Factor out a `GcStore` interface

Starts progress on #5729.

The idea is that we should not have these default methods throwing
"unimplemented". This is a small step in that direction.

I kept `addTempRoot` because it is a no-op, rather than failure. Also,
as a practical matter, it is called all over the place, while doing
other tasks, so the downcasting would be annoying.

Maybe in the future I could move the "real" `addTempRoot` to `GcStore`,
and the existing usecases use a `tryAddTempRoot` wrapper to downcast or
do nothing, but I wasn't sure whether that was a good idea so with a
bias to less churn I didn't do it yet.

16 files changed, 143 insertions, 93 deletions

diff --git a/src/libmain/shared.cc b/src/libmain/shared.cc
index a0b0f4cb3..562d1b414 100644
--- a/src/libmain/shared.cc
+++ b/src/libmain/shared.cc
@@ -1,6 +1,7 @@
 #include "globals.hh"
 #include "shared.hh"
 #include "store-api.hh"
+#include "gc-store.hh"
 #include "util.hh"
 #include "loggers.hh"
 
diff --git a/src/libstore/build/local-derivation-goal.cc b/src/libstore/build/local-derivation-goal.cc
index 7e69d4145..581d63d0e 100644
--- a/src/libstore/build/local-derivation-goal.cc
+++ b/src/libstore/build/local-derivation-goal.cc
@@ -1,4 +1,5 @@
 #include "local-derivation-goal.hh"
+#include "gc-store.hh"
 #include "hook-instance.hh"
 #include "worker.hh"
 #include "builtins.hh"
@@ -1127,7 +1128,7 @@ struct RestrictedStoreConfig : virtual LocalFSStoreConfig
 /* A wrapper around LocalStore that only allows building/querying of
    paths that are in the input closures of the build or were added via
    recursive Nix calls. */
-struct RestrictedStore : public virtual RestrictedStoreConfig, public virtual LocalFSStore
+struct RestrictedStore : public virtual RestrictedStoreConfig, public virtual LocalFSStore, public virtual GcStore
 {
     ref<LocalStore> next;
 
diff --git a/src/libstore/daemon.cc b/src/libstore/daemon.cc
index 9d4f6b4a4..89d9487da 100644
--- a/src/libstore/daemon.cc
+++ b/src/libstore/daemon.cc
@@ -3,6 +3,7 @@
 #include "worker-protocol.hh"
 #include "build-result.hh"
 #include "store-api.hh"
+#include "gc-store.hh"
 #include "path-with-outputs.hh"
 #include "finally.hh"
 #include "archive.hh"
@@ -623,9 +624,12 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
 
     case wopAddIndirectRoot: {
         Path path = absPath(readString(from));
+
         logger->startWork();
-        store->addIndirectRoot(path);
+        auto & gcStore = requireGcStore(*store);
+        gcStore.addIndirectRoot(path);
         logger->stopWork();
+
         to << 1;
         break;
     }
@@ -640,7 +644,8 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
 
     case wopFindRoots: {
         logger->startWork();
-        Roots roots = store->findRoots(!trusted);
+        auto & gcStore = requireGcStore(*store);
+        Roots roots = gcStore.findRoots(!trusted);
         logger->stopWork();
 
         size_t size = 0;
@@ -671,7 +676,8 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
         logger->startWork();
         if (options.ignoreLiveness)
             throw Error("you are not allowed to ignore liveness");
-        store->collectGarbage(options, results);
+        auto & gcStore = requireGcStore(*store);
+        gcStore.collectGarbage(options, results);
         logger->stopWork();
 
