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);