Copying /etc/resolv.conf to chroot while restoring the original file unless it has been changed during provisioning

builder/step_setup_chroot.go

@@ -1,8 +1,12 @@
 package builder
 
 import (
+	"bytes"
 	"context"
+	"errors"
 	"fmt"
+	"io"
+	"io/fs"
 	"io/ioutil"
 	"os"
 	"os/exec"
@@ -65,6 +69,58 @@ func getMounts() (map[string]bool, error) {
 	return selected, nil
 }
 
+// renameAndCheck courtesy of https://stackoverflow.com/a/25940392/51016
+func renameAndCheck(src, dst string) error {
+    err := os.Link(src, dst)
+    if err != nil {
+        return err
+    }
+    return os.Remove(src)
+}
+
+// deepCompare courtesy of https://stackoverflow.com/a/30038571/51016
+const chunkSize = 64000
+
+func deepCompare(file1, file2 string) (bool, error) {
+    // Check file size ...
+    f1, err := os.Open(file1)
+    if err != nil {
+        return false, err
+    }
+    defer f1.Close()
+
+    f2, err := os.Open(file2)
+    if err != nil {
+        return false, err
+    }
+    defer f2.Close()
+
+    for {
+        b1 := make([]byte, chunkSize)
+        _, err1 := f1.Read(b1)
+
+        b2 := make([]byte, chunkSize)
+        _, err2 := f2.Read(b2)
+
+        if err1 != nil || err2 != nil {
+            if err1 == io.EOF && err2 == io.EOF {
+                return true, nil
+            } else if err1 == io.EOF || err2 == io.EOF {
+                return false, nil
+            } else {
+				err := fmt.Errorf("file1: %w", err1)
+				err = fmt.Errorf("file2: %w", err2)
+				err = fmt.Errorf("error comparing files; %w", err)
+                return false, err
+            }
+        }
+
+        if !bytes.Equal(b1, b2) {
+            return false, nil
+        }
+    }
+}
+
 // StepSetupChroot prepares chroot environment by mounting specific locations (/dev /proc etc.)
 type StepSetupChroot struct {
 	ImageMountPointKey string
@@ -95,6 +151,47 @@ func (s *StepSetupChroot) Run(ctx context.Context, state multistep.StateBag) mul
 		}
 	}
 
+	ui.Message("patching file system for the chroot execution")
+
+	src := "/etc/resolv.conf"
+	dst := filepath.Join(imageMountpoint, src)
+	bak := dst + ".bak"
+
+	// backup /etc/resolv.conf if it exists
+	err := renameAndCheck(dst, bak)
+	if err == nil {
+		ui.Message(fmt.Sprintf("backed up '%s' to '%s'", dst, bak))
+	} else if errors.Is(err, fs.ErrNotExist) {
+		ui.Message(fmt.Sprintf("'%s' does not exist: %v", src, err))
+	} else if errors.Is(err, fs.ErrExist) {
+		ui.Error(fmt.Sprintf("'%s' already exists: %v", dst, err))
+	} else if errors.Is(err, fs.ErrPermission) {
+		ui.Error(fmt.Sprintf("could not create '%s': %v", bak, err))
+	}
+
+	source, err := os.Open(src)
+	if err != nil {
+		ui.Error(fmt.Sprintf("error while opening source: %v: '%s'", err, src))
+		return multistep.ActionHalt
+	}
+	defer source.Close()
+
+	destination, err := os.Create(dst)
+	if err != nil {
+		ui.Error(fmt.Sprintf("error while creating destination: %v: '%s'", err, dst))
+		return multistep.ActionHalt
+	}
+	defer destination.Close()
+
+	// copy /etc/resolv.conf to the chroot
+	_, err = io.Copy(destination, source)
+	if err == nil {
+		ui.Message(fmt.Sprintf("copied file from '%s' to '%s'", src, dst))
+	} else {
+		ui.Error(fmt.Sprintf("error while copying: %v: from '%s' to '%s'", err, src, dst))
+		return multistep.ActionHalt
+	}
+
 	return multistep.ActionContinue
 }
 
@@ -141,4 +238,30 @@ func (s *StepSetupChroot) Cleanup(state multistep.StateBag) {
 			}
 		}
 	}
+
+	// restore backed up /etc/resolv.conf in chroot
+	resolve := filepath.Join(imageMountpoint, "/etc/resolv.conf")
+	result, err := deepCompare("/etc/resolv.conf", resolve)
+	if err != nil {
+		ui.Error(fmt.Sprintf("error comparing host and chroot resolv.conf: %v", err))
+	}
+	// restore the backup only if resolv.conf is unchanged
+	if result {
+		bak := resolve + ".bak"
+
+		err = os.Remove(resolve)
+		if err != nil {
+			ui.Error(fmt.Sprintf("could not remove file %v: %s", err, resolve))
+		}
+		err = renameAndCheck(bak, resolve)
+		if err == nil {
+			ui.Message(fmt.Sprintf("restored '%s'", resolve))
+		} else if errors.Is(err, fs.ErrNotExist) {
+			ui.Error(fmt.Sprintf("'%s' does not exist: %v", bak, err))
+		} else if errors.Is(err, fs.ErrExist) {
+			ui.Error(fmt.Sprintf("'%s' already exists: %v", resolve, err))
+		} else if errors.Is(err, fs.ErrPermission) {
+			ui.Error(fmt.Sprintf("could not restore '%s': %v", resolve, err))
+		}
+	}
 }