core: Report reason for deferring data read until apply

We have two different reasons why a data resource might be read only
during apply, rather than during planning as usual: the configuration
contains unknown values, or the data resource as a whole depends on a
managed resource which itself has a change pending.

However, we didn't previously distinguish these two in a way that allowed
the UI to describe the difference, and so we confusingly reported both
as "config refers to values not yet known", which in turn led to a number
of reasonable questions about why Terraform was claiming that but then
immediately below showing the configuration entirely known.

Now we'll use our existing "ActionReason" mechanism to tell the UI layer
which of the two reasons applies to a particular data resource instance.
The "dependency pending" situation tends to happen in conjunction with
"config unknown", so we'll prefer to refer that the configuration is
unknown if both are true.

internal/command/format/diff.go

@@ -71,7 +71,13 @@ func ResourceChange(
 	case plans.Create:
 		buf.WriteString(fmt.Sprintf(color.Color("[bold]  # %s[reset] will be created"), dispAddr))
 	case plans.Read:
-		buf.WriteString(fmt.Sprintf(color.Color("[bold]  # %s[reset] will be read during apply\n  # (config refers to values not yet known)"), dispAddr))
+		buf.WriteString(fmt.Sprintf(color.Color("[bold]  # %s[reset] will be read during apply"), dispAddr))
+		switch change.ActionReason {
+		case plans.ResourceInstanceReadBecauseConfigUnknown:
+			buf.WriteString("\n  # (config refers to values not yet known)")
+		case plans.ResourceInstanceReadBecauseDependencyPending:
+			buf.WriteString("\n  # (depends on a resource or a module with changes pending)")
+		}
 	case plans.Update:
 		switch language {
 		case DiffLanguageProposedChange:
@@ -166,7 +172,7 @@ func ResourceChange(
 		))
 	case addrs.DataResourceMode:
 		buf.WriteString(fmt.Sprintf(
-			"data %q %q ",
+			"data %q %q",
 			addr.Resource.Resource.Type,
 			addr.Resource.Resource.Name,
 		))

internal/command/format/diff_test.go

@@ -532,6 +532,70 @@ new line
       + forced = "example" # forces replacement
         name   = "name"
     }
+`,
+		},
+		"read during apply because of unknown configuration": {
+			Action:       plans.Read,
+			ActionReason: plans.ResourceInstanceReadBecauseConfigUnknown,
+			Mode:         addrs.DataResourceMode,
+			Before: cty.ObjectVal(map[string]cty.Value{
+				"name": cty.StringVal("name"),
+			}),
+			After: cty.ObjectVal(map[string]cty.Value{
+				"name": cty.StringVal("name"),
+			}),
+			Schema: &configschema.Block{
+				Attributes: map[string]*configschema.Attribute{
+					"name": {Type: cty.String, Optional: true},
+				},
+			},
+			ExpectedOutput: `  # data.test_instance.example will be read during apply
+  # (config refers to values not yet known)
+ <= data "test_instance" "example" {
+        name = "name"
+    }
+`,
+		},
+		"read during apply because of pending changes to upstream dependency": {
+			Action:       plans.Read,
+			ActionReason: plans.ResourceInstanceReadBecauseDependencyPending,
+			Mode:         addrs.DataResourceMode,
+			Before: cty.ObjectVal(map[string]cty.Value{
+				"name": cty.StringVal("name"),
+			}),
+			After: cty.ObjectVal(map[string]cty.Value{
+				"name": cty.StringVal("name"),
+			}),
+			Schema: &configschema.Block{
+				Attributes: map[string]*configschema.Attribute{
+					"name": {Type: cty.String, Optional: true},
+				},
+			},
+			ExpectedOutput: `  # data.test_instance.example will be read during apply
+  # (depends on a resource or a module with changes pending)
+ <= data "test_instance" "example" {
+        name = "name"
+    }
+`,
+		},
+		"read during apply for unspecified reason": {
+			Action: plans.Read,
+			Mode:   addrs.DataResourceMode,
+			Before: cty.ObjectVal(map[string]cty.Value{
+				"name": cty.StringVal("name"),
+			}),
+			After: cty.ObjectVal(map[string]cty.Value{
+				"name": cty.StringVal("name"),
+			}),
+			Schema: &configschema.Block{
+				Attributes: map[string]*configschema.Attribute{
+					"name": {Type: cty.String, Optional: true},
+				},
+			},
+			ExpectedOutput: `  # data.test_instance.example will be read during apply
+ <= data "test_instance" "example" {
+        name = "name"
+    }
 `,
 		},
 		"show all identifying attributes even if unchanged": {