         to << results.paths << results.bytesFreed << 0 /* obsolete */;
diff --git a/src/libstore/gc-store.cc b/src/libstore/gc-store.cc
new file mode 100644
index 000000000..3dbdec53b
--- /dev/null
+++ b/src/libstore/gc-store.cc
@@ -0,0 +1,13 @@
+#include "gc-store.hh"
+
+namespace nix {
+
+GcStore & requireGcStore(Store & store)
+{
+    auto * gcStore = dynamic_cast<GcStore *>(&store);
+    if (!gcStore)
+        throw UsageError("Garbage collection not supported by this store");
+    return *gcStore;
+}
+
+}
diff --git a/src/libstore/gc-store.hh b/src/libstore/gc-store.hh
new file mode 100644
index 000000000..829f70dc4
--- /dev/null
+++ b/src/libstore/gc-store.hh
@@ -0,0 +1,84 @@
+#pragma once
+
+#include "store-api.hh"
+
+
+namespace nix {
+
+
+typedef std::unordered_map<StorePath, std::unordered_set<std::string>> Roots;
+
+
+struct GCOptions
+{
+    /* Garbage collector operation:
+
+       - `gcReturnLive': return the set of paths reachable from
+         (i.e. in the closure of) the roots.
+
+       - `gcReturnDead': return the set of paths not reachable from
+         the roots.
+
+       - `gcDeleteDead': actually delete the latter set.
+
+       - `gcDeleteSpecific': delete the paths listed in
+          `pathsToDelete', insofar as they are not reachable.
+    */
+    typedef enum {
+        gcReturnLive,
+        gcReturnDead,
+        gcDeleteDead,
+        gcDeleteSpecific,
+    } GCAction;
+
+    GCAction action{gcDeleteDead};
+
+    /* If `ignoreLiveness' is set, then reachability from the roots is
+       ignored (dangerous!).  However, the paths must still be
+       unreferenced *within* the store (i.e., there can be no other
+       store paths that depend on them). */
+    bool ignoreLiveness{false};
+
+    /* For `gcDeleteSpecific', the paths to delete. */
+    StorePathSet pathsToDelete;
+
+    /* Stop after at least `maxFreed' bytes have been freed. */
+    uint64_t maxFreed{std::numeric_limits<uint64_t>::max()};
+};
+
+
+struct GCResults
+{
+    /* Depending on the action, the GC roots, or the paths that would
+       be or have been deleted. */
+    PathSet paths;
+
+    /* For `gcReturnDead', `gcDeleteDead' and `gcDeleteSpecific', the
+       number of bytes that would be or was freed. */
+    uint64_t bytesFreed = 0;
+};
+
+
+struct GcStore : public virtual Store
+{
+    /* Add an indirect root, which is merely a symlink to `path' from
+       /nix/var/nix/gcroots/auto/<hash of `path'>.  `path' is supposed
+       to be a symlink to a store path.  The garbage collector will
+       automatically remove the indirect root when it finds that
+       `path' has disappeared. */
+    virtual void addIndirectRoot(const Path & path) = 0;
+
+    /* Find the roots of the garbage collector.  Each root is a pair
+       (link, storepath) where `link' is the path of the symlink
+       outside of the Nix store that point to `storePath'. If
+       'censor' is true, privacy-sensitive information about roots
+       found in /proc is censored. */
+    virtual Roots findRoots(bool censor) = 0;
+
+    /* Perform a garbage collection. */
+    virtual void collectGarbage(const GCOptions & options, GCResults & results) = 0;
+};
+
+GcStore & requireGcStore(Store & store);
+
+}
diff --git a/src/libstore/local-fs-store.hh b/src/libstore/local-fs-store.hh
index d34f0cb62..fbd49dc2c 100644
--- a/src/libstore/local-fs-store.hh
+++ b/src/libstore/local-fs-store.hh
@@ -1,6 +1,7 @@
 #pragma once
 
 #include "store-api.hh"
+#include "gc-store.hh"
 
 namespace nix {
 
@@ -23,7 +24,7 @@ struct LocalFSStoreConfig : virtual StoreConfig
         "physical path to the Nix store"};
 };
 
-class LocalFSStore : public virtual LocalFSStoreConfig, public virtual Store
+class LocalFSStore : public virtual LocalFSStoreConfig, public virtual Store, virtual GcStore
 {
 public:
 
diff --git a/src/libstore/local-store.hh b/src/libstore/local-store.hh
index 1a278c9a8..70d225be3 100644
--- a/src/libstore/local-store.hh
+++ b/src/libstore/local-store.hh
@@ -5,6 +5,7 @@
 #include "pathlocks.hh"
 #include "store-api.hh"
 #include "local-fs-store.hh"
+#include "gc-store.hh"
 #include "sync.hh"
 #include "util.hh"
 
