libfetchers: don't treat empty/zero hashes as valid locks

treating a file with an empty or zero hash as locked is not helpful.
these are placeholdes for "hash is not known", thus treating them as
a valid lock makes them completely useless (and confusing to users).

fixes #1233

Change-Id: If42b47281e6973fc86662b973db69e26f6346f5a

doc/manual/rl-next/fetch-empty-hash.md

@@ -0,0 +1,10 @@
+---
+synopsis: "don't treat tarball fetches with empty or zero hash as locked"
+cls: []
+category: "Fixes"
+credits: [horrors]
+issues: [fj#1233]
+---
+
+Lix no longer treats tarball fetches with empty or zero hashes as locked.
+All such fetches are now also affected by `tarball-ttl` as a consequence.

lix/libexpr/primops/fetchTree.cc

@@ -308,10 +308,16 @@ static Value fetch(
     //       https://github.com/NixOS/nix/issues/4313
     auto storePath = unpack
         ? state.aio
-              .blockOn(fetchers::downloadTarball(state.ctx.store, *url, name, (bool) expectedHash))
+              .blockOn(
+                  fetchers::downloadTarball(
+                      state.ctx.store, *url, name, expectedHash != Hash(HashType::SHA256)
+                  )
+              )
               .tree.storePath
         : state.aio
-              .blockOn(fetchers::downloadFile(state.ctx.store, *url, name, (bool) expectedHash))
+              .blockOn(
+                  fetchers::downloadFile(state.ctx.store, *url, name, expectedHash != Hash(HashType::SHA256))
+              )
               .storePath;
 
     if (expectedHash) {

tests/functional2/eval/fetchers/test_tarball_ttl.py

@@ -0,0 +1,81 @@
+from pathlib import Path
+
+import pytest
+
+from testlib.fixtures.nix import Nix
+
+
+@pytest.mark.no_daemon
+@pytest.mark.parametrize("sha", ["", "sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA="])
+def test_locked_tarball_ttl(nix: Nix, sha: str):
+    files = nix.env.dirs.home
+
+    (files / "file").write_text("contents")
+    res = (
+        nix.nix(
+            [
+                "eval",
+                "--expr",
+                f'builtins.fetchurl {{ url = "file://{files}/file"; sha256 = "{sha}"; }}',
+            ],
+            flake=True,
+        )
+        .run()
+        .expect(102)
+    )
+    assert "hash mismatch" in res.stderr_plain
+    assert "sha256-0bKln76n4gB3r5+Rsn6V6GUGGycL4D/1Oas7c1h4gug=" in res.stderr_plain
+
+    # empty or zero hash should count as not locked and be refetched
+    (files / "file").write_text("other contents")
+    res = (
+        nix.nix(
+            [
+                "eval",
+                *["--tarball-ttl", "0"],
+                "--expr",
+                f'builtins.fetchurl {{ url = "file://{files}/file"; sha256 = "{sha}"; }}',
+            ],
+            flake=True,
+        )
+        .run()
+        .expect(102)
+    )
+    assert "hash mismatch" in res.stderr_plain
+    assert "sha256-Ql7LXhAAyVRbEF8VR8bW8D0c1LONlNRX7AMpB+i1EHk=" in res.stderr_plain
+
+    # hash-locked tarballs should *not* be refetched as long as their store path exists
+    (files / "file").write_text("different contents")
+    res = (
+        nix.nix(
+            [
+                "eval",
+                *["--tarball-ttl", "0"],
+                "--raw",
+                "--expr",
+                f'builtins.fetchurl {{ url = "file://{files}/file"; sha256 = "sha256-Ql7LXhAAyVRbEF8VR8bW8D0c1LONlNRX7AMpB+i1EHk="; }}',
+            ],
+            flake=True,
+        )
+        .run()
+        .ok()
+    )
+    path = Path(res.stdout_plain)
+    assert path.read_text() == "other contents"
+
+    # but store paths that went away are always refetched and checked
+    nix.nix(["store", "delete", path], flake=True).run().ok()
+    res = (
+        nix.nix(
+            [
+                "eval",
+                "--expr",
+                f'builtins.fetchurl {{ url = "file://{files}/file"; sha256 = "sha256-Ql7LXhAAyVRbEF8VR8bW8D0c1LONlNRX7AMpB+i1EHk="; }}',
+            ],
+            flake=True,
+        )
+        .run()
+        .expect(102)
+    )
+    assert "hash mismatch" in res.stderr_plain
+    assert "sha256-DKElpSdXX/9t9X/8f4IfbwQccBptZZqVy1/6JW6uFoY=" in res.stderr_plain