internal/command/jsonplan/plan.go

@@ -405,6 +405,10 @@ func (p *plan) marshalResourceChanges(resources []*plans.ResourceInstanceChangeS
 			r.ActionReason = "delete_because_each_key"
 		case plans.ResourceInstanceDeleteBecauseNoModule:
 			r.ActionReason = "delete_because_no_module"
+		case plans.ResourceInstanceReadBecauseConfigUnknown:
+			r.ActionReason = "read_because_config_unknown"
+		case plans.ResourceInstanceReadBecauseDependencyPending:
+			r.ActionReason = "read_because_dependency_pending"
 		default:
 			return nil, fmt.Errorf("resource %s has an unsupported action reason %s", r.Address, rc.ActionReason)
 		}

internal/command/views/json/change.go

@@ -80,6 +80,8 @@ const (
 	ReasonDeleteBecauseCountIndex       ChangeReason = "delete_because_count_index"
 	ReasonDeleteBecauseEachKey          ChangeReason = "delete_because_each_key"
 	ReasonDeleteBecauseNoModule         ChangeReason = "delete_because_no_module"
+	ReasonReadBecauseConfigUnknown      ChangeReason = "read_because_config_unknown"
+	ReasonReadBecauseDependencyPending  ChangeReason = "read_because_dependency_pending"
 )
 
 func changeReason(reason plans.ResourceInstanceChangeActionReason) ChangeReason {
@@ -104,6 +106,10 @@ func changeReason(reason plans.ResourceInstanceChangeActionReason) ChangeReason
 		return ReasonDeleteBecauseEachKey
 	case plans.ResourceInstanceDeleteBecauseNoModule:
 		return ReasonDeleteBecauseNoModule
+	case plans.ResourceInstanceReadBecauseConfigUnknown:
+		return ReasonReadBecauseConfigUnknown
+	case plans.ResourceInstanceReadBecauseDependencyPending:
+		return ReasonReadBecauseDependencyPending
 	default:
 		// This should never happen, but there's no good way to guarantee
 		// exhaustive handling of the enum, so a generic fall back is better

internal/plans/changes.go

@@ -407,6 +407,18 @@ const (
 	// potentially multiple nested modules could all contribute conflicting
 	// specific reasons for a particular instance to no longer be declared.
 	ResourceInstanceDeleteBecauseNoModule ResourceInstanceChangeActionReason = 'M'
+
+	// ResourceInstanceReadBecauseConfigUnknown indicates that the resource
+	// must be read during apply (rather than during planning) because its
+	// configuration contains unknown values. This reason applies only to
+	// data resources.
+	ResourceInstanceReadBecauseConfigUnknown ResourceInstanceChangeActionReason = '?'
+
+	// ResourceInstanceReadBecauseDependencyPending indicates that the resource
+	// must be read during apply (rather than during planning) because it
+	// depends on a managed resource instance which has its own changes
+	// pending.
+	ResourceInstanceReadBecauseDependencyPending ResourceInstanceChangeActionReason = '!'
 )
 
 // OutputChange describes a change to an output value.

