configs: Decode preconditions and postconditions

This allows precondition and postcondition checks to be declared for
resources and output values as long as the preconditions_postconditions
experiment is enabled.

Terraform Core doesn't currently know anything about these features, so
as of this commit declaring them does nothing at all.

internal/configs/checks.go

@@ -64,6 +64,17 @@ func decodeCheckRuleBlock(block *hcl.Block, override bool) (*CheckRule, hcl.Diag
 
 	if attr, exists := content.Attributes["condition"]; exists {
 		cr.Condition = attr.Expr
+
+		if len(cr.Condition.Variables()) == 0 {
+			// A condition expression that doesn't refer to any variable is
+			// pointless, because its result would always be a constant.
+			diags = diags.Append(&hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  fmt.Sprintf("Invalid %s expression", block.Type),
+				Detail:   "The condition expression must refer to at least one object from elsewhere in the configuration, or else its result would not be checking anything.",
+				Subject:  cr.Condition.Range().Ptr(),
+			})
+		}
 	}
 
 	if attr, exists := content.Attributes["error_message"]; exists {

internal/configs/experiments.go

@@ -209,6 +209,55 @@ func checkModuleExperiments(m *Module) hcl.Diagnostics {
 		}
 	}
 
+	if !m.ActiveExperiments.Has(experiments.PreconditionsPostconditions) {
+		for _, r := range m.ManagedResources {
+			for _, c := range r.Preconditions {
+				diags = diags.Append(&hcl.Diagnostic{
+					Severity: hcl.DiagError,
+					Summary:  "Preconditions are experimental",
+					Detail:   "The resource preconditions feature is currently an opt-in experiment, subject to change in future releases based on feedback.\n\nActivate the feature for this module by adding preconditions_postconditions to the list of active experiments.",
+					Subject:  c.DeclRange.Ptr(),
+				})
+			}
+			for _, c := range r.Postconditions {
+				diags = diags.Append(&hcl.Diagnostic{
+					Severity: hcl.DiagError,
+					Summary:  "Postconditions are experimental",
+					Detail:   "The resource preconditions feature is currently an opt-in experiment, subject to change in future releases based on feedback.\n\nActivate the feature for this module by adding preconditions_postconditions to the list of active experiments.",
+					Subject:  c.DeclRange.Ptr(),
+				})
+			}
+		}
+		for _, r := range m.DataResources {
+			for _, c := range r.Preconditions {
+				diags = diags.Append(&hcl.Diagnostic{
+					Severity: hcl.DiagError,
+					Summary:  "Preconditions are experimental",
+					Detail:   "The resource preconditions feature is currently an opt-in experiment, subject to change in future releases based on feedback.\n\nActivate the feature for this module by adding preconditions_postconditions to the list of active experiments.",
+					Subject:  c.DeclRange.Ptr(),
+				})
+			}
+			for _, c := range r.Postconditions {
+				diags = diags.Append(&hcl.Diagnostic{
+					Severity: hcl.DiagError,
+					Summary:  "Postconditions are experimental",
+					Detail:   "The resource preconditions feature is currently an opt-in experiment, subject to change in future releases based on feedback.\n\nActivate the feature for this module by adding preconditions_postconditions to the list of active experiments.",
+					Subject:  c.DeclRange.Ptr(),
+				})
+			}
+		}
+		for _, o := range m.Outputs {
+			for _, c := range o.Preconditions {
+				diags = diags.Append(&hcl.Diagnostic{
+					Severity: hcl.DiagError,
+					Summary:  "Preconditions are experimental",
+					Detail:   "The output value preconditions feature is currently an opt-in experiment, subject to change in future releases based on feedback.\n\nActivate the feature for this module by adding preconditions_postconditions to the list of active experiments.",
+					Subject:  c.DeclRange.Ptr(),
+				})
+			}
+		}
+	}
+
 	return diags
 }
 

internal/configs/named_values.go

@@ -356,6 +356,8 @@ type Output struct {
 	DependsOn   []hcl.Traversal
 	Sensitive   bool
 
