Clean up the lock file handling flags

Added a flag --no-update-lock-file to barf if the lock file needs any
changes. This is useful for CI systems if you're building a
checkout. Fixes #2947.

Renamed --no-save-lock-file to --no-write-lock-file. It is now a fatal
error if the lock file needs changes but --no-write-lock-file is not
given.

7 files changed, 78 insertions, 75 deletions

diff --git a/flake.lock b/flake.lock
index 75c78d102..46e6c9dc5 100644
--- a/flake.lock
+++ b/flake.lock
@@ -2,9 +2,9 @@
     "inputs": {
         "nixpkgs": {
             "inputs": {},
-            "narHash": "sha256-HkMF+s/yqNOOxqZGp+rscaC8LPtOGc50nEAjLFsnJpg=",
+            "narHash": "sha256-V4jz8Hbt+mZkXhH+3KmUQcRGETOFd8mVPhgQlS4Lu5E=",
             "originalUrl": "flake:nixpkgs/release-19.09",
-            "url": "github:edolstra/nixpkgs/e7223c602152ee4544b05157fc9d88a3feed22c2"
+            "url": "github:edolstra/nixpkgs/dd45a16733f4469a0dded6ad0bf9a662ea39bdea"
         }
     },
     "version": 3
diff --git a/src/libexpr/flake/flake.cc b/src/libexpr/flake/flake.cc
index 2982f7718..adcb88dcc 100644
--- a/src/libexpr/flake/flake.cc
+++ b/src/libexpr/flake/flake.cc
@@ -251,17 +251,6 @@ static std::pair<fetchers::Tree, FlakeRef> getNonFlake(
     return std::make_pair(std::move(sourceInfo), resolvedRef);
 }
 
-bool allowedToUseRegistries(LockFileMode handle, bool isTopRef)
-{
-    if (handle == AllPure) return false;
-    else if (handle == TopRefUsesRegistries) return isTopRef;
-    else if (handle == UpdateLockFile) return true;
-    else if (handle == UseUpdatedLockFile) return true;
-    else if (handle == RecreateLockFile) return true;
-    else if (handle == UseNewLockFile) return true;
-    else assert(false);
-}
-
 static void flattenLockFile(
     const LockedInputs & inputs,
     const InputPath & prefix,
@@ -311,20 +300,17 @@ static std::string diffLockFiles(const LockedInputs & oldLocks, const LockedInpu
 LockedFlake lockFlake(
     EvalState & state,
     const FlakeRef & topRef,
-    LockFileMode lockFileMode,
     const LockFlags & lockFlags)
 {
     settings.requireExperimentalFeature("flakes");
 
     RefMap refMap;
 
-    auto flake = getFlake(state, topRef,
-        allowedToUseRegistries(lockFileMode, true), refMap);
+    auto flake = getFlake(state, topRef, lockFlags.useRegistries, refMap);
 
     LockFile oldLockFile;
 
-    if (lockFileMode != RecreateLockFile && lockFileMode != UseNewLockFile) {
-        // If recreateLockFile, start with an empty lockfile
+    if (!lockFlags.recreateLockFile) {
         // FIXME: symlink attack
         oldLockFile = LockFile::read(
             flake.sourceInfo->actualPath + "/" + flake.resolvedRef.subdir + "/flake.lock");
@@ -436,12 +422,13 @@ LockedFlake lockFlake(
                 } else {
                     /* We need to update/create a new lock file
                        entry. So fetch the flake/non-flake. */
-                    if (lockFileMode == AllPure || lockFileMode == TopRefUsesRegistries)
+
+                    if (!lockFlags.allowMutable && !input.ref.isImmutable())
                         throw Error("cannot update flake input '%s' in pure mode", inputPathS);
 
                     if (input.isFlake) {
                         auto inputFlake = getFlake(state, input.ref,
-                            allowedToUseRegistries(lockFileMode, false), refMap);
+                            lockFlags.useRegistries, refMap);
 