internal/plans/internal/planproto/planfile.pb.go

@@ -150,21 +150,25 @@ const (
 	ResourceInstanceActionReason_DELETE_BECAUSE_EACH_KEY           ResourceInstanceActionReason = 7
 	ResourceInstanceActionReason_DELETE_BECAUSE_NO_MODULE          ResourceInstanceActionReason = 8
 	ResourceInstanceActionReason_REPLACE_BY_TRIGGERS               ResourceInstanceActionReason = 9
+	ResourceInstanceActionReason_READ_BECAUSE_CONFIG_UNKNOWN       ResourceInstanceActionReason = 10
+	ResourceInstanceActionReason_READ_BECAUSE_DEPENDENCY_PENDING   ResourceInstanceActionReason = 11
 )
 
 // Enum value maps for ResourceInstanceActionReason.
 var (
 	ResourceInstanceActionReason_name = map[int32]string{
-		0: "NONE",
+		0:  "NONE",
-		1: "REPLACE_BECAUSE_TAINTED",
+		1:  "REPLACE_BECAUSE_TAINTED",
-		2: "REPLACE_BY_REQUEST",
+		2:  "REPLACE_BY_REQUEST",
-		3: "REPLACE_BECAUSE_CANNOT_UPDATE",
+		3:  "REPLACE_BECAUSE_CANNOT_UPDATE",
-		4: "DELETE_BECAUSE_NO_RESOURCE_CONFIG",
+		4:  "DELETE_BECAUSE_NO_RESOURCE_CONFIG",
-		5: "DELETE_BECAUSE_WRONG_REPETITION",
+		5:  "DELETE_BECAUSE_WRONG_REPETITION",
-		6: "DELETE_BECAUSE_COUNT_INDEX",
+		6:  "DELETE_BECAUSE_COUNT_INDEX",
-		7: "DELETE_BECAUSE_EACH_KEY",
+		7:  "DELETE_BECAUSE_EACH_KEY",
-		8: "DELETE_BECAUSE_NO_MODULE",
+		8:  "DELETE_BECAUSE_NO_MODULE",
-		9: "REPLACE_BY_TRIGGERS",
+		9:  "REPLACE_BY_TRIGGERS",
+		10: "READ_BECAUSE_CONFIG_UNKNOWN",
+		11: "READ_BECAUSE_DEPENDENCY_PENDING",
 	}
 	ResourceInstanceActionReason_value = map[string]int32{
 		"NONE":                              0,
@@ -177,6 +181,8 @@ var (
 		"DELETE_BECAUSE_EACH_KEY":           7,
 		"DELETE_BECAUSE_NO_MODULE":          8,
 		"REPLACE_BY_TRIGGERS":               9,
+		"READ_BECAUSE_CONFIG_UNKNOWN":       10,
+		"READ_BECAUSE_DEPENDENCY_PENDING":   11,
 	}
 )
 
