reinstate error wrapping for context errs

Author: kmoe

tfexec/cmd_default.go

@@ -10,6 +10,28 @@ import (
 	"sync"
 )
 
+// 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.Cancelled
+	}
+	return false
+}
+
+func (e *cmdErr) Error() string {
+	return e.err.Error()
+}
+
 func (tf *Terraform) runTerraformCmd(ctx context.Context, cmd *exec.Cmd) error {
 	var errBuf strings.Builder
 
@@ -40,8 +62,11 @@ 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