Fix compatibility with nix-2.3 not knowing --extra-experimental-features.

CHANGELOG.md

@@ -2,6 +2,9 @@
 
 ## 0.2.3 (unreleased)
 
+- Fix compatibility with nix-2.3 where `nix --extra-experimental-features` isn't
+  a known flag yet.  We have to switch on the version of Nix we've been given.
+
 ## 0.2.2 (2023-05-22)
 
 - Fixed crash when `nix-store --query --references` returns nothing (e.g. for a

src/nvd

@@ -53,6 +53,7 @@ signal(SIGPIPE, SIG_DFL)  # Python handles SIGPIPE improperly by default.
 NVD_VERSION = "0.2.3"
 
 NIX_BIN_DIR = None
+USE_NIX_COMMAND_FEATURE = False  # Whether to explicitly enable the nix-command feature.
 USE_COLOUR = False
 
 SGR_RESET = 0
@@ -440,11 +441,9 @@ def query_closure_disk_usage_bytes(target: Path) -> Optional[int]:
 
     try:
         stdout = subprocess.run(
-                [
-                    make_nix_bin_path("nix"), "path-info",
-                    "--extra-experimental-features", "nix-command",
-                    "--closure-size", target_str,
-                ],
+            [make_nix_bin_path("nix")]
+            + make_extra_experimental_features_args()
+            + ["path-info", "--closure-size", target_str],
             stdout=PIPE,
             text=True
         ).stdout
@@ -491,6 +490,32 @@ def make_nix_bin_path(exe_name: str) -> str:
     else:
         return exe_name
 
+def make_extra_experimental_features_args() -> List[str]:
+    global USE_NIX_COMMAND_FEATURE
+    if USE_NIX_COMMAND_FEATURE:
+        return ["--extra-experimental-features", "nix-command"]
+    else:
+        return []
+
+def query_nix_version() -> Version:
+    output = subprocess.run(
+        [make_nix_bin_path("nix-build"), "--version"],
+        stdout=PIPE,
+        text=True,
+        check=True,
+    ).stdout
+
+    first_line = output.split("\n", 1)[0]
+
+    words = first_line.split(" ")
+    regex = re.compile(r"^[0-9]+\.[0-9]")
+    for word in words:
+        if regex.search(word):
+            return Version(word)
+
+    raise RuntimeError(
+        "Could not determine Nix version from 'nix-store --version' output: {output!r}")
+
 def run_list(*, root, only_selected, name_patterns):
     path = Path(root)
 
@@ -722,6 +747,7 @@ def parse_args():
 
 def main():
     global NIX_BIN_DIR
+    global USE_NIX_COMMAND_FEATURE
     global USE_COLOUR
     global INST_ADDED
     global INST_REMOVED
@@ -748,6 +774,11 @@ def main():
         print("nvd: Subcommand required, see 'nvd --help'.")
         sys.exit(1)
 
+    # Nix 2.4 and later need "experimental-features: nix-command" in order to
+    # use the "nix ..." style commands.
+    if Version("2.4") < query_nix_version():
+        USE_NIX_COMMAND_FEATURE = True
+
     {
         "list": run_list,
         "diff": run_diff,