@@ -1300,7 +1306,7 @@ var file_planfile_proto_rawDesc = []byte{
 	0x45, 0x10, 0x05, 0x12, 0x16, 0x0a, 0x12, 0x44, 0x45, 0x4c, 0x45, 0x54, 0x45, 0x5f, 0x54, 0x48,
 	0x45, 0x4e, 0x5f, 0x43, 0x52, 0x45, 0x41, 0x54, 0x45, 0x10, 0x06, 0x12, 0x16, 0x0a, 0x12, 0x43,
 	0x52, 0x45, 0x41, 0x54, 0x45, 0x5f, 0x54, 0x48, 0x45, 0x4e, 0x5f, 0x44, 0x45, 0x4c, 0x45, 0x54,
-	0x45, 0x10, 0x07, 0x2a, 0xc0, 0x02, 0x0a, 0x1c, 0x52, 0x65, 0x73, 0x6f, 0x75, 0x72, 0x63, 0x65,
+	0x45, 0x10, 0x07, 0x2a, 0x86, 0x03, 0x0a, 0x1c, 0x52, 0x65, 0x73, 0x6f, 0x75, 0x72, 0x63, 0x65,
 	0x49, 0x6e, 0x73, 0x74, 0x61, 0x6e, 0x63, 0x65, 0x41, 0x63, 0x74, 0x69, 0x6f, 0x6e, 0x52, 0x65,
 	0x61, 0x73, 0x6f, 0x6e, 0x12, 0x08, 0x0a, 0x04, 0x4e, 0x4f, 0x4e, 0x45, 0x10, 0x00, 0x12, 0x1b,
 	0x0a, 0x17, 0x52, 0x45, 0x50, 0x4c, 0x41, 0x43, 0x45, 0x5f, 0x42, 0x45, 0x43, 0x41, 0x55, 0x53,
@@ -1320,18 +1326,23 @@ var file_planfile_proto_rawDesc = []byte{
 	0x1c, 0x0a, 0x18, 0x44, 0x45, 0x4c, 0x45, 0x54, 0x45, 0x5f, 0x42, 0x45, 0x43, 0x41, 0x55, 0x53,
 	0x45, 0x5f, 0x4e, 0x4f, 0x5f, 0x4d, 0x4f, 0x44, 0x55, 0x4c, 0x45, 0x10, 0x08, 0x12, 0x17, 0x0a,
 	0x13, 0x52, 0x45, 0x50, 0x4c, 0x41, 0x43, 0x45, 0x5f, 0x42, 0x59, 0x5f, 0x54, 0x52, 0x49, 0x47,
-	0x47, 0x45, 0x52, 0x53, 0x10, 0x09, 0x2a, 0x6c, 0x0a, 0x0d, 0x43, 0x6f, 0x6e, 0x64, 0x69, 0x74,
+	0x47, 0x45, 0x52, 0x53, 0x10, 0x09, 0x12, 0x1f, 0x0a, 0x1b, 0x52, 0x45, 0x41, 0x44, 0x5f, 0x42,
-	0x69, 0x6f, 0x6e, 0x54, 0x79, 0x70, 0x65, 0x12, 0x0b, 0x0a, 0x07, 0x49, 0x4e, 0x56, 0x41, 0x4c,
+	0x45, 0x43, 0x41, 0x55, 0x53, 0x45, 0x5f, 0x43, 0x4f, 0x4e, 0x46, 0x49, 0x47, 0x5f, 0x55, 0x4e,
-	0x49, 0x44, 0x10, 0x00, 0x12, 0x19, 0x0a, 0x15, 0x52, 0x45, 0x53, 0x4f, 0x55, 0x52, 0x43, 0x45,
+	0x4b, 0x4e, 0x4f, 0x57, 0x4e, 0x10, 0x0a, 0x12, 0x23, 0x0a, 0x1f, 0x52, 0x45, 0x41, 0x44, 0x5f,
-	0x5f, 0x50, 0x52, 0x45, 0x43, 0x4f, 0x4e, 0x44, 0x49, 0x54, 0x49, 0x4f, 0x4e, 0x10, 0x01, 0x12,
+	0x42, 0x45, 0x43, 0x41, 0x55, 0x53, 0x45, 0x5f, 0x44, 0x45, 0x50, 0x45, 0x4e, 0x44, 0x45, 0x4e,
