Filter out unwanted environment variables

When running commands, drop locale- and restic-related environment
variables.

shell.go

@@ -10,10 +10,37 @@ import (
 	"os/exec"
 	"path/filepath"
 	"strconv"
+	"strings"
 
 	"git.autistici.org/ale/tabacco/jobs"
 )
 
+// Environment variables starting with any of the following strings
+// are removed from the command execution environment.
+var filteredEnvVars = []string{
+	"LANG=",
+	"LC_",
+	"RESTIC_",
+}
+
+// Return a copy of os.Environ(), with filteredEnvVars removed.
+func safeEnv() []string {
+	var env []string
+	for _, s := range os.Environ() {
+		skip := false
+		for _, pfx := range filteredEnvVars {
+			if strings.HasPrefix(s, pfx) {
+				skip = true
+				break
+			}
+		}
+		if !skip {
+			env = append(env, s)
+		}
+	}
+	return env
+}
+
 // Shell runs commands, with some options (a global dry-run flag
 // preventing all executions, nice level, i/o class). As one may guess
 // by the name, commands are run using the shell, so variable
@@ -22,6 +49,7 @@ type Shell struct {
 	dryRun      bool
 	niceLevel   int
 	ioniceClass int
+	env         []string
 }
 
 // NewShell creates a new Shell.
@@ -30,6 +58,7 @@ func NewShell(dryRun bool) *Shell {
 		dryRun:      dryRun,
 		niceLevel:   10,
 		ioniceClass: 2,
+		env:         safeEnv(),
 	}
 }
 
@@ -47,8 +76,6 @@ func (s *Shell) SetIOClass(n int) {
 // context - notably it sets log output and working directory to be in
 // the working dir if the job has been wrapped in WithWorkDir().
 func (s *Shell) command(ctx context.Context, arg string) *exec.Cmd {
-	log.Printf("sh: %s", arg)
-
 	var args []string
 	if s.dryRun {
 		args = []string{"/bin/echo", arg}
@@ -70,8 +97,14 @@ func (s *Shell) command(ctx context.Context, arg string) *exec.Cmd {
 	}
 
 	c := exec.CommandContext(ctx, args[0], args[1:]...) // #nosec
+	var env []string
+	env = append(env, s.env...)
+	c.Env = env
 	c.Stderr = os.Stderr
 	c.Dir = getWorkDir(ctx)
+
+	log.Printf("sh: %s", arg)
+
 	return c
 }
 
@@ -121,12 +154,9 @@ func (s *Shell) Run(ctx context.Context, arg string) error {
 // RunWithEnv runs a command with additional environment variables.
 func (s *Shell) RunWithEnv(ctx context.Context, arg string, envMap map[string]string) error {
 	c := s.command(ctx, arg)
-	var env []string
-	copy(env, os.Environ())
 	for k, v := range envMap {
-		env = append(env, fmt.Sprintf("%s=%s", k, v))
+		c.Env = append(c.Env, fmt.Sprintf("%s=%s", k, v))
 	}
-	c.Env = env
 	c.Stdout = os.Stdout
 	return c.Run()
 }