Allow equivalence checking of filer errors to fs errors (#416)

## Changes

The pattern `errors.Is(err, fs.ErrNotExist)` is common to check for an
error type.

Errors can implement `Is(error) bool` with a custom equivalence checker.

## Tests

New asserts all pass in the integration test.

bundle/deploy/terraform/state_pull.go

@@ -2,14 +2,15 @@ package terraform
 
 import (
 	"context"
+	"errors"
 	"io"
+	"io/fs"
 	"os"
 	"path/filepath"
 
 	"github.com/databricks/cli/bundle"
 	"github.com/databricks/cli/libs/filer"
 	"github.com/databricks/cli/libs/log"
-	"github.com/databricks/databricks-sdk-go/apierr"
 )
 
 type statePull struct{}
@@ -34,7 +35,7 @@ func (l *statePull) Apply(ctx context.Context, b *bundle.Bundle) error {
 	remote, err := f.Read(ctx, TerraformStateFileName)
 	if err != nil {
 		// On first deploy this state file doesn't yet exist.
-		if apierr.IsMissing(err) {
+		if errors.Is(err, fs.ErrNotExist) {
 			log.Infof(ctx, "Remote state file does not exist")
 			return nil
 		}

internal/filer_test.go

@@ -46,10 +46,12 @@ func runFilerReadWriteTest(t *testing.T, ctx context.Context, f filer.Filer) {
 	// Write should fail because the root path doesn't yet exist.
 	err = f.Write(ctx, "/foo/bar", strings.NewReader(`hello world`))
 	assert.True(t, errors.As(err, &filer.NoSuchDirectoryError{}))
+	assert.True(t, errors.Is(err, fs.ErrNotExist))
 
 	// Read should fail because the root path doesn't yet exist.
 	_, err = f.Read(ctx, "/foo/bar")
 	assert.True(t, errors.As(err, &filer.FileDoesNotExistError{}))
+	assert.True(t, errors.Is(err, fs.ErrNotExist))
 
 	// Write with CreateParentDirectories flag should succeed.
 	err = f.Write(ctx, "/foo/bar", strings.NewReader(`hello world`), filer.CreateParentDirectories)
@@ -59,6 +61,7 @@ func runFilerReadWriteTest(t *testing.T, ctx context.Context, f filer.Filer) {
 	// Write should fail because there is an existing file at the specified path.
 	err = f.Write(ctx, "/foo/bar", strings.NewReader(`hello universe`))
 	assert.True(t, errors.As(err, &filer.FileAlreadyExistsError{}))
+	assert.True(t, errors.Is(err, fs.ErrExist))
 
 	// Write with OverwriteIfExists should succeed.
 	err = f.Write(ctx, "/foo/bar", strings.NewReader(`hello universe`), filer.OverwriteIfExists)
@@ -68,6 +71,7 @@ func runFilerReadWriteTest(t *testing.T, ctx context.Context, f filer.Filer) {
 	// Delete should fail if the file doesn't exist.
 	err = f.Delete(ctx, "/doesnt_exist")
 	assert.True(t, errors.As(err, &filer.FileDoesNotExistError{}))
+	assert.True(t, errors.Is(err, fs.ErrNotExist))
 
 	// Delete should succeed for file that does exist.
 	err = f.Delete(ctx, "/foo/bar")
@@ -102,6 +106,7 @@ func runFilerReadDirTest(t *testing.T, ctx context.Context, f filer.Filer) {
 	// Expect an error if the path doesn't exist.
 	_, err = f.ReadDir(ctx, "/dir/a/b/c/d/e")
 	assert.True(t, errors.As(err, &filer.NoSuchDirectoryError{}), err)
+	assert.True(t, errors.Is(err, fs.ErrNotExist))
 
 	// Expect two entries in the root.
 	entries, err = f.ReadDir(ctx, ".")

libs/filer/filer.go

@@ -22,10 +22,18 @@ func (err FileAlreadyExistsError) Error() string {
 	return fmt.Sprintf("file already exists: %s", err.path)
 }
 
+func (err FileAlreadyExistsError) Is(other error) bool {
+	return other == fs.ErrExist
+}
+
 type FileDoesNotExistError struct {
 	path string
 }
 
+func (err FileDoesNotExistError) Is(other error) bool {
+	return other == fs.ErrNotExist
+}
+
 func (err FileDoesNotExistError) Error() string {
 	return fmt.Sprintf("file does not exist: %s", err.path)
 }
@@ -38,6 +46,10 @@ func (err NoSuchDirectoryError) Error() string {
 	return fmt.Sprintf("no such directory: %s", err.path)
 }
 
+func (err NoSuchDirectoryError) Is(other error) bool {
+	return other == fs.ErrNotExist
+}
+
 // Filer is used to access files in a workspace.
 // It has implementations for accessing files in WSFS and in DBFS.
 type Filer interface {