states: Extract version check logic from read

Instead of always checking the Terraform version associated with a state
file when reading it, we add a CheckTerraformVersion method and call it
from locations where we care about enforcing this check.

For this commit, the check has been retained at all call sites for
states/statefile.Read, with these exceptions:

- Unit tests, which shouldn't care about the state file version;
- E2E test runner which should always be using valid state files;
- terraform.ShimLegacyState, where the check is pointless as the state
  file was just created by the current Terraform version.

backend/remote/backend_state.go

@@ -64,6 +64,9 @@ func (r *remoteClient) Put(state []byte) error {
 	if err != nil {
 		return fmt.Errorf("Error reading state: %s", err)
 	}
+	if err := stateFile.CheckTerraformVersion(); err != nil {
+		return fmt.Errorf("Incompatible statefile: %s", err)
+	}
 
 	options := tfe.StateVersionCreateOptions{
 		Lineage: tfe.String(stateFile.Lineage),

command/show.go

@@ -245,6 +245,9 @@ func getStateFromPath(path string) (*statefile.File, error) {
 	if err != nil {
 		return nil, fmt.Errorf("Error reading %s as a statefile: %s", path, err)
 	}
+	if err := stateFile.CheckTerraformVersion(); err != nil {
+		return nil, fmt.Errorf("Incompatible statefile %s: %s", path, err)
+	}
 	return stateFile, nil
 }
 

command/state_push.go

@@ -63,6 +63,10 @@ func (c *StatePushCommand) Run(args []string) int {
 		c.Ui.Error(fmt.Sprintf("Error reading source state %q: %s", args[0], err))
 		return 1
 	}
+	if err := srcStateFile.CheckTerraformVersion(); err != nil {
+		c.Ui.Error(fmt.Sprintf("Incompatible statefile %q: %s", args[0], err))
+		return 1
+	}
 
 	// Load the backend
 	b, backendDiags := c.Backend(nil)

command/workspace_new.go

@@ -141,6 +141,10 @@ func (c *WorkspaceNewCommand) Run(args []string) int {
 		c.Ui.Error(err.Error())
 		return 1
 	}
+	if err := stateFile.CheckTerraformVersion(); err != nil {
+		c.Ui.Error(err.Error())
+		return 1
+	}
 
 	// save the existing state in the new Backend.
 	err = stateMgr.WriteState(stateFile.State)

plans/planfile/reader.go

@@ -101,7 +101,11 @@ func (r *Reader) ReadStateFile() (*statefile.File, error) {
 			if err != nil {
 				return nil, fmt.Errorf("failed to extract state from plan file: %s", err)
 			}
-			return statefile.Read(r)
+			stateFile, err := statefile.Read(r)
+			if err == nil {
+				err = stateFile.CheckTerraformVersion()
+			}
+			return stateFile, err
 		}
 	}
 	return nil, statefile.ErrNoState

states/remote/state.go

@@ -125,6 +125,9 @@ func (s *State) refreshState() error {
 	if err != nil {
 		return err
 	}
+	if err := stateFile.CheckTerraformVersion(); err != nil {
+		return err
+	}
 
 	s.lineage = stateFile.Lineage
 	s.serial = stateFile.Serial

states/statefile/file.go

@@ -1,6 +1,8 @@
 package statefile
 
 import (
+	"fmt"
+
 	version "github.com/hashicorp/go-version"
 
 	"github.com/hashicorp/terraform/states"
@@ -60,3 +62,15 @@ func (f *File) DeepCopy() *File {
 		State:            f.State.DeepCopy(),
 	}
 }
+
+func (f *File) CheckTerraformVersion() error {
+	if f.TerraformVersion != nil && f.TerraformVersion.GreaterThan(tfversion.SemVer) {
+		return fmt.Errorf(
+			"state snapshot was created by Terraform v%s, which is newer than current v%s; upgrade to Terraform v%s or greater to work with this state",
+			f.TerraformVersion,
+			tfversion.SemVer,
+			f.TerraformVersion,
+		)
+	}
+	return nil
+}

states/statefile/read.go

@@ -62,15 +62,6 @@ func Read(r io.Reader) (*File, error) {
 		panic("readState returned nil state with no errors")
 	}
 
-	if state.TerraformVersion != nil && state.TerraformVersion.GreaterThan(tfversion.SemVer) {
-		return state, fmt.Errorf(
-			"state snapshot was created by Terraform v%s, which is newer than current v%s; upgrade to Terraform v%s or greater to work with this state",
-			state.TerraformVersion,
-			tfversion.SemVer,
-			state.TerraformVersion,
-		)
-	}
-
 	return state, diags.Err()
 }
 

states/statemgr/filesystem.go

@@ -280,6 +280,10 @@ func (s *Filesystem) refreshState() error {
 			return err
 		}
 		log.Printf("[TRACE] statemgr.Filesystem: snapshot file has nil snapshot, but that's okay")
+	} else {
+		if err := f.CheckTerraformVersion(); err != nil {
+			return err
+		}
 	}
 
 	s.file = f
@@ -459,6 +463,10 @@ func (s *Filesystem) createStateFiles() error {
 		}
 		log.Printf("[TRACE] statemgr.Filesystem: no previously-stored snapshot exists")
 	} else {
+		if err := s.backupFile.CheckTerraformVersion(); err != nil {
+			return err
+		}
+
 		log.Printf("[TRACE] statemgr.Filesystem: existing snapshot has lineage %q serial %d", s.backupFile.Lineage, s.backupFile.Serial)
 	}