+	Preconditions []*CheckRule
+
 	DescriptionSet bool
 	SensitiveSet   bool
 
@@ -409,6 +411,26 @@ func decodeOutputBlock(block *hcl.Block, override bool) (*Output, hcl.Diagnostic
 		o.DependsOn = append(o.DependsOn, deps...)
 	}
 
+	for _, block := range content.Blocks {
+		switch block.Type {
+		case "precondition":
+			cr, moreDiags := decodeCheckRuleBlock(block, override)
+			diags = append(diags, moreDiags...)
+			o.Preconditions = append(o.Preconditions, cr)
+		case "postcondition":
+			diags = append(diags, &hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  "Postconditions are not allowed",
+				Detail:   "Output values can only have preconditions, not postconditions.",
+				Subject:  block.TypeRange.Ptr(),
+			})
+		default:
+			// The cases above should be exhaustive for all block types
+			// defined in the block type schema, so this shouldn't happen.
+			panic(fmt.Sprintf("unexpected lifecycle sub-block type %q", block.Type))
+		}
+	}
+
 	return o, diags
 }
 
@@ -497,4 +519,8 @@ var outputBlockSchema = &hcl.BodySchema{
 			Name: "sensitive",
 		},
 	},
+	Blocks: []hcl.BlockHeaderSchema{
+		{Type: "precondition"},
+		{Type: "postcondition"},
+	},
 }

internal/configs/parser_config.go

@@ -142,14 +142,14 @@ func (p *Parser) loadConfigFile(path string, override bool) (*File, hcl.Diagnost
 			}
 
 		case "resource":
-			cfg, cfgDiags := decodeResourceBlock(block)
+			cfg, cfgDiags := decodeResourceBlock(block, override)
 			diags = append(diags, cfgDiags...)
 			if cfg != nil {
 				file.ManagedResources = append(file.ManagedResources, cfg)
 			}
 
 		case "data":
-			cfg, cfgDiags := decodeDataBlock(block)
+			cfg, cfgDiags := decodeDataBlock(block, override)
 			diags = append(diags, cfgDiags...)
 			if cfg != nil {
 				file.DataResources = append(file.DataResources, cfg)

internal/configs/parser_config_test.go

@@ -97,7 +97,7 @@ func TestParserLoadConfigFileFailureMessages(t *testing.T) {
 		{
 			"invalid-files/data-resource-lifecycle.tf",
 			hcl.DiagError,
-			"Unsupported lifecycle block",
+			"Invalid data resource lifecycle argument",
 		},
 		{
 			"invalid-files/variable-type-unknown.tf",

internal/configs/resource.go

@@ -22,6 +22,9 @@ type Resource struct {
 	ProviderConfigRef *ProviderConfigRef
 	Provider          addrs.Provider
 
+	Preconditions  []*CheckRule
+	Postconditions []*CheckRule
+
 	DependsOn []hcl.Traversal
 
 	// Managed is populated only for Mode = addrs.ManagedResourceMode,
@@ -81,7 +84,7 @@ func (r *Resource) ProviderConfigAddr() addrs.LocalProviderConfig {
 	}
 }
 
-func decodeResourceBlock(block *hcl.Block) (*Resource, hcl.Diagnostics) {
+func decodeResourceBlock(block *hcl.Block, override bool) (*Resource, hcl.Diagnostics) {
 	var diags hcl.Diagnostics
 	r := &Resource{
 		Mode:      addrs.ManagedResourceMode,
@@ -237,6 +240,24 @@ func decodeResourceBlock(block *hcl.Block) (*Resource, hcl.Diagnostics) {
 
 			}
 
+			for _, block := range lcContent.Blocks {
+				switch block.Type {
+				case "precondition", "postcondition":
+					cr, moreDiags := decodeCheckRuleBlock(block, override)
+					diags = append(diags, moreDiags...)
+					switch block.Type {
+					case "precondition":
+						r.Preconditions = append(r.Preconditions, cr)
+					case "postcondition":
+						r.Postconditions = append(r.Postconditions, cr)
+					}
+				default:
+					// The cases above should be exhaustive for all block types
+					// defined in the lifecycle schema, so this shouldn't happen.
+					panic(fmt.Sprintf("unexpected lifecycle sub-block type %q", block.Type))
+				}
+			}
+
 		case "connection":
 			if seenConnection != nil {
 				diags = append(diags, &hcl.Diagnostic{
@@ -307,7 +328,7 @@ func decodeResourceBlock(block *hcl.Block) (*Resource, hcl.Diagnostics) {
 	return r, diags
 }
 
