osbuild-worker/osbuild: move out execution helper

Move the execution helper for osbuild into the osbuild2 package so it can be
reused. In the process, generalise it slightly.

internal/osbuild2/osbuild-exec.go

@@ -0,0 +1,81 @@
+package osbuild2
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"io"
+	"os"
+	"os/exec"
+)
+
+// Run an instance of osbuild, returning a parsed osbuild.Result.
+//
+// Note that osbuild returns non-zero when the pipeline fails. This function
+// does not return an error in this case. Instead, the failure is communicated
+// with its corresponding logs through osbuild.Result.
+func RunOSBuild(manifest []byte, store, outputDirectory string, exports, checkpoints []string, result bool, errorWriter io.Writer) (*Result, error) {
+	var stdoutBuffer bytes.Buffer
+	var res Result
+
+	cmd := exec.Command(
+		"osbuild",
+		"--store", store,
+		"--output-directory", outputDirectory,
+		"-",
+	)
+
+	for _, export := range exports {
+		cmd.Args = append(cmd.Args, "--export", export)
+	}
+
+	for _, checkpoint := range checkpoints {
+		cmd.Args = append(cmd.Args, "--checkpoint", checkpoint)
+	}
+
+	if result {
+		cmd.Args = append(cmd.Args, "--json")
+		cmd.Stdout = &stdoutBuffer
+	} else {
+		cmd.Stdout = os.Stdout
+	}
+	cmd.Stderr = errorWriter
+	stdin, err := cmd.StdinPipe()
+	if err != nil {
+		return nil, fmt.Errorf("error setting up stdin for osbuild: %v", err)
+	}
+
+	err = cmd.Start()
+	if err != nil {
+		return nil, fmt.Errorf("error starting osbuild: %v", err)
+	}
+
+	_, err = stdin.Write(manifest)
+	if err != nil {
+		return nil, fmt.Errorf("error writing osbuild manifest: %v", err)
+	}
+
+	err = stdin.Close()
+	if err != nil {
+		return nil, fmt.Errorf("error closing osbuild's stdin: %v", err)
+	}
+
+	err = cmd.Wait()
+
+	if result {
+		// try to decode the output even though the job could have failed
+		decodeErr := json.Unmarshal(stdoutBuffer.Bytes(), &res)
+		if decodeErr != nil {
+			return nil, fmt.Errorf("error decoding osbuild output: %v\nthe raw output:\n%s", decodeErr, stdoutBuffer.String())
+		}
+	}
+
+	if err != nil {
+		// ignore ExitError if output could be decoded correctly
+		if _, isExitError := err.(*exec.ExitError); !isExitError {
+			return nil, fmt.Errorf("running osbuild failed: %v", err)
+		}
+	}
+
+	return &res, nil
+}