Add support for writing a backup to restore from

Since we are making changes to a user's configuration, we want to
be careful and let them have a way of restoring the old version in
case something goes wrong. In the future we could handle this
restore process via the tool as well.

pkg/tstune/config_file.go

@@ -5,21 +5,32 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"path"
 	"strings"
+	"time"
 )
 
 const (
-	osMac                = "darwin"
-	osLinux              = "linux"
-	fileNameMac          = "/usr/local/var/postgres/postgresql.conf"
-	fileNameDebianFmt    = "/etc/postgresql/%s/main/postgresql.conf"
-	fileNameRPMFmt       = "/var/lib/pgsql/%s/data/postgresql.conf"
-	fileNameArch         = "/var/lib/postgres/data/postgresql.conf"
-	errConfigNotFoundFmt = "could not find postgresql.conf at any of these locations:\n%v"
+	osMac   = "darwin"
+	osLinux = "linux"
+
+	fileNameMac       = "/usr/local/var/postgres/postgresql.conf"
+	fileNameDebianFmt = "/etc/postgresql/%s/main/postgresql.conf"
+	fileNameRPMFmt    = "/var/lib/pgsql/%s/data/postgresql.conf"
+	fileNameArch      = "/var/lib/postgres/data/postgresql.conf"
+
+	errConfigNotFoundFmt   = "could not find postgresql.conf at any of these locations:\n%v"
+	errBackupNotCreatedFmt = "could not create backup at %s: %v"
+
+	backupFilePrefix = "timescaledb_tune.backup"
+	backupDateFmt    = "200601021504"
 )
 
 // allows us to substitute mock versions in tests
 var osStatFn = os.Stat
+var osCreateFn = func(path string) (io.Writer, error) {
+	return os.Create(path)
+}
 
 // fileExists is a simple check for stating if a file exists and if any error
 // occurs it returns false.
@@ -119,6 +130,19 @@ func getConfigFileState(r io.Reader) (*configFileState, error) {
 	return cfs, nil
 }
 
+// Backup writes the conf file state to the system's temporary directory
+// with a well known name format so it can potentially be restored.
+func (cfs *configFileState) Backup() (string, error) {
+	backupName := backupFilePrefix + time.Now().Format(backupDateFmt)
+	backupPath := path.Join(os.TempDir(), backupName)
+	bf, err := osCreateFn(backupPath)
+	if err != nil {
+		return backupPath, fmt.Errorf(errBackupNotCreatedFmt, backupPath, err)
+	}
+	_, err = cfs.WriteTo(bf)
+	return backupPath, err
+}
+
 func (cfs *configFileState) WriteTo(w io.Writer) (int64, error) {
 	ret := int64(0)
 	for _, l := range cfs.lines {

pkg/tstune/config_file_test.go

@@ -1,11 +1,15 @@
 package tstune
 
 import (
+	"bufio"
 	"bytes"
 	"fmt"
+	"io"
 	"os"
+	"path"
 	"strings"
 	"testing"
+	"time"
 
 	"github.com/timescale/timescaledb-tune/pkg/pgtune"
 )
@@ -324,6 +328,61 @@ func (w *testWriter) Write(buf []byte) (int, error) {
 	return 0, nil
 }
 