-func decodeDataBlock(block *hcl.Block) (*Resource, hcl.Diagnostics) {
+func decodeDataBlock(block *hcl.Block, override bool) (*Resource, hcl.Diagnostics) {
 	var diags hcl.Diagnostics
 	r := &Resource{
 		Mode:      addrs.DataResourceMode,
@@ -368,6 +389,7 @@ func decodeDataBlock(block *hcl.Block) (*Resource, hcl.Diagnostics) {
 	}
 
 	var seenEscapeBlock *hcl.Block
+	var seenLifecycle *hcl.Block
 	for _, block := range content.Blocks {
 		switch block.Type {
 
@@ -391,21 +413,59 @@ func decodeDataBlock(block *hcl.Block) (*Resource, hcl.Diagnostics) {
 			// will see a blend of both.
 			r.Config = hcl.MergeBodies([]hcl.Body{r.Config, block.Body})
 
-		// The rest of these are just here to reserve block type names for future use.
 		case "lifecycle":
+			if seenLifecycle != nil {
 				diags = append(diags, &hcl.Diagnostic{
 					Severity: hcl.DiagError,
-				Summary:  "Unsupported lifecycle block",
+					Summary:  "Duplicate lifecycle block",
-				Detail:   "Data resources do not have lifecycle settings, so a lifecycle block is not allowed.",
+					Detail:   fmt.Sprintf("This resource already has a lifecycle block at %s.", seenLifecycle.DefRange),
-				Subject:  &block.DefRange,
+					Subject:  block.DefRange.Ptr(),
 				})
+				continue
+			}
+			seenLifecycle = block
+
+			lcContent, lcDiags := block.Body.Content(resourceLifecycleBlockSchema)
+			diags = append(diags, lcDiags...)
+
+			// All of the attributes defined for resource lifecycle are for
+			// managed resources only, so we can emit a common error message
+			// for any given attributes that HCL accepted.
+			for name, attr := range lcContent.Attributes {
+				diags = append(diags, &hcl.Diagnostic{
+					Severity: hcl.DiagError,
+					Summary:  "Invalid data resource lifecycle argument",
+					Detail:   fmt.Sprintf("The lifecycle argument %q is defined only for managed resources (\"resource\" blocks), and is not valid for data resources.", name),
+					Subject:  attr.NameRange.Ptr(),
+				})
+			}
+
+			for _, block := range lcContent.Blocks {
+				switch block.Type {
+				case "precondition", "postcondition":
+					cr, moreDiags := decodeCheckRuleBlock(block, override)
+					diags = append(diags, moreDiags...)
+					switch block.Type {
+					case "precondition":
+						r.Preconditions = append(r.Preconditions, cr)
+					case "postcondition":
+						r.Postconditions = append(r.Postconditions, cr)
+					}
+				default:
+					// The cases above should be exhaustive for all block types
+					// defined in the lifecycle schema, so this shouldn't happen.
+					panic(fmt.Sprintf("unexpected lifecycle sub-block type %q", block.Type))
+				}
+			}
 
 		default:
+			// Any other block types are ones we're reserving for future use,
+			// but don't have any defined meaning today.
 			diags = append(diags, &hcl.Diagnostic{
 				Severity: hcl.DiagError,
 				Summary:  "Reserved block type name in data block",
 				Detail:   fmt.Sprintf("The block type name %q is reserved for use by Terraform in a future version.", block.Type),
-				Subject:  &block.TypeRange,
+				Subject:  block.TypeRange.Ptr(),
 			})
 		}
 	}
@@ -551,13 +611,17 @@ var resourceBlockSchema = &hcl.BodySchema{
 var dataBlockSchema = &hcl.BodySchema{
 	Attributes: commonResourceAttributes,
 	Blocks: []hcl.BlockHeaderSchema{
-		{Type: "lifecycle"}, // reserved for future use
+		{Type: "lifecycle"},
 		{Type: "locals"}, // reserved for future use
 		{Type: "_"},      // meta-argument escaping block
 	},
 }
 
