Implement GC minimum age gate

Use the recently added `lastUsageTime` store path attribute to allow users to constrain the GC to only deleting paths which have been last used n days ago.
This is useful for ensuring that e.g. periodic GCs aren't too aggressive.

Author: Popax21

doc/manual/rl-next/gc-age-gate.md

@@ -0,0 +1,22 @@
+---
+synopsis: "Store GCs can now be restricted with a minimum age"
+prs: [14725]
+issues: [7572]
+---
+
+Nix store GCs invoked using either `nix-collect-garbage` or `nix store gc` may
+now be restricted to only deleting paths older than a certain minimum age, i.e.
+whose most recent usage is more than *n* days in the past. "Usage" is
+intentionally defined ambiguously, however in general all operations which
+produce/require the presence of a given store path count as "usage".
+
+
+Example usage:
+
+```bash
+# Delete store paths older than 7 days
+nix store gc --older-than 7d
+
+# Alternatively:
+nix-collect-garbage --path-older-than 7d
+```

doc/manual/source/command-ref/nix-collect-garbage.md

@@ -4,7 +4,7 @@
 
 # Synopsis
 
-`nix-collect-garbage` [`--delete-old`] [`-d`] [`--delete-older-than` *period*] [`--max-freed` *bytes*] [`--dry-run`]
+`nix-collect-garbage` [`--delete-old`] [`-d`] [`--delete-older-than` *period*] [`--delete-paths-older-than` *period*] [`--max-freed` *bytes*] [`--dry-run`]
 
 # Description
 
@@ -62,7 +62,17 @@ These options are for deleting old [profiles] prior to deleting unreachable [sto
   This is the equivalent of invoking [`nix-env --delete-generations <period>`](@docroot@/command-ref/nix-env/delete-generations.md#generations-time) on each found profile.
   See the documentation of that command for additional information about the *period* argument.
 
-  - <span id="opt-max-freed">[`--max-freed`](#opt-max-freed)</span> *bytes*
+- <span id="opt-paths-delete-older-than">[`--delete-paths-older-than`](#opt-delete-paths-older-than)</span> *period*
+
+  Only delete store paths older than the specified amount, i.e. restrict the GC
+  to only deleting store paths whose most recent usage is more than *N* days in
+  the past. *period* is a value such as `30d`, which would mean 30 days.
+
+  "Usage" is intentionally defined ambiguously, however in general all
+  operations which produce/require the presence of a given store path count as
+  "usage".
+
+- <span id="opt-max-freed">[`--max-freed`](#opt-max-freed)</span> *bytes*
 
 <!-- duplication from https://github.com/NixOS/nix/blob/442a2623e48357ff72c77bb11cf2cf06d94d2f90/doc/manual/source/command-ref/nix-store/gc.md?plain=1#L39-L44 -->
 

src/libstore/gc.cc

@@ -742,6 +742,12 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
             if (options.action == GCOptions::gcDeleteSpecific && !options.pathsToDelete.count(*path))
                 return;
 
+            /* If this path is too young, bail out */
+            if (options.olderThan.has_value() && options.olderThan.value() < queryPathInfo(*path)->lastUsageTime) {
+                debug("not deleting '%s' because it's too young", printStorePath(*path));
+                return markAlive();
+            }
+
             {
                 auto hashPart = path->hashPart();
                 auto shared(_shared.lock());

src/libstore/include/nix/store/gc-store.hh

@@ -55,6 +55,11 @@ struct GCOptions
      * Stop after at least `maxFreed` bytes have been freed.
      */
     uint64_t maxFreed{std::numeric_limits<uint64_t>::max()};
+
+    /**
+     * Only delete paths older than a certain point in time.
+     */
+    std::optional<time_t> olderThan;
 };
 
 struct GCResults

src/nix/nix-collect-garbage/nix-collect-garbage.cc

@@ -78,6 +78,8 @@ static int main_nix_collect_garbage(int argc, char ** argv)
             else if (*arg == "--delete-older-than") {
                 removeOld = true;
                 deleteOlderThan = getArg(*arg, arg, end);
+            } else if (*arg == "--delete-paths-older-than") {
+                options.olderThan = parseOlderThanTimeSpec(getArg(*arg, arg, end));
             } else if (*arg == "--dry-run")
                 dryRun = true;
             else if (*arg == "--max-freed")

src/nix/store-gc.cc

@@ -4,12 +4,14 @@
 #include "nix/store/store-api.hh"
 #include "nix/store/store-cast.hh"
 #include "nix/store/gc-store.hh"
+#include "nix/store/profiles.hh"
 
 using namespace nix;
 
 struct CmdStoreGC : StoreCommand, MixDryRun
 {
     GCOptions options;
+    std::optional<std::string> olderThan;
 
     CmdStoreGC()
     {
@@ -19,6 +21,14 @@ struct CmdStoreGC : StoreCommand, MixDryRun
             .labels = {"n"},
             .handler = {&options.maxFreed},
         });
+        addFlag({
+            .longName = "older-than",
+            .description = "Only delete paths older than the specified age. *age* "
+                           "must be in the format *N*`d`, where *N* denotes a number "
+                           "of days.",
+            .labels = {"age"},
+            .handler = {&olderThan},
+        });
     }
 
     std::string description() override
@@ -38,6 +48,7 @@ struct CmdStoreGC : StoreCommand, MixDryRun
         auto & gcStore = require<GcStore>(*store);
 
         options.action = dryRun ? GCOptions::gcReturnDead : GCOptions::gcDeleteDead;
+        options.olderThan = olderThan.transform(parseOlderThanTimeSpec);
         GCResults results;
         PrintFreed freed(options.action == GCOptions::gcDeleteDead, results);
         gcStore.collectGarbage(options, results);

src/nix/store-gc.md

@@ -14,8 +14,19 @@ R""(
   # nix store gc --max 1G
   ```
 
+* Delete store paths older than 7 days:
+
+  ```console
+  # nix store gc --older-than 7d
+  ```
+
 # Description
 
 This command deletes unreachable paths in the Nix store.
 
+`--older-than Nd` restricts the GC to only deleting store paths whose most
+recent usage is more than *N* days in the past. "Usage" is intentionally defined
+ambiguously, however in general all operations which produce/require the
+presence of a given store path count as "usage". 
+
 )""