reinstate error wrapping for context errs

tfexec/cmd_default.go

@@ -40,8 +40,11 @@ func (tf *Terraform) runTerraformCmd(ctx context.Context, cmd *exec.Cmd) error {
 	}
 
 	err = cmd.Start()
-	if err == nil && ctx.Err() != nil {
-		err = ctx.Err()
+	if ctx.Err() != nil {
+		return cmdErr{
+			err:    err,
+			ctxErr: ctx.Err(),
+		}
 	}
 	if err != nil {
 		return err
@@ -66,8 +69,11 @@ func (tf *Terraform) runTerraformCmd(ctx context.Context, cmd *exec.Cmd) error {
 	wg.Wait()
 
 	err = cmd.Wait()
-	if err == nil && ctx.Err() != nil {
-		err = ctx.Err()
+	if ctx.Err() != nil {
+		return cmdErr{
+			err:    err,
+			ctxErr: ctx.Err(),
+		}
 	}
 	if err != nil {
 		return err

tfexec/cmd_linux.go

@@ -45,8 +45,11 @@ func (tf *Terraform) runTerraformCmd(ctx context.Context, cmd *exec.Cmd) error {
 	}
 
 	err = cmd.Start()
-	if err == nil && ctx.Err() != nil {
-		err = ctx.Err()
+	if ctx.Err() != nil {
+		return cmdErr{
+			err:    err,
+			ctxErr: ctx.Err(),
+		}
 	}
 	if err != nil {
 		return err
@@ -71,8 +74,11 @@ func (tf *Terraform) runTerraformCmd(ctx context.Context, cmd *exec.Cmd) error {
 	wg.Wait()
 
 	err = cmd.Wait()
-	if err == nil && ctx.Err() != nil {
-		err = ctx.Err()
+	if ctx.Err() != nil {
+		return cmdErr{
+			err:    err,
+			ctxErr: ctx.Err(),
+		}
 	}
 	if err != nil {
 		return err

tfexec/errors.go

@@ -1,6 +1,9 @@
 package tfexec
 
-import "fmt"
+import (
+	"context"
+	"fmt"
+)
 
 // this file contains non-parsed exported errors
 
@@ -37,3 +40,25 @@ type ErrManualEnvVar struct {
 func (err *ErrManualEnvVar) Error() string {
 	return fmt.Sprintf("manual setting of env var %q detected", err.Name)
 }
+
+// cmdErr is a custom error type to be returned when a cmd exits with a context
+// error such as context.Canceled or context.DeadlineExceeded.
+// The type is specifically designed to respond true to errors.Is for these two
+// errors.
+// See https://github.com/golang/go/issues/21880 for why this is necessary.
+type cmdErr struct {
+	err    error
+	ctxErr error
+}
+
+func (e cmdErr) Is(target error) bool {
+	switch target {
+	case context.DeadlineExceeded, context.Canceled:
+		return e.ctxErr == context.DeadlineExceeded || e.ctxErr == context.Canceled
+	}
+	return false
+}
+
+func (e cmdErr) Error() string {
+	return e.err.Error()
+}