Simplify logic flow: everything is a constraint

Explicit version strings are actually also version constraints! And the special
comparisons we were doing to allow a range of compatible versions can also be
expressed as version constraints.

Bonus: also simplify the way we handle version check errors, by composing the
messages inline and only extracting the repetitive parts into a function.

internal/cloud/backend.go

@@ -781,66 +781,66 @@ func (b *Cloud) VerifyWorkspaceTerraformVersion(workspaceName string) tfdiags.Di
 		return nil
 	}
 
-	// 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.
-	remoteVersion, err := version.NewSemver(workspace.TerraformVersion)
+	remoteConstraint, err := version.NewConstraint(workspace.TerraformVersion)
 	if err != nil {
-		log.Printf("[DEBUG] Invalid Terraform version (%s); will try to parse as version constraint", workspace.TerraformVersion)
+		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
 	}
-	if remoteVersion == nil {
-		// If it's not a valid version, it might be a valid version constraint:
-		remoteConstraint, err := version.NewConstraint(workspace.TerraformVersion)
-		if err != nil {
-			diags = diags.Append(terraformInvalidVersionOrConstraint(b.ignoreVersionConflict, workspace.TerraformVersion))
-			return diags
-		}
 
-		// Avoiding tfversion.SemVer because it omits the prerelease prefix, and we
-		// want constraints like `~> 1.2.0-beta1` to be possible.
-		fullTfversion := version.Must(version.NewSemver(tfversion.String()))
+	// If the workspace has a literal Terraform version, see if we can use a
+	// looser version constraint.
+	remoteVersion, _ := version.NewSemver(workspace.TerraformVersion)
+	if remoteVersion != nil {
+		v014 := version.Must(version.NewSemver("0.14.0"))
+		v120 := version.Must(version.NewSemver("1.2.0"))
 
-		// If it's a constraint, we only ensure that the local version meets it.
-		// This can result in both false positives and false negatives, but in the
-		// most common case (~> x.y.z) it's useful enough.
-		if remoteConstraint.Check(fullTfversion) {
-			return diags
+		// Versions from 0.14 through the early 1.x series should be compatible
+		// (though we don't know about 1.2 yet).
+		if remoteVersion.GreaterThanOrEqual(v014) && remoteVersion.LessThan(v120) {
+			early1xCompatible, err := version.NewConstraint(fmt.Sprintf(">= 0.14.0, < %s", v120.String()))
+			if err != nil {
+				panic(err)
+			}
+			remoteConstraint = early1xCompatible
 		}
 
-		diags = diags.Append(terraformMismatchDiagnostic(b.ignoreVersionConflict, b.organization, workspace, tfversion.String()))
-		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
+		// 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(v120) {
+			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
+			}
 		}
 	}
-	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 1.2.0
-		v120 := version.Must(version.NewSemver("1.2.0"))
-		if tfversion.SemVer.LessThan(v120) && remoteVersion.LessThan(v120) {
-			return diags
-		}
-		// - 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
-		}
+
+	// Re-parsing tfversion.String because tfversion.SemVer omits the prerelease
+	// prefix, and we want to allow constraints like `~> 1.2.0-beta1`.
+	fullTfversion := version.Must(version.NewSemver(tfversion.String()))
+
+	if remoteConstraint.Check(fullTfversion) {
+		return diags
 	}
 
-	diags = diags.Append(terraformMismatchDiagnostic(b.ignoreVersionConflict, b.organization, workspace, tfversion.String()))
+	message := fmt.Sprintf(
+		"The local Terraform version (%s) does not meet the version requirements for remote workspace %s/%s (%s).",
+		tfversion.String(),
+		b.organization,
+		workspace.Name,
+		workspace.TerraformVersion,
+	)
+	diags = diags.Append(incompatibleWorkspaceTerraformVersion(message, b.ignoreVersionConflict))
 	return diags
 }
 