@@ -43,7 +44,7 @@ struct LocalStoreConfig : virtual LocalFSStoreConfig
 };
 
 
-class LocalStore : public virtual LocalStoreConfig, public virtual LocalFSStore
+class LocalStore : public virtual LocalStoreConfig, public virtual LocalFSStore, public virtual GcStore
 {
 private:
 
diff --git a/src/libstore/path-with-outputs.cc b/src/libstore/path-with-outputs.cc
index 97aa01b57..078c117bd 100644
--- a/src/libstore/path-with-outputs.cc
+++ b/src/libstore/path-with-outputs.cc
@@ -22,9 +22,9 @@ DerivedPath StorePathWithOutputs::toDerivedPath() const
 
 std::vector<DerivedPath> toDerivedPaths(const std::vector<StorePathWithOutputs> ss)
 {
-	std::vector<DerivedPath> reqs;
-	for (auto & s : ss) reqs.push_back(s.toDerivedPath());
-	return reqs;
+    std::vector<DerivedPath> reqs;
+    for (auto & s : ss) reqs.push_back(s.toDerivedPath());
+    return reqs;
 }
 
 
diff --git a/src/libstore/remote-store.cc b/src/libstore/remote-store.cc
index cbcb75c00..a25398ef2 100644
--- a/src/libstore/remote-store.cc
+++ b/src/libstore/remote-store.cc
@@ -1,6 +1,7 @@
 #include "serialise.hh"
 #include "util.hh"
 #include "path-with-outputs.hh"
+#include "gc-store.hh"
 #include "remote-fs-accessor.hh"
 #include "build-result.hh"
 #include "remote-store.hh"
diff --git a/src/libstore/remote-store.hh b/src/libstore/remote-store.hh
index b94216d31..2628206b1 100644
--- a/src/libstore/remote-store.hh
+++ b/src/libstore/remote-store.hh
@@ -4,6 +4,7 @@
 #include <string>
 
 #include "store-api.hh"
+#include "gc-store.hh"
 
 
 namespace nix {
@@ -29,7 +30,7 @@ struct RemoteStoreConfig : virtual StoreConfig
 
 /* FIXME: RemoteStore is a misnomer - should be something like
    DaemonStore. */
-class RemoteStore : public virtual RemoteStoreConfig, public virtual Store
+class RemoteStore : public virtual RemoteStoreConfig, public virtual Store, public virtual GcStore
 {
 public:
 
diff --git a/src/libstore/store-api.hh b/src/libstore/store-api.hh
index 8c57596d0..7bd21519c 100644
--- a/src/libstore/store-api.hh
+++ b/src/libstore/store-api.hh
@@ -76,59 +76,6 @@ enum AllowInvalidFlag : bool { DisallowInvalid = false, AllowInvalid = true };
 const uint32_t exportMagic = 0x4558494e;
 
 
-typedef std::unordered_map<StorePath, std::unordered_set<std::string>> Roots;
-
-
-struct GCOptions
-{
-    /* Garbage collector operation:
-
-       - `gcReturnLive': return the set of paths reachable from
-         (i.e. in the closure of) the roots.
-
-       - `gcReturnDead': return the set of paths not reachable from
-         the roots.
-
-       - `gcDeleteDead': actually delete the latter set.
-
-       - `gcDeleteSpecific': delete the paths listed in
-          `pathsToDelete', insofar as they are not reachable.
-    */
-    typedef enum {
-        gcReturnLive,
-        gcReturnDead,
-        gcDeleteDead,
-        gcDeleteSpecific,
-    } GCAction;
-
-    GCAction action{gcDeleteDead};
-
-    /* If `ignoreLiveness' is set, then reachability from the roots is
-       ignored (dangerous!).  However, the paths must still be
-       unreferenced *within* the store (i.e., there can be no other
-       store paths that depend on them). */
-    bool ignoreLiveness{false};
-
-    /* For `gcDeleteSpecific', the paths to delete. */
-    StorePathSet pathsToDelete;
-
-    /* Stop after at least `maxFreed' bytes have been freed. */
-    uint64_t maxFreed{std::numeric_limits<uint64_t>::max()};
-};
-
-
-struct GCResults
-{
-    /* Depending on the action, the GC roots, or the paths that would
-       be or have been deleted. */
-    PathSet paths;
-
-    /* For `gcReturnDead', `gcDeleteDead' and `gcDeleteSpecific', the
-       number of bytes that would be or was freed. */
-    uint64_t bytesFreed = 0;
-};
-
-
 enum BuildMode { bmNormal, bmRepair, bmCheck };
 
