Fix error reading remote workspace with version constraint

.changes/unreleased/BUG FIXES-20250117-104335.yaml

@@ -0,0 +1,4 @@
+kind: BUG FIXES
+body: "Fixes error ('Invalid Terraform version: Malformed version') reading workspace when the remote workspace has the terraform version set to a constraint."
+custom:
+    Issue: "21401"

internal/backend/remote/backend.go

@@ -952,45 +952,55 @@ func (b *Remote) VerifyWorkspaceTerraformVersion(workspaceName string) tfdiags.D
 		return nil
 	}
 
-	remoteVersion, err := version.NewSemver(workspace.TerraformVersion)
+	remoteConstraint, err := version.NewConstraint(workspace.TerraformVersion)
 	if err != nil {
-		diags = diags.Append(tfdiags.Sourceless(
-			tfdiags.Error,
-			"Error looking up workspace",
-			fmt.Sprintf("Invalid Terraform version: %s", err),
-		))
+		message := fmt.Sprintf(
+			"The remote workspace specified an invalid Terraform version or constraint (%s), "+
+				"and it isn't possible to determine whether the local Terraform version (%s) is compatible.",
+			workspace.TerraformVersion,
+			tfversion.String(),
+		)
+		diags = diags.Append(incompatibleWorkspaceTerraformVersion(message, b.ignoreVersionConflict))
 		return diags
 	}
 
-	v014 := version.Must(version.NewSemver("0.14.0"))
-	if tfversion.SemVer.LessThan(v014) || remoteVersion.LessThan(v014) {
-		// Versions of Terraform prior to 0.14.0 will refuse to load state files
-		// written by a newer version of Terraform, even if it is only a patch
-		// level difference. As a result we require an exact match.
-		if tfversion.SemVer.Equal(remoteVersion) {
-			return diags
-		}
-	}
-	if tfversion.SemVer.GreaterThanOrEqual(v014) && remoteVersion.GreaterThanOrEqual(v014) {
-		// Versions of Terraform after 0.14.0 should be compatible with each
-		// other.  At the time this code was written, the only constraints we
-		// are aware of are:
-		//
-		// - 0.14.0 is guaranteed to be compatible with versions up to but not
-		//   including 1.3.0
+	remoteVersion, _ := version.NewSemver(workspace.TerraformVersion)
+
+	if remoteVersion != nil && remoteVersion.Prerelease() == "" {
+		v014 := version.Must(version.NewSemver("0.14.0"))
 		v130 := version.Must(version.NewSemver("1.3.0"))
-		if tfversion.SemVer.LessThan(v130) && remoteVersion.LessThan(v130) {
-			return diags
+
+		// Versions from 0.14 through the early 1.x series should be compatible
+		// (though we don't know about 1.3 yet).
+		if remoteVersion.GreaterThanOrEqual(v014) && remoteVersion.LessThan(v130) {
+			early1xCompatible, err := version.NewConstraint(fmt.Sprintf(">= 0.14.0, < %s", v130.String()))
+			if err != nil {
+				panic(err)
+			}
+			remoteConstraint = early1xCompatible
 		}
-		// - Any new Terraform state version will require at least minor patch
-		//   increment, so x.y.* will always be compatible with each other
-		tfvs := tfversion.SemVer.Segments64()
-		rwvs := remoteVersion.Segments64()
-		if len(tfvs) == 3 && len(rwvs) == 3 && tfvs[0] == rwvs[0] && tfvs[1] == rwvs[1] {
-			return diags
+
+		// Any future new state format will require at least a minor version
+		// increment, so x.y.* will always be compatible with each other.
+		if remoteVersion.GreaterThanOrEqual(v130) {
+			rwvs := remoteVersion.Segments64()
+			if len(rwvs) >= 3 {
+				// ~> x.y.0
+				minorVersionCompatible, err := version.NewConstraint(fmt.Sprintf("~> %d.%d.0", rwvs[0], rwvs[1]))
+				if err != nil {
+					panic(err)
+				}
+				remoteConstraint = minorVersionCompatible
+			}
 		}
 	}
 
+	fullTfversion := version.Must(version.NewSemver(tfversion.String()))
+
+	if remoteConstraint.Check(fullTfversion) {
+		return diags
+	}
+
 	// Even if ignoring version conflicts, it may still be useful to call this
 	// method and warn the user about a mismatch between the local and remote
 	// Terraform versions.
@@ -1019,6 +1029,19 @@ func (b *Remote) VerifyWorkspaceTerraformVersion(workspaceName string) tfdiags.D
 	return diags
 }
 
+func incompatibleWorkspaceTerraformVersion(message string, ignoreVersionConflict bool) tfdiags.Diagnostic {
+	severity := tfdiags.Error
+	suggestion := ignoreRemoteVersionHelp
+	if ignoreVersionConflict {
+		severity = tfdiags.Warning
+		suggestion = ""
+	}
+	description := strings.TrimSpace(fmt.Sprintf("%s\n\n%s", message, suggestion))
+	return tfdiags.Sourceless(severity, "Incompatible Terraform version", description)
+}
+
+const ignoreRemoteVersionHelp = "If you're sure you want to upgrade the state, you can force Terraform to continue using the -ignore-remote-version flag. This may result in an unusable workspace."
+
 func (b *Remote) IsLocalOperations() bool {
 	return b.forceLocal
 }

internal/backend/remote/backend_test.go

@@ -666,7 +666,7 @@ func TestRemote_VerifyWorkspaceTerraformVersion_workspaceErrors(t *testing.T) {
 	if len(diags) != 1 {
 		t.Fatal("expected diag, but none returned")
 	}
-	if got := diags.Err().Error(); !strings.Contains(got, "Error looking up workspace: Invalid Terraform version") {
+	if got := diags.Err().Error(); !strings.Contains(got, "The remote workspace specified an invalid Terraform version or constraint") {
 		t.Fatalf("unexpected error: %s", got)
 	}
 }