command: Fix detection of necessary backend migration

In the refactoring for new HCL this codepath stopped taking into account
changes to the CLI -backend-config options when deciding if a backend
migration is required.

This restores that behavior in a different way than it used to be: rather
than re-hashing the merged config and comparing the hashes, we instead
just compare directly the configuration values, which must be exactly
equal in order to skip migration.

This change is covered by the test TestInit_inputFalse, although as of
this commit it is still not passing due a downstream problem within the
migration code itself.

command/meta_backend.go

@@ -450,11 +450,8 @@ func (m *Meta) backendFromConfig(opts *BackendOpts) (backend.Backend, tfdiags.Di
 	case c != nil && !s.Backend.Empty():
 		// If our configuration is the same, then we're just initializing
 		// a previously configured remote backend.
-		if !s.Backend.Empty() {
+		if !m.backendConfigNeedsMigration(c, s.Backend) {
-			storedHash := s.Backend.Hash
+			return m.backend_C_r_S_unchanged(c, cHash, sMgr)
-			if storedHash == cHash {
-				return m.backend_C_r_S_unchanged(c, cHash, sMgr)
-			}
 		}
 
 		if !opts.Init {
@@ -466,9 +463,7 @@ func (m *Meta) backendFromConfig(opts *BackendOpts) (backend.Backend, tfdiags.Di
 			return nil, diags
 		}
 
-		log.Printf(
+		log.Printf("[WARN] backend config has changed since last init")
-			"[WARN] command: backend config change! saved: %d, new: %d",
-			s.Backend.Hash, cHash)
 		return m.backend_C_r_S_changed(c, cHash, sMgr, true)
 
 	default:
@@ -929,6 +924,60 @@ func (m *Meta) backend_C_R_S_unchanged(c *configs.Backend, sMgr *state.LocalStat
 // Reusable helper functions for backend management
 //-------------------------------------------------------------------
 
+// backendConfigNeedsMigration returns true if migration might be required to
+// move from the configured backend to the given cached backend config.
+//
+// This must be called with the synthetic *configs.Backend that results from
+// merging in any command-line options for correct behavior.
+//
+// If either the given configuration or cached configuration are invalid then
+// this function will conservatively assume that migration is required,
+// expecting that the migration code will subsequently deal with the same
+// errors.
+func (m *Meta) backendConfigNeedsMigration(c *configs.Backend, s *terraform.BackendState) bool {
+	if s == nil || s.Empty() {
+		log.Print("[TRACE] backendConfigNeedsMigration: no cached config, so migration is required")
+		return true
+	}
+	if c.Type != s.Type {
+		log.Printf("[TRACE] backendConfigNeedsMigration: type changed from %q to %q, so migration is required", s.Type, c.Type)
+		return true
+	}
+
+	// We need the backend's schema to do our comparison here.
+	f := backendInit.Backend(c.Type)
+	if f == nil {
+		log.Printf("[TRACE] backendConfigNeedsMigration: no backend of type %q, which migration codepath must handle", c.Type)
+		return true // let the migration codepath deal with the missing backend
+	}
+	b := f()
+
+	schema := b.ConfigSchema()
+	decSpec := schema.NoneRequired().DecoderSpec()
+	givenVal, diags := hcldec.Decode(c.Config, decSpec, nil)
+	if diags.HasErrors() {
+		log.Printf("[TRACE] backendConfigNeedsMigration: failed to decode given config; migration codepath must handle problem: %s", diags.Error())
+		return true // let the migration codepath deal with these errors
+	}
+
+	cachedVal, err := s.Config(schema)
+	if err != nil {
+		log.Printf("[TRACE] backendConfigNeedsMigration: failed to decode cached config; migration codepath must handle problem: %s", err)
+		return true // let the migration codepath deal with the error
+	}
+
+	// If we get all the way down here then it's the exact equality of the
+	// two decoded values that decides our outcome. It's safe to use RawEquals
+	// here (rather than Equals) because we know that unknown values can
+	// never appear in backend configurations.
+	if cachedVal.RawEquals(givenVal) {
+		log.Print("[TRACE] backendConfigNeedsMigration: given configuration matches cached configuration, so no migration is required")
+		return false
+	}
+	log.Print("[TRACE] backendConfigNeedsMigration: configuration values have changed, so migration is required")
+	return true
+}
+
 func (m *Meta) backendInitFromConfig(c *configs.Backend) (backend.Backend, cty.Value, tfdiags.Diagnostics) {
 	var diags tfdiags.Diagnostics