 struct BuildResult;
@@ -531,26 +478,6 @@ public:
     virtual void addTempRoot(const StorePath & path)
     { debug("not creating temporary root, store doesn't support GC"); }
 
-    /* Add an indirect root, which is merely a symlink to `path' from
-       /nix/var/nix/gcroots/auto/<hash of `path'>.  `path' is supposed
-       to be a symlink to a store path.  The garbage collector will
-       automatically remove the indirect root when it finds that
-       `path' has disappeared. */
-    virtual void addIndirectRoot(const Path & path)
-    { unsupported("addIndirectRoot"); }
-
-    /* Find the roots of the garbage collector.  Each root is a pair
-       (link, storepath) where `link' is the path of the symlink
-       outside of the Nix store that point to `storePath'. If
-       'censor' is true, privacy-sensitive information about roots
-       found in /proc is censored. */
-    virtual Roots findRoots(bool censor)
-    { unsupported("findRoots"); }
-
-    /* Perform a garbage collection. */
-    virtual void collectGarbage(const GCOptions & options, GCResults & results)
-    { unsupported("collectGarbage"); }
-
     /* Return a string representing information about the path that
        can be loaded into the database using `nix-store --load-db' or
        `nix-store --register-validity'. */
diff --git a/src/libutil/tests/logging.cc b/src/libutil/tests/logging.cc
index cef3bd481..2ffdc2e9b 100644
--- a/src/libutil/tests/logging.cc
+++ b/src/libutil/tests/logging.cc
@@ -359,7 +359,7 @@ namespace nix {
 
       // constructing without access violation.
       ErrPos ep(invalid);
-    
+
       // assignment without access violation.
       ep = invalid;
 
diff --git a/src/nix-collect-garbage/nix-collect-garbage.cc b/src/nix-collect-garbage/nix-collect-garbage.cc
index 48c030b18..4b28ea6a4 100644
--- a/src/nix-collect-garbage/nix-collect-garbage.cc
+++ b/src/nix-collect-garbage/nix-collect-garbage.cc
@@ -1,4 +1,5 @@
 #include "store-api.hh"
+#include "gc-store.hh"
 #include "profiles.hh"
 #include "shared.hh"
 #include "globals.hh"
@@ -80,10 +81,11 @@ static int main_nix_collect_garbage(int argc, char * * argv)
         // Run the actual garbage collector.
         if (!dryRun) {
             auto store = openStore();
+            auto & gcStore = requireGcStore(*store);
             options.action = GCOptions::gcDeleteDead;
             GCResults results;
             PrintFreed freed(true, results);
-            store->collectGarbage(options, results);
+            gcStore.collectGarbage(options, results);
         }
 
         return 0;
diff --git a/src/nix-store/nix-store.cc b/src/nix-store/nix-store.cc
index 1ebc177f5..8ebaf9387 100644
--- a/src/nix-store/nix-store.cc
+++ b/src/nix-store/nix-store.cc
@@ -3,6 +3,7 @@
 #include "dotgraph.hh"
 #include "globals.hh"
 #include "build-result.hh"
+#include "gc-store.hh"
 #include "local-store.hh"
 #include "monitor-fd.hh"
 #include "serve-protocol.hh"
@@ -428,11 +429,12 @@ static void opQuery(Strings opFlags, Strings opArgs)
             store->computeFSClosure(
                 args, referrers, true, settings.gcKeepOutputs, settings.gcKeepDerivations);
 
