Add "nix profile rollback" command

src/libstore/profiles.cc

@@ -236,6 +236,37 @@ void switchLink(Path link, Path target)
 }
 
 
+void switchGeneration(
+    const Path & profile,
+    std::optional<GenerationNumber> dstGen,
+    bool dryRun)
+{
+    PathLocks lock;
+    lockProfile(lock, profile);
+
+    auto [gens, curGen] = findGenerations(profile);
+
+    std::optional<Generation> dst;
+    for (auto & i : gens)
+        if ((!dstGen && i.number < curGen) ||
+            (dstGen && i.number == *dstGen))
+            dst = i;
+
+    if (!dst) {
+        if (dstGen)
+            throw Error("generation %1% does not exist", *dstGen);
+        else
+            throw Error("no generation older than the current (%1%) exists", curGen.value_or(0));
+    }
+
+    notice("switching from generation %d to %d", curGen.value_or(0), dst->number);
+
+    if (dryRun) return;
+
+    switchLink(profile, dst->path);
+}
+
+
 void lockProfile(PathLocks & lock, const Path & profile)
 {
     lock.lockPaths({profile}, (format("waiting for lock on profile '%1%'") % profile).str());

src/libstore/profiles.hh

@@ -11,7 +11,7 @@ namespace nix {
 class StorePath;
 
 
-typedef unsigned int GenerationNumber;
+typedef uint64_t GenerationNumber;
 
 struct Generation
 {
@@ -46,6 +46,13 @@ void deleteGenerationsOlderThan(const Path & profile, const string & timeSpec, b
 
 void switchLink(Path link, Path target);
 
+/* Roll back a profile to the specified generation, or to the most
+   recent one older than the current. */
+void switchGeneration(
+    const Path & profile,
+    std::optional<GenerationNumber> dstGen,
+    bool dryRun);
+
 /* Ensure exclusive access to a profile.  Any command that modifies
    the profile first acquires this lock. */
 void lockProfile(PathLocks & lock, const Path & profile);

src/libutil/args.hh

@@ -91,6 +91,14 @@ protected:
               })
             , arity(1)
         { }
+
+        template<class I>
+        Handler(std::optional<I> * dest)
+            : fun([=](std::vector<std::string> ss) {
+                *dest = string2IntWithUnitPrefix<I>(ss[0]);
+            })
+            , arity(1)
+        { }
     };
 
     /* Options. */

src/nix-env/nix-env.cc

@@ -1204,37 +1204,6 @@ static void opSwitchProfile(Globals & globals, Strings opFlags, Strings opArgs)
 }
 
 
-static constexpr GenerationNumber prevGen = std::numeric_limits<GenerationNumber>::max();
-
-
-static void switchGeneration(Globals & globals, GenerationNumber dstGen)
-{
-    PathLocks lock;
-    lockProfile(lock, globals.profile);
-
-    auto [gens, curGen] = findGenerations(globals.profile);
-
-    std::optional<Generation> dst;
-    for (auto & i : gens)
-        if ((dstGen == prevGen && i.number < curGen) ||
-            (dstGen >= 0 && i.number == dstGen))
-            dst = i;
-
-    if (!dst) {
-        if (dstGen == prevGen)
-            throw Error("no generation older than the current (%1%) exists", curGen.value_or(0));
-        else
-            throw Error("generation %1% does not exist", dstGen);
-    }
-
-    printInfo("switching from generation %1% to %2%", curGen.value_or(0), dst->number);
-
-    if (globals.dryRun) return;
-
-    switchLink(globals.profile, dst->path);
-}
-
-
 static void opSwitchGeneration(Globals & globals, Strings opFlags, Strings opArgs)
 {
     if (opFlags.size() > 0)
@@ -1243,7 +1212,7 @@ static void opSwitchGeneration(Globals & globals, Strings opFlags, Strings opArg
         throw UsageError("exactly one argument expected");
 
     if (auto dstGen = string2Int<GenerationNumber>(opArgs.front()))
-        switchGeneration(globals, *dstGen);
+        switchGeneration(globals.profile, *dstGen, globals.dryRun);
     else
         throw UsageError("expected a generation number");
 }
@@ -1256,7 +1225,7 @@ static void opRollback(Globals & globals, Strings opFlags, Strings opArgs)
     if (opArgs.size() != 0)
         throw UsageError("no arguments expected");
 
-    switchGeneration(globals, prevGen);
+    switchGeneration(globals.profile, {}, globals.dryRun);
 }
 
 

src/nix/profile-rollback.md

@@ -0,0 +1,26 @@
+R""(
+
+# Examples
+
+* Roll back your default profile to the previous version:
+
+  ```console
+  # nix profile rollback
+  switching from generation 519 to 518
+  ```
+
+* Switch your default profile to version 510:
+
+  ```console
+  # nix profile rollback --to 510
+  switching from generation 518 to 510
+  ```
+
+# Description
+
+This command switches a profile to the most recent version older
+than the currently active version, or if `--to` *N* is given, to
+version *N* of the profile. To see the available versions of a
+profile, use `nix profile history`.
+
+)""

src/nix/profile.cc

@@ -543,6 +543,38 @@ struct CmdProfileHistory : virtual StoreCommand, EvalCommand, MixDefaultProfile
     }
 };
 
+struct CmdProfileRollback : virtual StoreCommand, MixDefaultProfile, MixDryRun
+{
+    std::optional<GenerationNumber> version;
+
+    CmdProfileRollback()
+    {
+        addFlag({
+            .longName = "to",
+            .description = "The profile version to roll back to.",
+            .labels = {"version"},
+            .handler = {&version},
+        });
+    }
+
+    std::string description() override
+    {
+        return "roll back to the previous version or a specified version of this profile";
+    }
+
+    std::string doc() override
+    {
+        return
+          #include "profile-rollback.md"
+          ;
+    }
+
+    void run(ref<Store> store) override
+    {
+        switchGeneration(*profile, version, dryRun);
+    }
+};
+
 struct CmdProfile : NixMultiCommand
 {
     CmdProfile()
@@ -553,6 +585,7 @@ struct CmdProfile : NixMultiCommand
               {"list", []() { return make_ref<CmdProfileList>(); }},
               {"diff-closures", []() { return make_ref<CmdProfileDiffClosures>(); }},
               {"history", []() { return make_ref<CmdProfileHistory>(); }},
+              {"rollback", []() { return make_ref<CmdProfileRollback>(); }},
           })
     { }