-	0x1a, 0x0a, 0x16, 0x52, 0x45, 0x53, 0x4f, 0x55, 0x52, 0x43, 0x45, 0x5f, 0x50, 0x4f, 0x53, 0x54,
+	0x43, 0x59, 0x5f, 0x50, 0x45, 0x4e, 0x44, 0x49, 0x4e, 0x47, 0x10, 0x0b, 0x2a, 0x6c, 0x0a, 0x0d,
-	0x43, 0x4f, 0x4e, 0x44, 0x49, 0x54, 0x49, 0x4f, 0x4e, 0x10, 0x02, 0x12, 0x17, 0x0a, 0x13, 0x4f,
+	0x43, 0x6f, 0x6e, 0x64, 0x69, 0x74, 0x69, 0x6f, 0x6e, 0x54, 0x79, 0x70, 0x65, 0x12, 0x0b, 0x0a,
-	0x55, 0x54, 0x50, 0x55, 0x54, 0x5f, 0x50, 0x52, 0x45, 0x43, 0x4f, 0x4e, 0x44, 0x49, 0x54, 0x49,
+	0x07, 0x49, 0x4e, 0x56, 0x41, 0x4c, 0x49, 0x44, 0x10, 0x00, 0x12, 0x19, 0x0a, 0x15, 0x52, 0x45,
-	0x4f, 0x4e, 0x10, 0x03, 0x42, 0x42, 0x5a, 0x40, 0x67, 0x69, 0x74, 0x68, 0x75, 0x62, 0x2e, 0x63,
+	0x53, 0x4f, 0x55, 0x52, 0x43, 0x45, 0x5f, 0x50, 0x52, 0x45, 0x43, 0x4f, 0x4e, 0x44, 0x49, 0x54,
-	0x6f, 0x6d, 0x2f, 0x68, 0x61, 0x73, 0x68, 0x69, 0x63, 0x6f, 0x72, 0x70, 0x2f, 0x74, 0x65, 0x72,
+	0x49, 0x4f, 0x4e, 0x10, 0x01, 0x12, 0x1a, 0x0a, 0x16, 0x52, 0x45, 0x53, 0x4f, 0x55, 0x52, 0x43,
-	0x72, 0x61, 0x66, 0x6f, 0x72, 0x6d, 0x2f, 0x69, 0x6e, 0x74, 0x65, 0x72, 0x6e, 0x61, 0x6c, 0x2f,
+	0x45, 0x5f, 0x50, 0x4f, 0x53, 0x54, 0x43, 0x4f, 0x4e, 0x44, 0x49, 0x54, 0x49, 0x4f, 0x4e, 0x10,
-	0x70, 0x6c, 0x61, 0x6e, 0x73, 0x2f, 0x69, 0x6e, 0x74, 0x65, 0x72, 0x6e, 0x61, 0x6c, 0x2f, 0x70,
+	0x02, 0x12, 0x17, 0x0a, 0x13, 0x4f, 0x55, 0x54, 0x50, 0x55, 0x54, 0x5f, 0x50, 0x52, 0x45, 0x43,
-	0x6c, 0x61, 0x6e, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x62, 0x06, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x33,
+	0x4f, 0x4e, 0x44, 0x49, 0x54, 0x49, 0x4f, 0x4e, 0x10, 0x03, 0x42, 0x42, 0x5a, 0x40, 0x67, 0x69,
+	0x74, 0x68, 0x75, 0x62, 0x2e, 0x63, 0x6f, 0x6d, 0x2f, 0x68, 0x61, 0x73, 0x68, 0x69, 0x63, 0x6f,
+	0x72, 0x70, 0x2f, 0x74, 0x65, 0x72, 0x72, 0x61, 0x66, 0x6f, 0x72, 0x6d, 0x2f, 0x69, 0x6e, 0x74,
+	0x65, 0x72, 0x6e, 0x61, 0x6c, 0x2f, 0x70, 0x6c, 0x61, 0x6e, 0x73, 0x2f, 0x69, 0x6e, 0x74, 0x65,
+	0x72, 0x6e, 0x61, 0x6c, 0x2f, 0x70, 0x6c, 0x61, 0x6e, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x62, 0x06,
+	0x70, 0x72, 0x6f, 0x74, 0x6f, 0x33,
 }
 