 var resourceLifecycleBlockSchema = &hcl.BodySchema{
+	// We tell HCL that these elements are all valid for both "resource"
+	// and "data" lifecycle blocks, but the rules are actually more restrictive
+	// than that. We deal with that after decoding so that we can return
+	// more specific error messages than HCL would typically return itself.
 	Attributes: []hcl.AttributeSchema{
 		{
 			Name: "create_before_destroy",
@@ -569,4 +633,8 @@ var resourceLifecycleBlockSchema = &hcl.BodySchema{
 			Name: "ignore_changes",
 		},
 	},
+	Blocks: []hcl.BlockHeaderSchema{
+		{Type: "precondition"},
+		{Type: "postcondition"},
+	},
 }

internal/configs/testdata/error-files/precondition-postcondition-constant.tf

@@ -0,0 +1,34 @@
+resource "test" "test" {
+  lifecycle {
+    precondition {
+      condition     = true # ERROR: Invalid precondition expression
+      error_message = "Must be true."
+    }
+    postcondition {
+      condition     = true # ERROR: Invalid postcondition expression
+      error_message = "Must be true."
+    }
+  }
+}
+
+data "test" "test" {
+  lifecycle {
+    precondition {
+      condition     = true # ERROR: Invalid precondition expression
+      error_message = "Must be true."
+    }
+    postcondition {
+      condition     = true # ERROR: Invalid postcondition expression
+      error_message = "Must be true."
+    }
+  }
+}
+
+output "test" {
+  value = ""
+ 
+  precondition {
+    condition     = true # ERROR: Invalid precondition expression
+    error_message = "Must be true."
+  }
+}

internal/configs/testdata/invalid-files/data-reserved-lifecycle.tf

@@ -1,3 +0,0 @@
-data "test" "foo" {
-  lifecycle {}
-}

internal/configs/testdata/invalid-files/data-resource-lifecycle.tf

@@ -1,5 +1,7 @@
 data "example" "example" {
   lifecycle {
-    # This block intentionally left blank
+    # The lifecycle arguments are not valid for data resources:
+    # only the precondition and postcondition blocks are allowed.
+    ignore_changes = []
   }
 }

internal/configs/testdata/invalid-modules/preconditions-postconditions-experiment/preconditions-postconditions-experiment.tf

@@ -0,0 +1,34 @@
+resource "test" "test" {
+  lifecycle {
+    precondition { # ERROR: Preconditions are experimental
+      condition     = path.module != ""
+      error_message = "Must be true."
+    }
+    postcondition { # ERROR: Postconditions are experimental
+      condition     = path.module != ""
+      error_message = "Must be true."
+    }
+  }
+}
+
+data "test" "test" {
+  lifecycle {
+    precondition { # ERROR: Preconditions are experimental
+      condition     = path.module != ""
+      error_message = "Must be true."
+    }
+    postcondition { # ERROR: Postconditions are experimental
+      condition     = path.module != ""
+      error_message = "Must be true."
+    }
+  }
+}
+
+output "test" {
+  value = ""
+
+  precondition { # ERROR: Preconditions are experimental
+    condition     = path.module != ""
+    error_message = "Must be true."
+  }
+}

internal/configs/testdata/warning-files/preconditions-postconditions-experiment.tf

@@ -0,0 +1,38 @@
+terraform {
+  experiments = [preconditions_postconditions] # WARNING: Experimental feature "preconditions_postconditions" is active
+}
+
+resource "test" "test" {
+  lifecycle {
+    precondition {
+      condition     = path.module != ""
+      error_message = "Must be true."
+    }
+    postcondition {
+      condition     = path.module != ""
+      error_message = "Must be true."
+    }
+  }
+}
+
+data "test" "test" {
+  lifecycle {
+    precondition {
+      condition     = path.module != ""
+      error_message = "Must be true."
+    }
+    postcondition {
+      condition     = path.module != ""
+      error_message = "Must be true."
+    }
+  }
+}
+
+output "test" {
+  value = ""
+
+  precondition {
+    condition     = path.module != ""
+    error_message = "Must be true."
+  }
+}