libexpr/flake: user friendly error if parsing flake.lock fails

CHERRY-PICK: repeated bug reports

Fix: https://git.lix.systems/lix-project/lix/issues/559
Change-Id: I75f56c801d7f16a2e1ef8e702f16f1ef91f7b01f
(cherry picked from commit 68373f8664)

lix/libexpr/flake/lockfile.cc

@@ -1,5 +1,7 @@
 #include "lix/libexpr/flake/lockfile.hh"
+#include "json-to-value.hh"
 #include "lix/libstore/store-api.hh"
+#include "lix/libutil/error.hh"
 #include "lix/libutil/url-parts.hh"
 
 #include <iomanip>
@@ -186,8 +188,16 @@ std::string LockFile::to_string() const
 
 LockFile LockFile::read(const Path & path)
 {
-    if (!pathExists(path)) return LockFile();
-    return LockFile(nlohmann::json::parse(readFile(path)), path);
+    if (!pathExists(path)) {
+        return LockFile();
+    }
+    try {
+        return LockFile(nlohmann::json::parse(readFile(path)), path);
+    } catch (nlohmann::json::parse_error &nlohmann_json_parse_exc) {
+       auto json_parse_error = JSONParseError(nlohmann_json_parse_exc.what());
+       json_parse_error.addTrace(nullptr, "while parsing the lock file at %s", path);
+       throw json_parse_error;
+    }
 }
 
 std::ostream & operator <<(std::ostream & stream, const LockFile & lockFile)

tests/functional2/flakes/test_invalid_flake_lock.py

@@ -0,0 +1,30 @@
+from pathlib import Path
+from textwrap import dedent
+from functional2.testlib.fixtures import Nix
+import re
+
+def test_invalid_flake_lock(nix: Nix, tmp_path: Path):
+    flake_dir = tmp_path / 'flake'
+    flake_dir.mkdir()
+
+    (flake_dir / 'flake.nix').write_text(dedent("""
+        {
+            inputs = {};
+            outputs = inputs: {};
+        }
+    """))
+    (flake_dir / 'flake.lock').write_text(dedent("""
+        {
+            this flake.lock is obviously invalid
+        }
+    """))
+
+    cmd = nix.nix(["build"], flake=True)
+    cmd.cwd = flake_dir
+    res = cmd.run().expect(1)
+    print(res.stderr_plain)
+
+    ERROR_RE1 = re.compile(fr"while updating the lock file of flake 'path:{flake_dir}.+'")
+    ERROR_RE2 = re.compile(fr"while parsing the lock file at .+")
+    assert ERROR_RE1.search(res.stderr_plain)
+    assert ERROR_RE2.search(res.stderr_plain)