Emit basic dry run message for garbage collection

src/libmain/include/nix/main/shared.hh

@@ -89,19 +89,7 @@ extern volatile ::sig_atomic_t blockInt;
 
 struct GCResults;
 
-struct PrintFreed
-{
-    bool show;
-    const GCResults & results;
-
-    PrintFreed(bool show, const GCResults & results)
-        : show(show)
-        , results(results)
-    {
-    }
-
-    ~PrintFreed();
-};
+void printFreed(bool dryRun, const GCResults & results);
 
 #ifndef _WIN32
 /**

src/libmain/shared.cc

@@ -391,9 +391,12 @@ RunPager::~RunPager()
     }
 }
 
-PrintFreed::~PrintFreed()
+void printFreed(bool dryRun, const GCResults & results)
 {
-    if (show)
+    /* bytesFreed cannot be reliably computed without actually deleting store paths because of hardlinking. */
+    if (dryRun)
+        std::cout << fmt("%d store paths would be deleted\n", results.paths.size());
+    else
         std::cout << fmt("%d store paths deleted, %s freed\n", results.paths.size(), renderSize(results.bytesFreed));
 }
 

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

@@ -69,8 +69,7 @@ struct GCResults
     PathSet paths;
 
     /**
-     * For `gcReturnDead`, `gcDeleteDead` and `gcDeleteSpecific`, the
-     * number of bytes that would be or was freed.
+     * For `gcDeleteDead` and `gcDeleteSpecific`, the number of bytes that were freed.
      */
     uint64_t bytesFreed = 0;
 };

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

@@ -1,5 +1,6 @@
 #include "nix/util/file-system.hh"
 #include "nix/util/signals.hh"
+#include "nix/util/error.hh"
 #include "nix/store/store-open.hh"
 #include "nix/store/store-cast.hh"
 #include "nix/store/gc-store.hh"
@@ -87,6 +88,9 @@ static int main_nix_collect_garbage(int argc, char ** argv)
             return true;
         });
 
+        if (options.maxFreed != std::numeric_limits<uint64_t>::max() && dryRun)
+            throw UsageError("options --max-freed and --dry-run cannot be combined");
+
         if (removeOld) {
             std::set<std::filesystem::path> dirsToClean = {
                 profilesDir(),
@@ -97,15 +101,12 @@ static int main_nix_collect_garbage(int argc, char ** argv)
                 removeOldGenerations(dir);
         }
 
-        // Run the actual garbage collector.
-        if (!dryRun) {
-            auto store = openStore();
-            auto & gcStore = require<GcStore>(*store);
-            options.action = GCOptions::gcDeleteDead;
-            GCResults results;
-            PrintFreed freed(true, results);
-            gcStore.collectGarbage(options, results);
-        }
+        auto store = openStore();
+        auto & gcStore = require<GcStore>(*store);
+        options.action = dryRun ? GCOptions::gcReturnDead : GCOptions::gcDeleteDead;
+        GCResults results;
+        Finally printer([&] { printFreed(dryRun, results); });
+        gcStore.collectGarbage(options, results);
 
         return 0;
     }

src/nix/nix-store/nix-store.cc

@@ -18,6 +18,8 @@
 #include "nix/store/export-import.hh"
 #include "nix/util/strings.hh"
 #include "nix/store/posix-fs-canonicalise.hh"
+#include "nix/util/error.hh"
+#include "nix/store/gc-store.hh"
 
 #include "man-pages.hh"
 
@@ -679,6 +681,10 @@ static void opGC(Strings opFlags, Strings opArgs)
     if (!opArgs.empty())
         throw UsageError("no arguments expected");
 
+    if (options.maxFreed != std::numeric_limits<uint64_t>::max()
+        && (options.action == GCOptions::gcReturnDead || options.action == GCOptions::gcReturnLive || printRoots))
+        throw UsageError("option --max-freed cannot be combined with --print-live, --print-dead, or --print-roots");
+
     auto & gcStore = require<GcStore>(*store);
 
     if (printRoots) {
@@ -693,12 +699,14 @@ static void opGC(Strings opFlags, Strings opArgs)
     }
 
     else {
-        PrintFreed freed(options.action == GCOptions::gcDeleteDead, results);
+        Finally printer([&] {
+            if (options.action != GCOptions::gcDeleteDead)
+                for (auto & i : results.paths)
+                    cout << i << std::endl;
+            else
+                printFreed(false, results);
+        });
         gcStore.collectGarbage(options, results);
-
-        if (options.action != GCOptions::gcDeleteDead)
-            for (auto & i : results.paths)
-                cout << i << std::endl;
     }
 }
 
@@ -722,7 +730,7 @@ static void opDelete(Strings opFlags, Strings opArgs)
     auto & gcStore = require<GcStore>(*store);
 
     GCResults results;
-    PrintFreed freed(true, results);
+    Finally printer([&] { printFreed(false, results); });
     gcStore.collectGarbage(options, results);
 }
 

src/nix/store-delete.cc

@@ -40,7 +40,7 @@ struct CmdStoreDelete : StorePathsCommand
             options.pathsToDelete.insert(path);
 
         GCResults results;
-        PrintFreed freed(true, results);
+        Finally printer([&] { printFreed(false, results); });
         gcStore.collectGarbage(options, results);
     }
 };

src/nix/store-gc.cc

@@ -4,6 +4,8 @@
 #include "nix/store/store-api.hh"
 #include "nix/store/store-cast.hh"
 #include "nix/store/gc-store.hh"
+#include "nix/util/error.hh"
+#include "nix/util/logging.hh"
 
 using namespace nix;
 
@@ -15,7 +17,7 @@ struct CmdStoreGC : StoreCommand, MixDryRun
     {
         addFlag({
             .longName = "max",
-            .description = "Stop after freeing *n* bytes of disk space.",
+            .description = "Stop after freeing *n* bytes of disk space. Cannot be combined with --dry-run.",
             .labels = {"n"},
             .handler = {&options.maxFreed},
         });
@@ -35,11 +37,14 @@ struct CmdStoreGC : StoreCommand, MixDryRun
 
     void run(ref<Store> store) override
     {
+        if (options.maxFreed != std::numeric_limits<uint64_t>::max() && dryRun)
+            throw UsageError("options --max and --dry-run cannot be combined");
+
         auto & gcStore = require<GcStore>(*store);
 
         options.action = dryRun ? GCOptions::gcReturnDead : GCOptions::gcDeleteDead;
         GCResults results;
-        PrintFreed freed(options.action == GCOptions::gcDeleteDead, results);
+        Finally printer([&] { printFreed(dryRun, results); });
         gcStore.collectGarbage(options, results);
     }
 };