Implement --delete-generations + flag for keeping last N number of generations

doc/manual/command-ref/nix-env.xml

@@ -1346,9 +1346,10 @@ $ nix-env --list-generations
 <para>This operation deletes the specified generations of the current
 profile.  The generations can be a list of generation numbers, the
 special value <literal>old</literal> to delete all non-current
-generations, or a value such as <literal>30d</literal> to delete all
+generations,  a value such as <literal>30d</literal> to delete all
 generations older than the specified number of days (except for the
-generation that was active at that point in time).
+generation that was active at that point in time), or a value such as.
+<literal>+5</literal> to delete all but the number of items specified.
 Periodically deleting old generations is important to make garbage
 collection effective.</para>
 
@@ -1359,6 +1360,8 @@ collection effective.</para>
 <screen>
 $ nix-env --delete-generations 3 4 8
 
+$ nix-env --delete-generations +5
+
 $ nix-env --delete-generations 30d
 
 $ nix-env -p other_profile --delete-generations old</screen>

src/libstore/profiles.cc

@@ -157,6 +157,29 @@ void deleteGenerations(const Path & profile, const std::set<unsigned int> & gens
     }
 }
 
+void deleteGenerationsGreaterThan(const Path & profile, const string & max, bool dryRun)
+{
+    int max_keep = 0;
+    PathLocks lock;
+    if(max.size() < 2)
+     throw Error(format("invalid number of generations ‘%1%’") % max);
+    string str_max = string(max, 1, max.size());
+    if (!string2Int(str_max, max_keep) || max_keep == 0)
+      throw Error(format("invalid number of generations to keep ‘%1%’") % max);
+
+    lockProfile(lock, profile);
+
+    int curGen;
+    Generations gens = findGenerations(profile, curGen);
+
+    for (auto i = gens.rbegin(); i != gens.rend(); ++i) {
+      if (max_keep) {
+	max_keep--;
+	continue;
+      }
+      deleteGeneration2(profile, i->number, dryRun);
+    }
+}
 
 void deleteOldGenerations(const Path & profile, bool dryRun)
 {

src/libstore/profiles.hh

@@ -39,6 +39,8 @@ void deleteGeneration(const Path & profile, unsigned int gen);
 
 void deleteGenerations(const Path & profile, const std::set<unsigned int> & gensToDelete, bool dryRun);
 
+void deleteGenerationsGreaterThan(const Path & profile, const string & max, bool dryRun);
+
 void deleteOldGenerations(const Path & profile, bool dryRun);
 
 void deleteGenerationsOlderThan(const Path & profile, time_t t, bool dryRun);

src/nix-env/nix-env.cc

@@ -1284,6 +1284,8 @@ static void opDeleteGenerations(Globals & globals, Strings opFlags, Strings opAr
         deleteOldGenerations(globals.profile, globals.dryRun);
     } else if (opArgs.size() == 1 && opArgs.front().find('d') != string::npos) {
         deleteGenerationsOlderThan(globals.profile, opArgs.front(), globals.dryRun);
+    } else if (opArgs.size() == 1 && opArgs.front().find('+') != string::npos) {
+        deleteGenerationsGreaterThan(globals.profile, opArgs.front(), globals.dryRun);
     } else {
         std::set<unsigned int> gens;
         for (auto & i : opArgs) {