 var (

internal/plans/internal/planproto/planfile.proto

@@ -147,6 +147,8 @@ enum ResourceInstanceActionReason {
     DELETE_BECAUSE_EACH_KEY = 7;
     DELETE_BECAUSE_NO_MODULE = 8;
     REPLACE_BY_TRIGGERS = 9;
+    READ_BECAUSE_CONFIG_UNKNOWN = 10;
+    READ_BECAUSE_DEPENDENCY_PENDING = 11;
 }
 
 message ResourceInstanceChange {

internal/plans/planfile/tfplan.go

@@ -278,6 +278,10 @@ func resourceChangeFromTfplan(rawChange *planproto.ResourceInstanceChange) (*pla
 		ret.ActionReason = plans.ResourceInstanceDeleteBecauseEachKey
 	case planproto.ResourceInstanceActionReason_DELETE_BECAUSE_NO_MODULE:
 		ret.ActionReason = plans.ResourceInstanceDeleteBecauseNoModule
+	case planproto.ResourceInstanceActionReason_READ_BECAUSE_CONFIG_UNKNOWN:
+		ret.ActionReason = plans.ResourceInstanceReadBecauseConfigUnknown
+	case planproto.ResourceInstanceActionReason_READ_BECAUSE_DEPENDENCY_PENDING:
+		ret.ActionReason = plans.ResourceInstanceReadBecauseDependencyPending
 	default:
 		return nil, fmt.Errorf("resource has invalid action reason %s", rawChange.ActionReason)
 	}
@@ -625,6 +629,10 @@ func resourceChangeToTfplan(change *plans.ResourceInstanceChangeSrc) (*planproto
 		ret.ActionReason = planproto.ResourceInstanceActionReason_DELETE_BECAUSE_EACH_KEY
 	case plans.ResourceInstanceDeleteBecauseNoModule:
 		ret.ActionReason = planproto.ResourceInstanceActionReason_DELETE_BECAUSE_NO_MODULE
+	case plans.ResourceInstanceReadBecauseConfigUnknown:
+		ret.ActionReason = planproto.ResourceInstanceActionReason_READ_BECAUSE_CONFIG_UNKNOWN
+	case plans.ResourceInstanceReadBecauseDependencyPending:
+		ret.ActionReason = planproto.ResourceInstanceActionReason_READ_BECAUSE_DEPENDENCY_PENDING
 	default:
 		return nil, fmt.Errorf("resource %s has unsupported action reason %s", change.Addr, change.ActionReason)
 	}

internal/plans/resourceinstancechangeactionreason_string.go

@@ -18,38 +18,46 @@ func _() {
 	_ = x[ResourceInstanceDeleteBecauseCountIndex-67]
 	_ = x[ResourceInstanceDeleteBecauseEachKey-69]
 	_ = x[ResourceInstanceDeleteBecauseNoModule-77]
+	_ = x[ResourceInstanceReadBecauseConfigUnknown-63]
+	_ = x[ResourceInstanceReadBecauseDependencyPending-33]
 }
 
 const (
 	_ResourceInstanceChangeActionReason_name_0 = "ResourceInstanceChangeNoReason"
-	_ResourceInstanceChangeActionReason_name_1 = "ResourceInstanceDeleteBecauseCountIndexResourceInstanceReplaceByTriggersResourceInstanceDeleteBecauseEachKeyResourceInstanceReplaceBecauseCannotUpdate"
+	_ResourceInstanceChangeActionReason_name_1 = "ResourceInstanceReadBecauseDependencyPending"
-	_ResourceInstanceChangeActionReason_name_2 = "ResourceInstanceDeleteBecauseNoModuleResourceInstanceDeleteBecauseNoResourceConfig"
+	_ResourceInstanceChangeActionReason_name_2 = "ResourceInstanceReadBecauseConfigUnknown"
-	_ResourceInstanceChangeActionReason_name_3 = "ResourceInstanceReplaceByRequest"
+	_ResourceInstanceChangeActionReason_name_3 = "ResourceInstanceDeleteBecauseCountIndexResourceInstanceReplaceByTriggersResourceInstanceDeleteBecauseEachKeyResourceInstanceReplaceBecauseCannotUpdate"
-	_ResourceInstanceChangeActionReason_name_4 = "ResourceInstanceReplaceBecauseTainted"
+	_ResourceInstanceChangeActionReason_name_4 = "ResourceInstanceDeleteBecauseNoModuleResourceInstanceDeleteBecauseNoResourceConfig"
-	_ResourceInstanceChangeActionReason_name_5 = "ResourceInstanceDeleteBecauseWrongRepetition"
+	_ResourceInstanceChangeActionReason_name_5 = "ResourceInstanceReplaceByRequest"
+	_ResourceInstanceChangeActionReason_name_6 = "ResourceInstanceReplaceBecauseTainted"
+	_ResourceInstanceChangeActionReason_name_7 = "ResourceInstanceDeleteBecauseWrongRepetition"
 )
 
