Add setting 'allow-dirty-locks'

This allows writing lock files with dirty inputs, so long as they have
a NAR hash. (Currently they always have a NAR hash, but with lazy
trees that may not always be the case.)

Generally dirty locks are bad for reproducibility (we can detect if
the dirty input has changed, but we have no way to fetch it except
substitution). Hence we don't allow them by default.

Fixes #11181.

Author: edolstra

src/libcmd/installables.cc

@@ -450,7 +450,7 @@ ref<eval_cache::EvalCache> openEvalCache(
     std::shared_ptr<flake::LockedFlake> lockedFlake)
 {
     auto fingerprint = evalSettings.useEvalCache && evalSettings.pureEval
-        ? lockedFlake->getFingerprint(state.store)
+        ? lockedFlake->getFingerprint(state.store, state.fetchSettings)
         : std::nullopt;
     auto rootLoader = [&state, lockedFlake]()
         {

src/libexpr/primops/fetchTree.cc

@@ -182,7 +182,7 @@ static void fetchTree(
     if (!state.settings.pureEval && !input.isDirect() && experimentalFeatureSettings.isEnabled(Xp::Flakes))
         input = lookupInRegistries(state.store, input).first;
 
-    if (state.settings.pureEval && !input.isLocked()) {
+    if (state.settings.pureEval && !input.isConsideredLocked(state.fetchSettings)) {
         auto fetcher = "fetchTree";
         if (params.isFetchGit)
             fetcher = "fetchGit";

src/libfetchers/fetch-settings.hh

@@ -70,6 +70,22 @@ struct Settings : public Config
     Setting<bool> warnDirty{this, true, "warn-dirty",
         "Whether to warn about dirty Git/Mercurial trees."};
 
+    Setting<bool> allowDirtyLocks{
+        this,
+        false,
+        "allow-dirty-locks",
+        R"(
+          Whether to allow dirty inputs (such as dirty Git workdirs)
+          to be locked via their NAR hash. This is generally bad
+          practice since Nix has no way to obtain such inputs if they
+          are subsequently modified. Therefore lock files with dirty
+          locks should generally only be used for local testing, and
+          should not be pushed to other users.
+        )",
+        {},
+        true,
+        Xp::Flakes};
+
     Setting<bool> trustTarballsFromGitForges{
         this, true, "trust-tarballs-from-git-forges",
         R"(

src/libfetchers/fetchers.cc

@@ -4,6 +4,7 @@
 #include "fetch-to-store.hh"
 #include "json-utils.hh"
 #include "store-path-accessor.hh"
+#include "fetch-settings.hh"
 
 #include <nlohmann/json.hpp>
 
@@ -154,6 +155,12 @@ bool Input::isLocked() const
     return scheme && scheme->isLocked(*this);
 }
 
+bool Input::isConsideredLocked(
+    const Settings & settings) const
+{
+    return isLocked() || (settings.allowDirtyLocks && getNarHash());
+}
+
 bool Input::isFinal() const
 {
     return maybeGetBoolAttr(attrs, "__final").value_or(false);

src/libfetchers/fetchers.hh

@@ -95,6 +95,15 @@ public:
      */
     bool isLocked() const;
 
+    /**
+     * Return whether the input is either locked, or, if
+     * `allow-dirty-locks` is enabled, it has a NAR hash. In the
+     * latter case, we can verify the input but we may not be able to
+     * fetch it from anywhere.
+     */
+    bool isConsideredLocked(
+        const Settings & settings) const;
+
     /**
      * Return whether this is a "final" input, meaning that fetching
      * it will not add, remove or change any attributes. (See

src/libflake/flake/flake.cc

@@ -345,6 +345,7 @@ Flake getFlake(EvalState & state, const FlakeRef & originalRef, bool allowLookup
 }
 
 static LockFile readLockFile(
+    const Settings & settings,
     const fetchers::Settings & fetchSettings,
     const SourcePath & lockFilePath)
 {
@@ -380,6 +381,7 @@ LockedFlake lockFlake(
         }
 
         auto oldLockFile = readLockFile(
+            settings,
             state.fetchSettings,
             lockFlags.referenceLockFilePath.value_or(
                 flake.lockFilePath()));
@@ -616,7 +618,7 @@ LockedFlake lockFlake(
                                 inputFlake.inputs, childNode, inputPath,
                                 oldLock
                                 ? std::dynamic_pointer_cast<const Node>(oldLock)
-                                : readLockFile(state.fetchSettings, inputFlake.lockFilePath()).root.get_ptr(),
+                                : readLockFile(settings, state.fetchSettings, inputFlake.lockFilePath()).root.get_ptr(),
                                 oldLock ? lockRootPath : inputPath,
                                 localPath,
                                 false);
@@ -678,9 +680,11 @@ LockedFlake lockFlake(
 
