Test and fix that we only load images when needed

src/haskell/lib/Arion/Images.hs

@@ -22,7 +22,11 @@ loadImages requestedImages = do
   loaded <- getDockerImages
 
   let
-    isNew i = (imageName i <> ":" <> imageTag i) `notElem` loaded
+    isNew i = 
+      -- On docker, the image name is unmodified
+      (imageName i <> ":" <> imageTag i) `notElem` loaded
+      -- -- On podman, you automatically get a localhost prefix
+        && ("localhost/" <> imageName i <> ":" <> imageTag i) `notElem` loaded
 
   traverse_ loadImage . filter isNew $ requestedImages
 

tests/arion-test/default.nix

@@ -70,6 +70,44 @@ in
         )
         machine.wait_until_fails("curl --fail localhost:8000")
 
+    # Tests
+    #  - running same image again doesn't require a `docker load`
+    with subtest("docker load only once"):
+        # We assume image loading relies on the `docker images` and `docker load` commands, so this should fail
+        machine.fail(
+            "export REAL_DOCKER=$(which docker); rm -rf work && cp -frT ${../../examples/minimal} work && cd work && NIX_PATH=nixpkgs='${pkgs.path}' PATH=\"${pkgs.writeScriptBin "docker" ''
+              #!${pkgs.runtimeShell} -eu
+              echo 1>&2 "This failure is expected. Args were" "$@"
+              echo "$@" >/tmp/docker-args
+              exit 1
+            ''}/bin:$PATH\" arion up -d"
+        )
+        machine.succeed(
+            "export REAL_DOCKER=$(which docker); rm -rf work && cp -frT ${../../examples/minimal} work && cd work && NIX_PATH=nixpkgs='${pkgs.path}' PATH=\"${pkgs.writeScriptBin "docker" ''
+              #!${pkgs.runtimeShell} -eu
+              case $1 in
+                load)
+                  echo 1>&2 "arion must not docker load when upping the same deployment for the second time"
+                  exit 1
+                  ;;
+                images)
+                  echo 1>&2 "execing docker to list images"
+                  exec $REAL_DOCKER "$@"
+                  ;;
+                *)
+                  echo 1>&2 "Unknown docker invocation. This may be a shortcoming of this docker mock."
+                  echo 1>&2 "Invocation: docker" "$@"
+                  ;;
+              esac
+            ''}/bin:$PATH\" arion up -d"
+        )
+        machine.wait_until_succeeds("curl --fail localhost:8000")
+        machine.succeed(
+            "cd work && NIX_PATH=nixpkgs='${pkgs.path}' arion down"
+        )
+        machine.wait_until_fails("curl --fail localhost:8000")
+
+
     # Tests
     #  - examples/flake
     # This _test_ doesn't work because flake-compat fetches the github