datasource: add file datasource

The file datasource is meant to be used to pre-create a file that can
then be used elsewhere in the build process by its path.

This is useful for example when building a configuration file from a
template, so then the resulting file can be referenced by components
which only accept file paths.

command/execute.go

@@ -15,6 +15,7 @@ import (
 
 	filebuilder "github.com/hashicorp/packer/builder/file"
 	nullbuilder "github.com/hashicorp/packer/builder/null"
+	filedatasource "github.com/hashicorp/packer/datasource/file"
 	hcppackerartifactdatasource "github.com/hashicorp/packer/datasource/hcp-packer-artifact"
 	hcppackerimagedatasource "github.com/hashicorp/packer/datasource/hcp-packer-image"
 	hcppackeriterationdatasource "github.com/hashicorp/packer/datasource/hcp-packer-iteration"
@@ -65,6 +66,7 @@ var PostProcessors = map[string]packersdk.PostProcessor{
 }
 
 var Datasources = map[string]packersdk.Datasource{
+	"file":                 new(filedatasource.Datasource),
 	"hcp-packer-artifact":  new(hcppackerartifactdatasource.Datasource),
 	"hcp-packer-image":     new(hcppackerimagedatasource.Datasource),
 	"hcp-packer-iteration": new(hcppackeriterationdatasource.Datasource),

datasource/file/data.go

@@ -0,0 +1,131 @@
+// Copyright (c) HashiCorp, Inc.
+// SPDX-License-Identifier: MPL-2.0
+
+//go:generate packer-sdc struct-markdown
+//go:generate packer-sdc mapstructure-to-hcl2 -type DatasourceOutput,Config
+package file
+
+import (
+	"fmt"
+	"log"
+	"os"
+	"path/filepath"
+
+	"github.com/hashicorp/hcl/v2/hcldec"
+	"github.com/hashicorp/packer-plugin-sdk/common"
+	"github.com/hashicorp/packer-plugin-sdk/hcl2helper"
+	"github.com/hashicorp/packer-plugin-sdk/template/config"
+	"github.com/zclconf/go-cty/cty"
+)
+
+type Config struct {
+	common.PackerConfig `mapstructure:",squash"`
+	// The contents of the file to create
+	//
+	// This is useful especially for files that involve templating so that
+	// Packer can dynamically create files and expose them for later importing
+	// as attributes in another entity.
+	//
+	// If no contents are specified, the resulting file will be empty.
+	Contents string `mapstructure:"contents" required:"false"`
+	// The file or directory to write the contents to.
+	Destination string `mapstructure:"destination" required:"false"`
+}
+
+type Datasource struct {
+	config Config
+}
+
+type DatasourceOutput struct {
+	// The path of the file created
+	Path string `mapstructure:"path"`
+}
+
+func (d *Datasource) ConfigSpec() hcldec.ObjectSpec {
+	return d.config.FlatMapstructure().HCL2Spec()
+}
+
+func (d *Datasource) Configure(raws ...interface{}) error {
+	err := config.Decode(&d.config, nil, raws...)
+	if err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func (d *Datasource) OutputSpec() hcldec.ObjectSpec {
+	return (&DatasourceOutput{}).FlatMapstructure().HCL2Spec()
+}
+
+func (d *Datasource) Execute() (cty.Value, error) {
+	nulVal := cty.NullVal(cty.EmptyObject)
+
+	dest, err := d.createTempOutputFile()
+	if err != nil {
+		return nulVal, fmt.Errorf("failed to create output file: %s", err)
+	}
+	defer dest.Close()
+
+	log.Printf("[INFO] data/file - Writing to %q", dest.Name())
+
+	written, err := dest.Write([]byte(d.config.Contents))
+	if err != nil {
+		defer os.Remove(d.config.Destination)
+		return nulVal, fmt.Errorf("failed to write contents to %q: %s", d.config.Destination, err)
+	}
+
+	if written != len(d.config.Contents) {
+		defer os.Remove(d.config.Destination)
+		return nulVal, fmt.Errorf(
+			"failed to write contents to %q: expected to write %d bytes, but wrote %d instead",
+			d.config.Destination,
+			len(d.config.Contents),
+			written)
+	}
+
+	output := DatasourceOutput{
+		Path: dest.Name(),
+	}
+
+	return hcl2helper.HCL2ValueFromConfig(output, d.OutputSpec()), nil
+}
+
+func (d *Datasource) createTempOutputFile() (*os.File, error) {
+	// If we did not get a destination, we'll create a temp file in the
+	// system's temporary directory
+	if d.config.Destination == "" {
+		return os.CreateTemp("", "")
+	}
+
+	// First try to stat the destination, to determine if it already exists and its type
+	st, statErr := os.Stat(d.config.Destination)
+	if statErr == nil {
+		if st.IsDir() {
+			return os.CreateTemp(d.config.Destination, "")
+		}
+
+		return os.OpenFile(d.config.Destination, os.O_TRUNC|os.O_RDWR, 0644)
+	}
+
+	outDir := filepath.Dir(d.config.Destination)
+
+	// In case the destination does not exist, we'll get the dirpath,
+	// and create it if it doesn't already exist
+	err := os.MkdirAll(outDir, 0755)
+	if err != nil {
+		return nil, fmt.Errorf("failed to create destination directory %q: %s", outDir, err)
+	}
+
+	// Check if the destination is a directory after the previous step.
+	//
+	// This happens if the path specified ends with a `/`, in which case the
+	// destination is a directory, and we must create a temporary file in
+	// this destination directory.
+	destStat, statErr := os.Stat(d.config.Destination)
+	if statErr == nil && destStat.IsDir() {
+		return os.CreateTemp(d.config.Destination, "")
+	}
+
+	return os.Create(d.config.Destination)
+}