 var (
-	_ResourceInstanceChangeActionReason_index_1 = [...]uint8{0, 39, 72, 108, 150}
+	_ResourceInstanceChangeActionReason_index_3 = [...]uint8{0, 39, 72, 108, 150}
-	_ResourceInstanceChangeActionReason_index_2 = [...]uint8{0, 37, 82}
+	_ResourceInstanceChangeActionReason_index_4 = [...]uint8{0, 37, 82}
 )
 
 func (i ResourceInstanceChangeActionReason) String() string {
 	switch {
 	case i == 0:
 		return _ResourceInstanceChangeActionReason_name_0
+	case i == 33:
+		return _ResourceInstanceChangeActionReason_name_1
+	case i == 63:
+		return _ResourceInstanceChangeActionReason_name_2
 	case 67 <= i && i <= 70:
 		i -= 67
-		return _ResourceInstanceChangeActionReason_name_1[_ResourceInstanceChangeActionReason_index_1[i]:_ResourceInstanceChangeActionReason_index_1[i+1]]
+		return _ResourceInstanceChangeActionReason_name_3[_ResourceInstanceChangeActionReason_index_3[i]:_ResourceInstanceChangeActionReason_index_3[i+1]]
 	case 77 <= i && i <= 78:
 		i -= 77
-		return _ResourceInstanceChangeActionReason_name_2[_ResourceInstanceChangeActionReason_index_2[i]:_ResourceInstanceChangeActionReason_index_2[i+1]]
+		return _ResourceInstanceChangeActionReason_name_4[_ResourceInstanceChangeActionReason_index_4[i]:_ResourceInstanceChangeActionReason_index_4[i+1]]
 	case i == 82:
-		return _ResourceInstanceChangeActionReason_name_3
-	case i == 84:
-		return _ResourceInstanceChangeActionReason_name_4
-	case i == 87:
 		return _ResourceInstanceChangeActionReason_name_5
+	case i == 84:
+		return _ResourceInstanceChangeActionReason_name_6
+	case i == 87:
+		return _ResourceInstanceChangeActionReason_name_7
 	default:
 		return "ResourceInstanceChangeActionReason(" + strconv.FormatInt(int64(i), 10) + ")"
 	}

internal/terraform/context_plan_test.go

@@ -1786,6 +1786,9 @@ func TestContext2Plan_computedDataResource(t *testing.T) {
 		}),
 		rc.After,
 	)
+	if got, want := rc.ActionReason, plans.ResourceInstanceReadBecauseConfigUnknown; got != want {
+		t.Errorf("wrong ActionReason\ngot:  %s\nwant: %s", got, want)
+	}
 }
 
 func TestContext2Plan_computedInFunction(t *testing.T) {
@@ -1987,6 +1990,10 @@ func TestContext2Plan_dataResourceBecomesComputed(t *testing.T) {
 		t.Fatal(err)
 	}
 
+	if got, want := rc.ActionReason, plans.ResourceInstanceReadBecauseConfigUnknown; got != want {
+		t.Errorf("wrong ActionReason\ngot:  %s\nwant: %s", got, want)
+	}
+
 	// foo should now be unknown
 	foo := rc.After.GetAttr("foo")
 	if foo.IsKnown() {
@@ -6295,8 +6302,21 @@ data "test_data_source" "e" {
 		},
 	})
 