-            Roots roots = store->findRoots(false);
+            auto & gcStore = requireGcStore(*store);
+            Roots roots = gcStore.findRoots(false);
             for (auto & [target, links] : roots)
                 if (referrers.find(target) != referrers.end())
                     for (auto & link : links)
-                        cout << fmt("%1% -> %2%\n", link, store->printStorePath(target));
+                        cout << fmt("%1% -> %2%\n", link, gcStore.printStorePath(target));
             break;
         }
 
@@ -588,20 +590,22 @@ static void opGC(Strings opFlags, Strings opArgs)
 
     if (!opArgs.empty()) throw UsageError("no arguments expected");
 
+    auto & gcStore = requireGcStore(*store);
+
     if (printRoots) {
-        Roots roots = store->findRoots(false);
+        Roots roots = gcStore.findRoots(false);
         std::set<std::pair<Path, StorePath>> roots2;
         // Transpose and sort the roots.
         for (auto & [target, links] : roots)
             for (auto & link : links)
                 roots2.emplace(link, target);
         for (auto & [link, target] : roots2)
-            std::cout << link << " -> " << store->printStorePath(target) << "\n";
+            std::cout << link << " -> " << gcStore.printStorePath(target) << "\n";
     }
 
     else {
         PrintFreed freed(options.action == GCOptions::gcDeleteDead, results);
-        store->collectGarbage(options, results);
+        gcStore.collectGarbage(options, results);
 
         if (options.action != GCOptions::gcDeleteDead)
             for (auto & i : results.paths)
@@ -625,9 +629,11 @@ static void opDelete(Strings opFlags, Strings opArgs)
     for (auto & i : opArgs)
         options.pathsToDelete.insert(store->followLinksToStorePath(i));
 
+    auto & gcStore = requireGcStore(*store);
+
     GCResults results;
     PrintFreed freed(true, results);
-    store->collectGarbage(options, results);
+    gcStore.collectGarbage(options, results);
 }
 
 
diff --git a/src/nix/store-delete.cc b/src/nix/store-delete.cc
index e4a3cb554..aa7a8b12f 100644
--- a/src/nix/store-delete.cc
+++ b/src/nix/store-delete.cc
@@ -2,6 +2,7 @@
 #include "common-args.hh"
 #include "shared.hh"
 #include "store-api.hh"
+#include "gc-store.hh"
 
 using namespace nix;
 
@@ -32,12 +33,14 @@ struct CmdStoreDelete : StorePathsCommand
 
     void run(ref<Store> store, std::vector<StorePath> && storePaths) override
     {
+        auto & gcStore = requireGcStore(*store);
+
         for (auto & path : storePaths)
             options.pathsToDelete.insert(path);
 
         GCResults results;
         PrintFreed freed(true, results);
-        store->collectGarbage(options, results);
+        gcStore.collectGarbage(options, results);
     }
 };
 
diff --git a/src/nix/store-gc.cc b/src/nix/store-gc.cc
index a2d74066e..21718dc0c 100644
--- a/src/nix/store-gc.cc
+++ b/src/nix/store-gc.cc
@@ -2,6 +2,7 @@
 #include "common-args.hh"
 #include "shared.hh"
 #include "store-api.hh"
+#include "gc-store.hh"
 
 using namespace nix;
 
@@ -33,10 +34,12 @@ struct CmdStoreGC : StoreCommand, MixDryRun
 
     void run(ref<Store> store) override
     {
+        auto & gcStore = requireGcStore(*store);
+
         options.action = dryRun ? GCOptions::gcReturnDead : GCOptions::gcDeleteDead;
         GCResults results;
         PrintFreed freed(options.action == GCOptions::gcDeleteDead, results);
-        store->collectGarbage(options, results);
+        gcStore.collectGarbage(options, results);
     }
 };