internal/cloud/backend_test.go

@@ -656,7 +656,7 @@ func TestCloud_VerifyWorkspaceTerraformVersion(t *testing.T) {
 				if len(diags) != 1 {
 					t.Fatal("expected diag, but none returned")
 				}
-				if got := diags.Err().Error(); !strings.Contains(got, "Terraform version mismatch") {
+				if got := diags.Err().Error(); !strings.Contains(got, "Incompatible Terraform version") {
 					t.Fatalf("unexpected error: %s", got)
 				}
 			} else {
@@ -705,7 +705,7 @@ func TestCloud_VerifyWorkspaceTerraformVersion_workspaceErrors(t *testing.T) {
 	if len(diags) != 1 {
 		t.Fatal("expected diag, but none returned")
 	}
-	if got := diags.Err().Error(); !strings.Contains(got, "Terraform version error: The remote workspace specified") {
+	if got := diags.Err().Error(); !strings.Contains(got, "Incompatible Terraform version: The remote workspace specified") {
 		t.Fatalf("unexpected error: %s", got)
 	}
 }
@@ -757,7 +757,7 @@ func TestCloud_VerifyWorkspaceTerraformVersion_ignoreFlagSet(t *testing.T) {
 	if got, want := diags[0].Severity(), tfdiags.Warning; got != want {
 		t.Errorf("wrong severity: got %#v, want %#v", got, want)
 	}
-	if got, want := diags[0].Description().Summary, "Terraform version mismatch"; got != want {
+	if got, want := diags[0].Description().Summary, "Incompatible Terraform version"; got != want {
 		t.Errorf("wrong summary: got %s, want %s", got, want)
 	}
 	wantDetail := "The local Terraform version (0.14.0) does not meet the version requirements for remote workspace hashicorp/app-prod (0.13.5)."

internal/cloud/errors.go

@@ -4,7 +4,6 @@ import (
 	"fmt"
 	"strings"
 
-	tfe "github.com/hashicorp/go-tfe"
 	"github.com/hashicorp/terraform/internal/tfdiags"
 	"github.com/zclconf/go-cty/cty"
 )
@@ -32,41 +31,15 @@ var (
 	)
 )
 
-func terraformMismatchDiagnostic(ignoreVersionConflict bool, organization string, workspace *tfe.Workspace, tfversion string) tfdiags.Diagnostic {
-	severity := tfdiags.Error
-	if ignoreVersionConflict {
-		severity = tfdiags.Warning
-	}
-
-	suggestion := "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."
-	if ignoreVersionConflict {
-		suggestion = ""
-	}
+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."
 
-	description := fmt.Sprintf(
-		"The local Terraform version (%s) does not meet the version requirements for remote workspace %s/%s (%s).\n\n%s",
-		tfversion,
-		organization,
-		workspace.Name,
-		workspace.TerraformVersion,
-		suggestion,
-	)
-	description = strings.TrimSpace(description)
-	return tfdiags.Sourceless(severity, "Terraform version mismatch", description)
-}
-
-func terraformInvalidVersionOrConstraint(ignoreVersionConflict bool, tfversion string) tfdiags.Diagnostic {
+func incompatibleWorkspaceTerraformVersion(message string, ignoreVersionConflict bool) tfdiags.Diagnostic {
 	severity := tfdiags.Error
+	suggestion := ignoreRemoteVersionHelp
 	if ignoreVersionConflict {
 		severity = tfdiags.Warning
-	}
-
-	suggestion := "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."
-	if ignoreVersionConflict {
 		suggestion = ""
 	}
-
-	description := fmt.Sprintf("The remote workspace specified an invalid Terraform version or version constraint: %s\n\n%s", tfversion, suggestion)
-	description = strings.TrimSpace(description)
-	return tfdiags.Sourceless(severity, "Terraform version error", description)
+	description := strings.TrimSpace(fmt.Sprintf("%s\n\n%s", message, suggestion))
+	return tfdiags.Sourceless(severity, "Incompatible Terraform version", description)
 }