+func TestConfigFileStateBackup(t *testing.T) {
+	oldOSCreateFn := osCreateFn
+	now := time.Now()
+	lines := []string{"foo", "bar", "baz", "quaz"}
+	cfs := &configFileState{lines: lines}
+	wantFileName := backupFilePrefix + now.Format(backupDateFmt)
+	wantPath := path.Join(os.TempDir(), wantFileName)
+
+	osCreateFn = func(_ string) (io.Writer, error) {
+		return nil, fmt.Errorf("erroring")
+	}
+
+	path, err := cfs.Backup()
+	if path != wantPath {
+		t.Errorf("incorrect path in error case: got\n%s\nwant\n%s", path, wantPath)
+	}
+	if err == nil {
+		t.Errorf("unexpected lack of error for bad create")
+	}
+	want := fmt.Sprintf(errBackupNotCreatedFmt, wantPath, "erroring")
+	if got := err.Error(); got != want {
+		t.Errorf("incorrect error: got\n%s\nwant\n%s", got, want)
+	}
+
+	var buf bytes.Buffer
+	osCreateFn = func(p string) (io.Writer, error) {
+		if p != wantPath {
+			t.Errorf("incorrect backup path: got %s want %s", p, wantPath)
+		}
+		return &buf, nil
+	}
+	path, err = cfs.Backup()
+	if path != wantPath {
+		t.Errorf("incorrect path in correct case: got\n%s\nwant\n%s", path, wantPath)
+	}
+	if err != nil {
+		t.Errorf("unexpected error for backup: %v", err)
+	}
+
+	scanner := bufio.NewScanner(bytes.NewReader(buf.Bytes()))
+	i := 0
+	for scanner.Scan() {
+		if scanner.Err() != nil {
+			t.Errorf("unexpected error while scanning: %v", scanner.Err())
+		}
+		got := scanner.Text()
+		if want := lines[i]; got != want {
+			t.Errorf("incorrect line at %d: got\n%s\nwant\n%s", i, got, want)
+		}
+		i++
+	}
+
+	osCreateFn = oldOSCreateFn
+}
+
 func TestConfigFileStateWriteTo(t *testing.T) {
 	cases := []struct {
 		desc      string

pkg/tstune/tuner.go

@@ -49,8 +49,8 @@ const (
 	fmtTunableParam = "%s = %s%s\n"
 	fmtLastTuned    = "timescaledb.last_tuned = '%s'"
 
-	dateFmt     = "2006-01-02 15:04"
-	fudgeFactor = 0.05
+	lastTunedDateFmt = "2006-01-02 15:04"
+	fudgeFactor      = 0.05
 
 	pgMajor96 = "9.6"
 	pgMajor10 = "10"
@@ -192,6 +192,12 @@ func (t *Tuner) Run(flags *TunerFlags, in io.Reader, out io.Writer, outErr io.Wr
 	ifErrHandle(err)
 	t.cfs = cfs
 
+	// Write backup
+	backupPath, err := cfs.Backup()
+	t.handler.p.Statement("Writing backup to:")
+	printFn(os.Stderr, backupPath+"\n\n")
+	ifErrHandle(err)
+
 	// Process the tuning of settings
 	if t.flags.Quiet {
 		err = t.processQuiet(config)
@@ -211,7 +217,7 @@ func (t *Tuner) Run(flags *TunerFlags, in io.Reader, out io.Writer, outErr io.Wr
 	}
 
 	// Append the current time to mark when database was last tuned
-	lastTunedLine := fmt.Sprintf(fmtLastTuned, time.Now().Format(dateFmt))
+	lastTunedLine := fmt.Sprintf(fmtLastTuned, time.Now().Format(lastTunedDateFmt))
 	cfs.lines = append(cfs.lines, lastTunedLine)
 
 	// Wrap up: Either write it out, or show success in --dry-run
@@ -519,7 +525,7 @@ func (t *Tuner) processQuiet(config *pgtune.SystemConfig) error {
 		return err
 	}
 	if changedSettings > 0 {
-		printFn(os.Stdout, fmtLastTuned+"\n", time.Now().Format(dateFmt))
+		printFn(os.Stdout, fmtLastTuned+"\n", time.Now().Format(lastTunedDateFmt))
 		checker := newYesNoChecker("not using these settings could lead to suboptimal performance")
 		err = t.promptUntilValidInput("Use these recommendations? "+promptYesNo, checker)
 		if err != nil {

pkg/tstune/tuner_test.go

@@ -1266,7 +1266,7 @@ var (
 )
 
 func TestTunerProcessQuiet(t *testing.T) {
-	lastTuned := fmt.Sprintf(fmtLastTuned, time.Now().Format(dateFmt))
+	lastTuned := fmt.Sprintf(fmtLastTuned, time.Now().Format(lastTunedDateFmt))
 	cases := []struct {
 		desc          string
 		lines         []string