-	_, diags := ctx.Plan(m, states.NewState(), DefaultPlanOpts)
+	plan, diags := ctx.Plan(m, states.NewState(), DefaultPlanOpts)
 	assertNoErrors(t, diags)
+
+	rc := plan.Changes.ResourceInstance(addrs.Resource{
+		Mode: addrs.DataResourceMode,
+		Type: "test_data_source",
+		Name: "d",
+	}.Instance(addrs.NoKey).Absolute(addrs.RootModuleInstance))
+	if rc != nil {
+		if got, want := rc.ActionReason, plans.ResourceInstanceReadBecauseDependencyPending; got != want {
+			t.Errorf("wrong ActionReason\ngot:  %s\nwant: %s", got, want)
+		}
+	} else {
+		t.Error("no change for test_data_source.e")
+	}
 }
 
 func TestContext2Plan_skipRefresh(t *testing.T) {

internal/terraform/node_resource_abstract_instance.go

@@ -1525,15 +1525,23 @@ func (n *NodeAbstractResourceInstance) planDataSource(ctx EvalContext, checkRule
 	unmarkedConfigVal, configMarkPaths := configVal.UnmarkDeepWithPaths()
 
 	configKnown := configVal.IsWhollyKnown()
+	depsPending := n.dependenciesHavePendingChanges(ctx)
 	// If our configuration contains any unknown values, or we depend on any
 	// unknown values then we must defer the read to the apply phase by
 	// producing a "Read" change for this resource, and a placeholder value for
 	// it in the state.
-	if n.forcePlanReadData(ctx) || !configKnown {
+	if depsPending || !configKnown {
-		if configKnown {
+		var reason plans.ResourceInstanceChangeActionReason
-			log.Printf("[TRACE] planDataSource: %s configuration is fully known, but we're forcing a read plan to be created", n.Addr)
+		switch {
-		} else {
+		case !configKnown:
 			log.Printf("[TRACE] planDataSource: %s configuration not fully known yet, so deferring to apply phase", n.Addr)
+			reason = plans.ResourceInstanceReadBecauseConfigUnknown
+		case depsPending:
+			// NOTE: depsPending can be true at the same time as configKnown
+			// is false; configKnown takes precedence because it's more
+			// specific.
+			log.Printf("[TRACE] planDataSource: %s configuration is fully known, at least one dependency has changes pending", n.Addr)
+			reason = plans.ResourceInstanceReadBecauseDependencyPending
 		}
 
 		proposedNewVal := objchange.PlannedDataResourceObject(schema, unmarkedConfigVal)
@@ -1550,6 +1558,7 @@ func (n *NodeAbstractResourceInstance) planDataSource(ctx EvalContext, checkRule
 				Before: priorVal,
 				After:  proposedNewVal,
 			},
+			ActionReason: reason,
 		}
 
 		plannedNewState := &states.ResourceInstanceObject{
@@ -1580,10 +1589,11 @@ func (n *NodeAbstractResourceInstance) planDataSource(ctx EvalContext, checkRule
 	return nil, plannedNewState, keyData, diags
 }
 
-// forcePlanReadData determines if we need to override the usual behavior of
+// dependenciesHavePendingChanges determines whether any managed resource the
-// immediately reading from the data source where possible, instead forcing us
+// receiver depends on has a change pending in the plan, in which case we'd
-// to generate a plan.
+// need to override the usual behavior of immediately reading from the data
-func (n *NodeAbstractResourceInstance) forcePlanReadData(ctx EvalContext) bool {
+// source where possible, and instead defer the read until the apply step.
+func (n *NodeAbstractResourceInstance) dependenciesHavePendingChanges(ctx EvalContext) bool {
 	nModInst := n.Addr.Module
 	nMod := nModInst.Module()
 

website/docs/internals/json-format.mdx

@@ -173,6 +173,13 @@ For ease of consumption by callers, the plan representation includes a partial r
       // - "delete_because_each_key": The corresponding resource uses for_each,
       //   but the instance key doesn't match any of the keys in the
       //   currently-configured for_each value.
+      // - "read_because_config_unknown": For a data resource, Terraform cannot
+      //   read the data during the plan phase because of values in the
+      //   configuration that won't be known until the apply phase.
+      // - "read_because_dependency_pending": For a data resource, Terraform
+      //   cannot read the data during the plan phase because the data
+      //   resource depends on at least one managed resource that also has
+      //   a pending change in the same plan.
       //
       // If there is no special reason to note, Terraform will omit this
       // property altogether.