Support archive priorities

If a package to be downloaded exists in archive A and archive B, and
archive A has higher priority than archive B, the package is downloaded
from archive A even if archive B has a newer version. If two or more
archives have the same priority, the most recent version of the package
from all of them is selected, if any. Otherwise, lower priority archives
are tried, if any.

cmd/chisel/cmd_cut.go

@@ -91,6 +91,7 @@ func (cmd *cmdCut) Execute(args []string) error {
 			Suites:     archiveInfo.Suites,
 			Components: archiveInfo.Components,
 			CacheDir:   cache.DefaultDir("chisel"),
+			Priority:   archiveInfo.Priority,
 		})
 		if err != nil {
 			return err

internal/archive/archive.go

@@ -34,6 +34,7 @@ type Options struct {
 	Suites     []string
 	Components []string
 	CacheDir   string
+	Priority   int
 }
 
 func Open(options *Options) (Archive, error) {

internal/setup/setup.go

@@ -29,6 +29,7 @@ type Archive struct {
 	Version    string
 	Suites     []string
 	Components []string
+	Priority   int
 }
 
 // Package holds a collection of slices that represent parts of themselves.
@@ -321,6 +322,7 @@ type yamlArchive struct {
 	Version    string   `yaml:"version"`
 	Suites     []string `yaml:"suites"`
 	Components []string `yaml:"components"`
+	Priority   int      `yaml:"priority"`
 }
 
 type yamlPackage struct {
@@ -427,6 +429,7 @@ func parseRelease(baseDir, filePath string, data []byte) (*Release, error) {
 			Version:    details.Version,
 			Suites:     details.Suites,
 			Components: details.Components,
+			Priority:   details.Priority,
 		}
 	}
 

internal/slicer/slicer.go

@@ -5,8 +5,10 @@ import (
 	"bytes"
 	"fmt"
 	"io"
+	"math"
 	"os"
 	"path/filepath"
+	"sort"
 	"strings"
 	"syscall"
 
@@ -69,13 +71,48 @@ func Run(options *RunOptions) error {
 		targetDirAbs = filepath.Join(dir, targetDir)
 	}
 
+	// Archives are ordered in descending order by priority. The
+	// default priority is 0 and the maximum allowed priority is
+	// the upper limit of int.
+	//
+	// If a package to be downloaded exists in archive A and
+	// archive B, and archive A has higher priority than archive
+	// B, the package is downloaded from archive A even if archive
+	// B has a newer version.
+	//
+	// If two or more archives have the same priority, the most
+	// recent version of the package from all of them is selected,
+	// if any. Otherwise, archives with lower priority are tried,
+	// if any.
+	//
+	// If two or more archives with the same priority contain the
+	// package with the same version, the first archive from the
+	// options.Archives iterator is used. In effect, the choice is
+	// random.
+	orderedArchives := make([]archive.Archive, 0, len(options.Archives))
+	for _, archive := range options.Archives {
+		orderedArchives = append(orderedArchives, archive)
+	}
+	sort.Slice(orderedArchives, func(a, b int) bool {
+		prioA := orderedArchives[a].Options().Priority
+		prioB := orderedArchives[b].Options().Priority
+		return prioA > prioB
+	})
+
 	// Build information to process the selection.
 	for _, slice := range options.Selection.Slices {
 		extractPackage := extract[slice.Package]
 		if extractPackage == nil {
 			var selectedVersion string
 			var selectedArchive archive.Archive
-			for _, currentArchive := range options.Archives {
+			currentPrio := math.MaxInt
+			for _, currentArchive := range orderedArchives {
+				if prio := currentArchive.Options().Priority; prio < currentPrio {
+					if selectedVersion != "" {
+						break
+					}
+					currentPrio = prio
+				}
 				pkgInfo := currentArchive.Info(slice.Package)
 				if pkgInfo == nil {
 					continue

internal/slicer/slicer_test.go

@@ -687,6 +687,92 @@ var slicerTests = []slicerTest{{
 		"/speed/cheetah": "file 0644 e98b0879",
 		"/speed/ostrich": "file 0644 c8fa2806",
 	},
+}, {
+	summary: "Pick package from archive with highest priority",
+	pkgs: map[string]map[string]testPackage{
+		"edge": {
+			"hello": testPackage{
+				info: map[string]string{
+					"Version": "2.0-beta",
+				},
+				content: testutil.MustMakeDeb([]testutil.TarEntry{
+					DIR(0755, "./"),
+					REG(0644, "./hello", "Hello, The Edge\n"),
+				}),
+			},
+		},
+		"candidate": {
+			"hello": testPackage{
+				info: map[string]string{
+					"Version": "1.8",
+				},
+				content: testutil.MustMakeDeb([]testutil.TarEntry{
+					DIR(0755, "./"),
+					REG(0644, "./hello", "Hello, The Candidate\n"),
+				}),
+			},
+		},
+		"stable": {
+			"hello": testPackage{
+				info: map[string]string{
+					"Version": "1.2",
+				},
+				content: testutil.MustMakeDeb([]testutil.TarEntry{
+					DIR(0755, "./"),
+					REG(0644, "./hello", "Hello, The Stable\n"),
+				}),
+			},
+		},
+		"obsolete": {
+			"hello": testPackage{
+				info: map[string]string{
+					"Version": "1.0",
+				},
+				content: testutil.MustMakeDeb([]testutil.TarEntry{
+					DIR(0755, "./"),
+					REG(0644, "./hello", "Hello, The Obsolete\n"),
+				}),
+			},
+		},
+	},
+	release: map[string]string{
+		"chisel.yaml": `
+			format: chisel-v1
+			archives:
+				edge:
+					version: 1
+					suites: [main]
+					components: [main]
+				candidate:
+					version: 1
+					suites: [main]
+					components: [main]
+					priority: 5
+				stable:
+					version: 1
+					suites: [main]
+					components: [main]
+					priority: 10
+				obsolete:
+					version: 1
+					suites: [main]
+					components: [main]
+					priority: 10
+		`,
+		"slices/mydir/hello.yaml": `
+			package: hello
+			slices:
+				all:
+					contents:
+						/hello:
+		`,
+	},
+	slices: []setup.SliceKey{
+		{"hello", "all"},
+	},
+	result: map[string]string{
+		"/hello": "file 0644 b5621b65",
+	},
 }}
 
 const defaultChiselYaml = `
@@ -792,6 +878,7 @@ func (s *S) TestRun(c *C) {
 					Version:    setupArchive.Version,
 					Suites:     setupArchive.Suites,
 					Components: setupArchive.Components,
+					Priority:   setupArchive.Priority,
 					Arch:       test.arch,
 				},
 				pkgs: archivePkgs,