             if (lockFlags.writeLockFile) {
                 if (sourcePath || lockFlags.outputLockFilePath) {
-                    if (auto unlockedInput = newLockFile.isUnlocked()) {
+                    if (auto unlockedInput = newLockFile.isUnlocked(state.fetchSettings)) {
                         if (lockFlags.failOnUnlocked)
-                            throw Error("cannot write lock file of flake '%s' because it has an unlocked input ('%s').\n", topRef, *unlockedInput);
+                            throw Error(
+                                "Will not write lock file of flake '%s' because it has an unlocked input ('%s'). "
+                                "Use '--allow-dirty-locks' to allow this anyway.", topRef, *unlockedInput);
                         if (state.fetchSettings.warnDirty)
                             warn("will not write lock file of flake '%s' because it has an unlocked input ('%s')", topRef, *unlockedInput);
                     } else {
@@ -979,9 +983,11 @@ static RegisterPrimOp r4({
 
 }
 
-std::optional<Fingerprint> LockedFlake::getFingerprint(ref<Store> store) const
+std::optional<Fingerprint> LockedFlake::getFingerprint(
+    ref<Store> store,
+    const fetchers::Settings & fetchSettings) const
 {
-    if (lockFile.isUnlocked()) return std::nullopt;
+    if (lockFile.isUnlocked(fetchSettings)) return std::nullopt;
 
     auto fingerprint = flake.lockedRef.input.getFingerprint(store);
     if (!fingerprint) return std::nullopt;

src/libflake/flake/flake.hh

@@ -129,7 +129,9 @@ struct LockedFlake
      */
     std::map<ref<Node>, SourcePath> nodePaths;
 
-    std::optional<Fingerprint> getFingerprint(ref<Store> store) const;
+    std::optional<Fingerprint> getFingerprint(
+        ref<Store> store,
+        const fetchers::Settings & fetchSettings) const;
 };
 
 struct LockFlags

src/libflake/flake/lockfile.cc

@@ -10,6 +10,7 @@
 #include <nlohmann/json.hpp>
 
 #include "strings.hh"
+#include "flake/settings.hh"
 
 namespace nix::flake {
 
@@ -43,8 +44,8 @@ LockedNode::LockedNode(
     , originalRef(getFlakeRef(fetchSettings, json, "original", nullptr))
     , isFlake(json.find("flake") != json.end() ? (bool) json["flake"] : true)
 {
-    if (!lockedRef.input.isLocked())
-        throw Error("lock file contains unlocked input '%s'",
+    if (!lockedRef.input.isConsideredLocked(fetchSettings))
+        throw Error("Lock file contains unlocked input '%s'. Use '--allow-dirty-locks' to accept this lock file.",
             fetchers::attrsToJSON(lockedRef.input.toAttrs()));
 
     // For backward compatibility, lock file entries are implicitly final.
@@ -228,7 +229,7 @@ std::ostream & operator <<(std::ostream & stream, const LockFile & lockFile)
     return stream;
 }
 
-std::optional<FlakeRef> LockFile::isUnlocked() const
+std::optional<FlakeRef> LockFile::isUnlocked(const fetchers::Settings & fetchSettings) const
 {
     std::set<ref<const Node>> nodes;
 
@@ -247,7 +248,7 @@ std::optional<FlakeRef> LockFile::isUnlocked() const
     for (auto & i : nodes) {
         if (i == ref<const Node>(root)) continue;
         auto node = i.dynamic_pointer_cast<const LockedNode>();
-        if (node && (!node->lockedRef.input.isLocked() || !node->lockedRef.input.isFinal()))
+        if (node && (!node->lockedRef.input.isConsideredLocked(fetchSettings) || !node->lockedRef.input.isFinal()))
             return node->lockedRef;
     }
 

src/libflake/flake/lockfile.hh

@@ -71,7 +71,7 @@ struct LockFile
      * Check whether this lock file has any unlocked or non-final
      * inputs. If so, return one.
      */
-    std::optional<FlakeRef> isUnlocked() const;
+    std::optional<FlakeRef> isUnlocked(const fetchers::Settings & fetchSettings) const;
 
     bool operator ==(const LockFile & other) const;
 

src/libflake/flake/settings.hh

@@ -29,7 +29,7 @@ struct Settings : public Config
         this,
         false,
         "accept-flake-config",
-        "Whether to accept nix configuration from a flake without prompting.",
+        "Whether to accept Nix configuration settings from a flake without prompting.",
         {},
         true,
         Xp::Flakes};