(std::string s, const StringMap & rewrites)
     return Rewriter(rewrites)(s);
 }
 
-
+/**
+ * Escape NUL bytes and `\` in a string by replacing them with `\0` and `\\` respectively.
+ */
+std::string escapeNul(const std::string & in);
+/**
+ * Undo replacements done by `escapeNul`. Other escape sequences (eg `\n`) are replaced by
+ * the second character in the sequence (eg `n`), `\` at the end of the string is dropped.
+ */
+std::string unescapeNul(const std::string & in);
 
 /**
  * Parse a string into an integer.
diff --git a/tests/unit/libutil/tests.cc b/tests/unit/libutil/tests.cc
index ff96b565b..24ddbb56f 100644
--- a/tests/unit/libutil/tests.cc
+++ b/tests/unit/libutil/tests.cc
@@ -817,4 +817,21 @@ namespace nix {
         ASSERT_TRUE(e.is<std::invalid_argument>());
         ASSERT_NE(e.as<std::invalid_argument>(), nullptr);
     }
+
+    TEST(NulEscaping, function)
+    {
+        for (int c1 = CHAR_MIN; c1 < CHAR_MAX; c1++) {
+            for (int c2 = CHAR_MIN; c2 < CHAR_MAX; c2++) {
+                auto sequence = std::format("{}{}", char(c1), char(c2));
+                for (auto str : {sequence, "a" + sequence + "b"}) {
+                    ASSERT_EQ(unescapeNul(escapeNul(str)), str);
+                }
+            }
+        }
+
+        // unknown escapes are character identity for simplicity of use
+        ASSERT_EQ(unescapeNul("\\a"), "a");
+        // escape of end-of-string is dropped silently
+        ASSERT_EQ(unescapeNul("a\\"), "a");
+    }
 }