Give a build step's commands only the environment its caller names (issue 1497)

go/forgekey/runner.go

@@ -37,18 +37,31 @@ func ExecRunner(ctx context.Context, env []string, name string, args ...string)
 // step's own context ends a command, and a fetch may outlast a tool call's thirty seconds.
 func ExecRunnerFor(bound time.Duration) Runner {
 	return func(ctx context.Context, env []string, name string, args ...string) Ran {
-		return execRun(ctx, bound, env, name, args...)
+		return execRun(ctx, bound, false, env, name, args...)
 	}
 }
 
-func execRun(ctx context.Context, bound time.Duration, env []string, name string, args ...string) Ran {
+// ExecRunnerClean is ExecRunnerFor whose commands get exactly the env argument as their environment, and nothing of
+// this process's (issue 1497; novox/hq ADR 0466 §4: no registry password in a fetch's environment). The caller
+// gives PATH and HOME itself. A directory is the command's own business (`git -C`, `go -C`).
+func ExecRunnerClean(bound time.Duration) Runner {
+	return func(ctx context.Context, env []string, name string, args ...string) Ran {
+		return execRun(ctx, bound, true, env, name, args...)
+	}
+}
+
+func execRun(ctx context.Context, bound time.Duration, clean bool, env []string, name string, args ...string) Ran {
 	if bound > 0 {
 		var cancel context.CancelFunc
 		ctx, cancel = context.WithTimeout(ctx, bound)
 		defer cancel()
 	}
 	cmd := exec.CommandContext(ctx, name, args...)
-	cmd.Env = append(os.Environ(), env...)
+	if clean {
+		cmd.Env = append([]string{}, env...) // non-nil, so an empty list is an empty environment
+	} else {
+		cmd.Env = append(os.Environ(), env...)
+	}
 	var out, errb bytes.Buffer
 	cmd.Stdout, cmd.Stderr = &out, &errb
 	err := cmd.Run()

go/forgekey/runner_test.go

@@ -2,6 +2,8 @@ package forgekey
 
 import (
 	"context"
+	"os"
+	"strings"
 	"testing"
 	"time"
 )
@@ -43,3 +45,21 @@ func TestExecRunnerForZeroIsCutByItsContext(t *testing.T) {
 		t.Errorf("ran = %+v after %v, want it cut", r, time.Since(began))
 	}
 }
+
+// 1497/AC4: ExecRunnerClean(0) runs env with only the given words: PATH and HOME, not the process's secret.
+func TestExecRunnerCleanGivesTheCommandOnlyTheEnvArgument(t *testing.T) {
+	t.Setenv("MESH_TEST_SECRET", "x")
+	r := ExecRunnerClean(0)(context.Background(), []string{"PATH=" + os.Getenv("PATH"), "HOME=/h"}, "env")
+	if r.Status != 0 || !strings.Contains(r.Stdout, "HOME=/h") || !strings.Contains(r.Stdout, "PATH=") || strings.Contains(r.Stdout, "MESH_TEST_SECRET") {
+		t.Errorf("ran = %+v", r)
+	}
+}
+
+// 1497/AC5: ExecRunnerFor(0) is unchanged: the env argument is added to the process's environment.
+func TestExecRunnerForAddsToTheProcessEnvironment(t *testing.T) {
+	t.Setenv("MESH_TEST_SECRET", "x")
+	r := ExecRunnerFor(0)(context.Background(), []string{"HOME=/h"}, "env")
+	if !strings.Contains(r.Stdout, "MESH_TEST_SECRET=x") || !strings.Contains(r.Stdout, "HOME=/h") {
+		t.Errorf("ran = %+v", r)
+	}
+}