                         newLocks.inputs.insert_or_assign(id,
                             LockedInput(inputFlake.resolvedRef, inputFlake.originalRef, inputFlake.sourceInfo->narHash));
@@ -461,7 +448,7 @@ LockedFlake lockFlake(
 
                     else {
                         auto [sourceInfo, resolvedRef] = getNonFlake(state, input.ref,
-                            allowedToUseRegistries(lockFileMode, false), refMap);
+                            lockFlags.useRegistries, refMap);
                         newLocks.inputs.insert_or_assign(id,
                             LockedInput(resolvedRef, input.ref, sourceInfo.narHash));
                     }
@@ -494,12 +481,15 @@ LockedFlake lockFlake(
         if (!(oldLockFile == LockFile()))
             printInfo("inputs of flake '%s' changed:\n%s", topRef, chomp(diffLockFiles(oldLockFile, newLockFile)));
 
-        if (lockFileMode == UpdateLockFile || lockFileMode == RecreateLockFile) {
+        if (lockFlags.writeLockFile) {
             if (auto sourcePath = topRef.input->getSourcePath()) {
                 if (!newLockFile.isImmutable()) {
                     if (settings.warnDirty)
                         warn("will not write lock file of flake '%s' because it has a mutable input", topRef);
                 } else {
+                    if (!lockFlags.updateLockFile)
+                        throw Error("flake '%s' requires lock file changes but they're not allowed due to '--no-update-lock-file'", topRef);
+
                     auto path = *sourcePath + (topRef.subdir == "" ? "" : "/" + topRef.subdir) + "/flake.lock";
 
                     if (pathExists(path))
@@ -522,9 +512,9 @@ LockedFlake lockFlake(
                     #endif
                 }
             } else
-                warn("cannot write lock file of remote flake '%s'", topRef);
-        } else if (lockFileMode != AllPure && lockFileMode != TopRefUsesRegistries)
-            warn("using updated lock file without writing it to file");
+                throw Error("cannot write modified lock file of flake '%s' (use '--no-write-lock-file' to ignore)", topRef);
+        } else
+            warn("not writing modified lock file of flake '%s'", topRef);
     }
 
     return LockedFlake { .flake = std::move(flake), .lockFile = std::move(newLockFile) };
@@ -655,9 +645,14 @@ void callFlake(EvalState & state,
 // This function is exposed to be used in nix files.
 static void prim_getFlake(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    LockFlags lockFlags;
-    callFlake(state, lockFlake(state, parseFlakeRef(state.forceStringNoCtx(*args[0], pos)),
-            evalSettings.pureEval ? AllPure : UseUpdatedLockFile, lockFlags), v);
+    callFlake(state,
+        lockFlake(state, parseFlakeRef(state.forceStringNoCtx(*args[0], pos)),
+            LockFlags {
+                .updateLockFile = false,
+                .useRegistries = !evalSettings.pureEval,
+                .allowMutable  = !evalSettings.pureEval,
+            }),
+        v);
 }
 
 static RegisterPrimOp r2("getFlake", 1, prim_getFlake);
diff --git a/src/libexpr/flake/flake.hh b/src/libexpr/flake/flake.hh
index eb013c4ba..355e22adc 100644
--- a/src/libexpr/flake/flake.hh
+++ b/src/libexpr/flake/flake.hh
@@ -13,15 +13,6 @@ namespace fetchers { struct Tree; }
 
 namespace flake {
 
-enum LockFileMode : unsigned int
-    { AllPure // Everything is handled 100% purely
-    , TopRefUsesRegistries // The top FlakeRef uses the registries, apart from that, everything happens 100% purely
-    , UpdateLockFile // Update the existing lockfile and write it to file
-    , UseUpdatedLockFile // `UpdateLockFile` without writing to file
-    , RecreateLockFile // Recreate the lockfile from scratch and write it to file
-    , UseNewLockFile // `RecreateLockFile` without writing to file
-    };
-
 struct FlakeInput;
 
