pkg/utils: Ensure that the distro CLI and config file option is valid

Currently, if an invalid or unsupported string is specified as the
distro on the command line or in the configuration file, then it would
silently fallback to Fedora.  This shouldn't happen.

It should only fallback to Fedora when no distro was specified and
there's no supported Toolbox image matching the host operating system.
If a distro was explicitly specified then it should either be supported
or it should error out.

containers#937

src/pkg/utils/utils.go

@@ -250,10 +250,6 @@ func getDefaultImageForDistro(distro, release string) string {
 		panic("distro not specified")
 	}
 
-	if _, supportedDistro := supportedDistros[distro]; !supportedDistro {
-		distro = distroFallback
-	}
-
 	distroObj, supportedDistro := supportedDistros[distro]
 	if !supportedDistro {
 		panicMsg := fmt.Sprintf("failed to find %s in the list of supported distributions", distro)
@@ -717,6 +713,10 @@ func ResolveContainerAndImageNames(container, distroCLI, imageCLI, releaseCLI st
 		}
 	}
 
+	if _, ok := supportedDistros[distro]; !ok {
+		return "", "", "", fmt.Errorf("distribution %s is unsupported", distro)
+	}
+
 	if distro != distroDefault && releaseCLI == "" && !viper.IsSet("general.release") {
 		return "", "", "", fmt.Errorf("release not found for non-default distribution %s", distro)
 	}

test/system/101-create.bats

@@ -71,6 +71,16 @@ teardown() {
   assert_output --regexp "Created[[:blank:]]+fedora-toolbox-32"
 }
 
+@test "create: Try to create a container based on unsupported distribution" {
+  local distro="foo"
+
+  run $TOOLBOX --assumeyes create --distro "$distro"
+
+  assert_failure
+  assert_line --index 0 "Error: distribution $distro is unsupported"
+  assert [ ${#lines[@]} -eq 1 ]
+}
+
 @test "create: Try to create a container based on non-existent image" {
   run $TOOLBOX -y create -i foo.org/bar
 

test/system/104-run.bats

@@ -48,6 +48,16 @@ teardown() {
   assert_line --index 2 "Run 'toolbox --help' for usage."
 }
 
+@test "run: Try to run a command in a container based on unsupported distribution" {
+  local distro="foo"
+
+  run $TOOLBOX --assumeyes run --distro "$distro" ls
+
+  assert_failure
+  assert_line --index 0 "Error: distribution $distro is unsupported"
+  assert [ ${#lines[@]} -eq 1 ]
+}
+
 @test "run: Try to run a command in a container based on Fedora but with wrong version" {
   run $TOOLBOX run -d fedora -r foobar ls
 

test/system/105-enter.bats

@@ -54,6 +54,16 @@ teardown() {
   assert_line --index 2 "Run 'toolbox --help' for usage."
 }
 
+@test "enter: Try to enter a container based on unsupported distribution" {
+  local distro="foo"
+
+  run $TOOLBOX --assumeyes enter --distro "$distro"
+
+  assert_failure
+  assert_line --index 0 "Error: distribution $distro is unsupported"
+  assert [ ${#lines[@]} -eq 1 ]
+}
+
 @test "enter: Try to enter a container based on Fedora but with wrong version" {
   run $TOOLBOX enter -d fedora -r foobar