configs/configload: some loaders can't install modules

Originally the hope was to use the afero filesystem abstraction for all
loader operations, but since we install modules using go-getter we cannot
(without a lot of refactoring) support vfs for installation.

The vfs use-case is for reading configuration from plan zip files anyway,
and so we have no real reason to support installation into a vfs. For now
at least we will just add the possibility that a loader might not be
install-capable. At the moment we have no non-install-capable loaders, but
we'll add one later once we get to loading configuration from plan files.

configs/configload/loader.go

@@ -59,11 +59,12 @@ func NewLoader(config *Config) (*Loader, error) {
 	ret := &Loader{
 		parser: parser,
 		modules: moduleMgr{
-			FS:       afero.Afero{fs},
-			Dir:      config.ModulesDir,
-			Services: config.Services,
-			Creds:    config.Creds,
-			Registry: reg,
+			FS:         afero.Afero{Fs: fs},
+			CanInstall: true,
+			Dir:        config.ModulesDir,
+			Services:   config.Services,
+			Creds:      config.Creds,
+			Registry:   reg,
 		},
 	}
 

configs/configload/loader_install.go

@@ -38,7 +38,16 @@ import (
 // may have wholly or partially completed. Modules must be loaded in order
 // to find their dependencies, so this function does many of the same checks
 // as LoadConfig as a side-effect.
+//
+// This function will panic if called on a loader that cannot install modules.
+// Use CanInstallModules to determine if a loader can install modules, or
+// refer to the documentation for that method for situations where module
+// installation capability is guaranteed.
 func (l *Loader) InstallModules(rootDir string, upgrade bool, hooks InstallHooks) hcl.Diagnostics {
+	if !l.CanInstallModules() {
+		panic(fmt.Errorf("InstallModules called on loader that cannot install modules"))
+	}
+
 	rootMod, diags := l.parser.LoadConfigDir(rootDir)
 	if rootMod == nil {
 		return diags
@@ -183,6 +192,16 @@ func (l *Loader) InstallModules(rootDir string, upgrade bool, hooks InstallHooks
 	return diags
 }
 
+// CanInstallModules returns true if InstallModules can be used with this
+// loader.
+//
+// Loaders created with NewLoader can always install modules. Loaders created
+// from plan files (where the configuration is embedded in the plan file itself)
+// cannot install modules, because the plan file is read-only.
+func (l *Loader) CanInstallModules() bool {
+	return l.modules.CanInstall
+}
+
 func (l *Loader) installLocalModule(req *configs.ModuleRequest, key string, hooks InstallHooks) (*configs.Module, hcl.Diagnostics) {
 	var diags hcl.Diagnostics
 

configs/configload/module_mgr.go

@@ -10,6 +10,13 @@ import (
 type moduleMgr struct {
 	FS afero.Afero
 
+	// CanInstall is true for a module manager that can support installation.
+	//
+	// This must be set only if FS is an afero.OsFs, because the installer
+	// (which uses go-getter) is not aware of the virtual filesystem
+	// abstraction and will always write into the "real" filesystem.
+	CanInstall bool
+
 	// Dir is the path where descendent modules are (or will be) installed.
 	Dir string