 typedef std::map<FlakeId, FlakeInput> FlakeInputs;
@@ -61,21 +52,48 @@ struct LockedFlake
 
 struct LockFlags
 {
+    /* Whether to ignore the existing lock file, creating a new one
+       from scratch. */
+    bool recreateLockFile = false;
+
+    /* Whether to update the lock file at all. If set to false, if any
+       change to the lock file is needed (e.g. when an input has been
+       added to flake.nix), you get a fatal error. */
+    bool updateLockFile = true;
+
+    /* Whether to write the lock file to disk. If set to true, if the
+       any changes to the lock file are needed and the flake is not
+       writable (i.e. is not a local Git working tree or similar), you
+       get a fatal error. If set to false, Nix will use the modified
+       lock file in memory only, without writing it to disk. */
+    bool writeLockFile = true;
+
+    /* Whether to use the registries to lookup indirect flake
+       references like 'nixpkgs'. */
+    bool useRegistries = true;
+
+    /* Whether mutable flake references (i.e. those without a Git
+       revision or similar) without a corresponding lock are
+       allowed. Mutable flake references with a lock are always
+       allowed. */
+    bool allowMutable = true;
+
     std::map<InputPath, FlakeRef> inputOverrides;
 };
 
 LockedFlake lockFlake(
-    EvalState &,
-    const FlakeRef &,
-    LockFileMode,
-    const LockFlags &);
+    EvalState & state,
+    const FlakeRef & flakeRef,
+    const LockFlags & lockFlags);
 
-void callFlake(EvalState & state,
+void callFlake(
+    EvalState & state,
     const Flake & flake,
     const LockedInputs & inputs,
     Value & v);
 
-void callFlake(EvalState & state,
+void callFlake(
+    EvalState & state,
     const LockedFlake & resFlake,
     Value & v);
 
diff --git a/src/nix/command.hh b/src/nix/command.hh
index eb44caf05..305ce5588 100644
--- a/src/nix/command.hh
+++ b/src/nix/command.hh
@@ -16,10 +16,6 @@ class EvalState;
 struct Pos;
 class Store;
 
-namespace flake {
-enum LockFileMode : unsigned int;
-}
-
 /* A command that requires a Nix store. */
 struct StoreCommand : virtual Command
 {
@@ -42,15 +38,9 @@ struct EvalCommand : virtual StoreCommand, MixEvalArgs
 
 struct MixFlakeOptions : virtual Args
 {
-    bool recreateLockFile = false;
-    bool saveLockFile = true;
-    bool useRegistries = true;
-
     flake::LockFlags lockFlags;
 
     MixFlakeOptions();
-
-    flake::LockFileMode getLockFileMode();
 };
 
 struct SourceExprCommand : virtual Args, EvalCommand, MixFlakeOptions
diff --git a/src/nix/flake.cc b/src/nix/flake.cc
index 2852a627d..49c0d30f0 100644
--- a/src/nix/flake.cc
+++ b/src/nix/flake.cc
@@ -38,12 +38,12 @@ public:
     Flake getFlake()
     {
         auto evalState = getEvalState();
-        return flake::getFlake(*evalState, getFlakeRef(), useRegistries);
+        return flake::getFlake(*evalState, getFlakeRef(), lockFlags.useRegistries);
     }
 
