2 files changed, 60 insertions, 1 deletions

diff --git a/overlays/morph/default.nix b/overlays/morph/default.nix
index ee59e25..448f051 100644
--- a/overlays/morph/default.nix
+++ b/overlays/morph/default.nix
@@ -1,5 +1,5 @@
 self: super: {
   morph = super.morph.overrideAttrs(old: {
-    patches = (old.patches or []) ++ [ ./verbose_nix.patch ];
+    patches = (old.patches or []) ++ [ ./verbose_nix.patch ./dry-run.patch ];
   });
 }
diff --git a/overlays/morph/dry-run.patch b/overlays/morph/dry-run.patch
new file mode 100644
index 0000000..2ff099e
--- /dev/null
+++ b/overlays/morph/dry-run.patch
@@ -0,0 +1,59 @@
+diff --git a/morph.go b/morph.go
+index a88414b..e5894df 100644
+--- a/morph.go
++++ b/morph.go
+@@ -542,6 +542,7 @@ func getNixContext() *nix.NixContext {
+        return &nix.NixContext{
+                EvalMachines:    filepath.Join(assetRoot, assets.Friendly, "eval-machines.nix"),
+                ShowTrace:       showTrace,
++               DryRun:          *dryRun,
+                KeepGCRoot:      *keepGCRoot,
+                AllowBuildShell: *allowBuildShell,
+        }
+@@ -574,8 +575,10 @@ func buildHosts(hosts []nix.Host) (resultPath string, err error) {
+                return
+        }
+ 
+-       fmt.Fprintln(os.Stderr, "nix result path: ")
+-       fmt.Println(resultPath)
++       if resultPath != "" {
++               fmt.Fprintln(os.Stderr, "nix result path: ")
++               fmt.Println(resultPath)
++       }
+        return
+ }
+ 
+diff --git a/nix/nix.go b/nix/nix.go
+index 6a9cfd5..0f7884b 100644
+--- a/nix/nix.go
++++ b/nix/nix.go
+@@ -49,6 +49,7 @@ type Deployment struct {
+ type NixContext struct {
+        EvalMachines    string
+        ShowTrace       bool
++       DryRun          bool
+        KeepGCRoot      bool
+        AllowBuildShell bool
+ }
+@@ -269,6 +270,10 @@ func (ctx *NixContext) BuildMachines(deploymentPath string, hosts []Host, nixArg
+                args = append(args, "--show-trace")
+        }
+ 
++       if ctx.DryRun {
++               args = append(args, "--dry-run")
++       }
++
+        if nixBuildTargets != "" {
+                args = append(args,
+                        "--arg", "buildTargets", nixBuildTargets)
+@@ -308,6 +313,10 @@ func (ctx *NixContext) BuildMachines(deploymentPath string, hosts []Host, nixArg
+                return resultPath, errors.New(errorMessage)
+        }
+ 
++       if ctx.DryRun {
++               return "", err
++       }
++
+        resultPath, err = os.Readlink(resultLinkPath)
+        if err != nil {
+                return "", err