fix remote build isolation when server runs as root

I am really not sure why the caller even should have the option to set
this. We should always use the correct isolation type based on the
privileges the server runs under never the client. podman-remote build
seems to send the default based on its local privs which was wrong as
well. To fix this I also changed the client to send the default if the
isolation flag is not set.

Fixes #22109

Signed-off-by: Paul Holzinger <pholzing@redhat.com>

cmd/podman/common/build.go

@@ -400,9 +400,14 @@ func buildFlagsWrapperToOptions(c *cobra.Command, contextDir string, flags *Buil
 		compression = buildahDefine.Uncompressed
 	}
 
-	isolation, err := parse.IsolationOption(flags.Isolation)
-	if err != nil {
-		return nil, err
+	isolation := buildahDefine.IsolationDefault
+	// Only parse the isolation when it is actually needed as we do not want to send a wrong default
+	// to the server in the remote case (root vs rootless).
+	if flags.Isolation != "" {
+		isolation, err = parse.IsolationOption(flags.Isolation)
+		if err != nil {
+			return nil, err
+		}
 	}
 
 	usernsOption, idmappingOptions, err := parse.IDMappingOptions(c, isolation)

pkg/api/handlers/compat/images_build.go

@@ -383,10 +383,19 @@ func BuildImage(w http.ResponseWriter, r *http.Request) {
 			return
 		}
 
-		// make sure to force rootless as rootless otherwise buildah runs code which is intended to be run only as root.
-		if isolation == buildah.IsolationOCI && rootless.IsRootless() {
-			isolation = buildah.IsolationOCIRootless
+		// Make sure to force rootless as rootless otherwise buildah runs code which is intended to be run only as root.
+		// Same the other way around: https://github.com/containers/podman/issues/22109
+		switch isolation {
+		case buildah.IsolationOCI:
+			if rootless.IsRootless() {
+				isolation = buildah.IsolationOCIRootless
+			}
+		case buildah.IsolationOCIRootless:
+			if !rootless.IsRootless() {
+				isolation = buildah.IsolationOCI
+			}
 		}
+
 		registry = ""
 		format = query.OutputFormat
 	} else {

pkg/machine/e2e/basic_test.go

@@ -54,6 +54,16 @@ var _ = Describe("run basic podman commands", func() {
 		Expect(runAlp).To(Exit(0))
 		Expect(runAlp.outputToString()).To(ContainSubstring("Alpine Linux"))
 
+		contextDir := GinkgoT().TempDir()
+		cfile := filepath.Join(contextDir, "Containerfile")
+		err = os.WriteFile(cfile, []byte("FROM quay.io/libpod/alpine_nginx\nRUN ip addr\n"), 0o644)
+		Expect(err).ToNot(HaveOccurred())
+
+		build, err := mb.setCmd(bm.withPodmanCommand([]string{"build", contextDir})).run()
+		Expect(err).ToNot(HaveOccurred())
+		Expect(build).To(Exit(0))
+		Expect(build.outputToString()).To(ContainSubstring("COMMIT"))
+
 		rmCon, err := mb.setCmd(bm.withPodmanCommand([]string{"rm", "-a"})).run()
 		Expect(err).ToNot(HaveOccurred())
 		Expect(rmCon).To(Exit(0))