     LockedFlake lockFlake()
     {
-        return flake::lockFlake(*getEvalState(), getFlakeRef(), getLockFileMode(), lockFlags);
+        return flake::lockFlake(*getEvalState(), getFlakeRef(), lockFlags);
     }
 };
 
diff --git a/src/nix/installables.cc b/src/nix/installables.cc
index 7d59a25ee..24eb739f5 100644
--- a/src/nix/installables.cc
+++ b/src/nix/installables.cc
@@ -22,17 +22,22 @@ MixFlakeOptions::MixFlakeOptions()
     mkFlag()
         .longName("recreate-lock-file")
         .description("recreate lock file from scratch")
-        .set(&recreateLockFile, true);
+        .set(&lockFlags.recreateLockFile, true);
 
     mkFlag()
-        .longName("no-save-lock-file")
-        .description("do not save the newly generated lock file")
-        .set(&saveLockFile, false);
+        .longName("no-update-lock-file")
+        .description("do not allow any updates to the lock file")
+        .set(&lockFlags.updateLockFile, false);
+
+    mkFlag()
+        .longName("no-write-lock-file")
+        .description("do not write the newly generated lock file")
+        .set(&lockFlags.writeLockFile, false);
 
     mkFlag()
         .longName("no-registries")
         .description("don't use flake registries")
-        .set(&useRegistries, false);
+        .set(&lockFlags.useRegistries, false);
 
     mkFlag()
         .longName("override-input")
@@ -46,17 +51,6 @@ MixFlakeOptions::MixFlakeOptions()
         });
 }
 
-flake::LockFileMode MixFlakeOptions::getLockFileMode()
-{
-    using namespace flake;
-    return
-        useRegistries
-        ? recreateLockFile
-          ? (saveLockFile ? RecreateLockFile : UseNewLockFile)
-          : (saveLockFile ? UpdateLockFile : UseUpdatedLockFile)
-        : AllPure;
-}
-
 SourceExprCommand::SourceExprCommand()
 {
     mkFlag()
@@ -332,7 +326,7 @@ std::tuple<std::string, FlakeRef, flake::EvalCache::Derivation> InstallableFlake
 {
     auto state = cmd.getEvalState();
 
-    auto lockedFlake = lockFlake(*state, flakeRef, cmd.getLockFileMode(), cmd.lockFlags);
+    auto lockedFlake = lockFlake(*state, flakeRef, cmd.lockFlags);
 
     Value * vOutputs = nullptr;
 
@@ -386,7 +380,7 @@ std::vector<flake::EvalCache::Derivation> InstallableFlake::toDerivations()
 
 Value * InstallableFlake::toValue(EvalState & state)
 {
-    auto lockedFlake = lockFlake(state, flakeRef, cmd.getLockFileMode(), cmd.lockFlags);
+    auto lockedFlake = lockFlake(state, flakeRef, cmd.lockFlags);
 
     auto vOutputs = getFlakeOutputs(state, lockedFlake);
 
diff --git a/tests/flakes.sh b/tests/flakes.sh
index 1b1912129..3eae73cdf 100644
--- a/tests/flakes.sh
+++ b/tests/flakes.sh
@@ -156,9 +156,15 @@ nix path-info $flake1Dir/result
 (! nix eval --expr "builtins.getFlake \"$flake2Dir\"")
 
 # But should succeed in impure mode.
-nix build -o $TEST_ROOT/result flake2#bar --impure
+(! nix build -o $TEST_ROOT/result flake2#bar --impure)
+nix build -o $TEST_ROOT/result flake2#bar --impure --no-write-lock-file
 
-# Test automatic lock file generation.
+# Building a local flake with an unlocked dependency should fail with --no-update-lock-file.
+nix build -o $TEST_ROOT/result $flake2Dir#bar --no-update-lock-file 2>&1 | grep 'requires lock file changes'
+
+# But it should succeed without that flag.
+nix build -o $TEST_ROOT/result $flake2Dir#bar --no-write-lock-file
+nix build -o $TEST_ROOT/result $flake2Dir#bar --no-update-lock-file 2>&1 | grep 'requires lock file changes'
 nix build -o $TEST_ROOT/result $flake2Dir#bar
 [[ -e $flake2Dir/flake.lock ]]
 git -C $flake2Dir add flake.lock