internal/pkg: replace outcomes from external cache

This is primarily useful for implementing a mirror service. This change also works around the zero-dependency FloodArtifact edge case.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pkg/pkg_test.go

@@ -197,6 +197,35 @@ func newStubFile(
 	}
 }
 
+// stubExtern implements [External] with hardcoded prepared outcomes.
+type stubExtern struct {
+	artifact map[unique.Handle[pkg.ID]]pkg.Checksum
+	checksum map[unique.Handle[pkg.Checksum]]fstest.MapFS
+	status   map[unique.Handle[pkg.ID]]string
+}
+
+func (e stubExtern) Artifact(id unique.Handle[pkg.ID]) (*pkg.Checksum, error) {
+	if checksum, ok := e.artifact[id]; ok {
+		return &checksum, nil
+	}
+	return nil, nil
+}
+
+func (e stubExtern) Checksum(checksum unique.Handle[pkg.Checksum]) pkg.Artifact {
+	var buf bytes.Buffer
+	if err := pkg.Write(e.checksum[checksum], ".", &buf); err != nil {
+		panic(err)
+	}
+	return pkg.NewArchive(pkg.NewFile("", buf.Bytes()))
+}
+
+func (e stubExtern) Status(id unique.Handle[pkg.ID]) (io.ReadCloser, error) {
+	if status, ok := e.status[id]; ok {
+		return io.NopCloser(strings.NewReader(status)), nil
+	}
+	return nil, nil
+}
+
 // destroyArtifact removes all traces of an [Artifact] from the on-disk cache.
 // Do not use this in a test case without a very good reason to do so.
 func destroyArtifact(
@@ -1490,6 +1519,108 @@ func TestCache(t *testing.T) {
 
 			"work": {Mode: fs.ModeDir | 0700},
 		}},
+
+		{"extern", 0, nil, func(t *testing.T, base *check.Absolute, c *pkg.Cache) {
+			a := &stubArtifactF{
+				kind:   pkg.KindExec,
+				params: []byte("extern"),
+			}
+			wantIdent := c.Ident(a)
+			wantOutput := expectsFS{
+				".":      {Mode: fs.ModeDir | 0500},
+				"result": {Mode: fs.ModeSymlink | 0777, Data: []byte("/proc/nonexistent")},
+			}
+			var wantChecksum pkg.Checksum
+			if err := pkg.SumFS(
+				&wantChecksum,
+				fstest.MapFS(wantOutput),
+				".",
+			); err != nil {
+				t.Fatal(err)
+			}
+			wantChecksumH := unique.Make(wantChecksum)
+
+			_a := &stubArtifactF{
+				kind:   pkg.KindExec,
+				params: []byte("extern substitute"),
+				deps:   []pkg.Artifact{pkg.NewFile("", nil)},
+			}
+			_wantIdent := c.Ident(_a)
+			_wantOutput := expectsFS{
+				".": {Mode: fs.ModeDir | 0500},
+			}
+			var _wantChecksum pkg.Checksum
+			if err := pkg.SumFS(
+				&_wantChecksum,
+				fstest.MapFS(_wantOutput),
+				".",
+			); err != nil {
+				t.Fatal(err)
+			}
+			_wantChecksumH := unique.Make(_wantChecksum)
+
+			kca := pkg.NewExec(
+				"", "",
+				new(pkg.Checksum), 0, false, false,
+				fhs.AbsRoot, nil, fhs.AbsRoot, nil,
+			)
+			kcIdent := c.Ident(kca)
+
+			c.SetExternal(stubExtern{
+				artifact: map[unique.Handle[pkg.ID]]pkg.Checksum{
+					wantIdent:  wantChecksum,
+					_wantIdent: _wantChecksum,
+					kcIdent:    wantChecksum,
+				},
+				checksum: map[unique.Handle[pkg.Checksum]]fstest.MapFS{
+					wantChecksumH:  fstest.MapFS(wantOutput),
+					_wantChecksumH: fstest.MapFS(_wantOutput),
+				},
+				status: map[unique.Handle[pkg.ID]]string{
+					wantIdent: "\x00",
+					kcIdent:   "unreachable",
+				},
+			})
+
+			cureMany(t, c, []cureStep{
+				{"extern", a, base.Append(
+					"identifier",
+					pkg.Encode(wantIdent.Value()),
+				), wantOutput, nil},
+
+				{"substitute", _a, base.Append(
+					"identifier",
+					pkg.Encode(_wantIdent.Value()),
+				), _wantOutput, nil},
+
+				{"mismatch", kca, nil, nil, &pkg.ChecksumMismatchError{
+					Got: wantChecksum,
+				}},
+			})
+		}, expectsFS{
+			".": {Mode: fs.ModeDir | 0700},
+
+			"checksum": {Mode: fs.ModeDir | 0700},
+			"checksum/MGWmEfjut2QE2xPJwTsmUzpff4BN_FEnQ7T0j7gvUCCiugJQNwqt9m151fm9D1yU":        {Mode: fs.ModeDir | 0500},
+			"checksum/OLBgp1GsljhM2TJ-sbHjaiH9txEUvgdDTAzHv2P24donTt6_529l-9Ua0vFImLlb":        {Mode: 0400},
+			"checksum/fHkl_RuHOoc4rso__nV-qreikovd6Yhrq5mpBlkf5hmPGaxDlik2bYOQ4dhUQjtl":        {Mode: fs.ModeDir | 0500},
+			"checksum/fHkl_RuHOoc4rso__nV-qreikovd6Yhrq5mpBlkf5hmPGaxDlik2bYOQ4dhUQjtl/result": {Mode: fs.ModeSymlink | 0777, Data: []byte("/proc/nonexistent")},
+
+			"identifier": {Mode: fs.ModeDir | 0700},
+			"identifier/4HqRo4uTwRQjfy3d2cujMoDC_pC4iv20h4a7NYlx0UdbVuky18o5iK78TFEfPX2U": {Mode: fs.ModeSymlink | 0777, Data: []byte("../checksum/MGWmEfjut2QE2xPJwTsmUzpff4BN_FEnQ7T0j7gvUCCiugJQNwqt9m151fm9D1yU")},
+			"identifier/7AZcJm58ghFyTVf_v2baSntgpsxkP5el7ti9dC77C29n8YTEqQW9jRW92KGNdYnz": {Mode: fs.ModeSymlink | 0777, Data: []byte("../checksum/fHkl_RuHOoc4rso__nV-qreikovd6Yhrq5mpBlkf5hmPGaxDlik2bYOQ4dhUQjtl")},
+			"identifier/c4aCI00C-ZVyo_FQDQLl1OYK4U_kjzxwrLdFDiXMHnbMcZXCkXo_nxUWauScZ_4V": {Mode: fs.ModeSymlink | 0777, Data: []byte("../checksum/MGWmEfjut2QE2xPJwTsmUzpff4BN_FEnQ7T0j7gvUCCiugJQNwqt9m151fm9D1yU")},
+			"identifier/cNoG77frXGRCJa7fUi1INKUEQg7L4qrX5acsSv-wqZdGZT7dQwM93rD3at6kSFFF": {Mode: fs.ModeSymlink | 0777, Data: []byte("../checksum/OLBgp1GsljhM2TJ-sbHjaiH9txEUvgdDTAzHv2P24donTt6_529l-9Ua0vFImLlb")},
+			"identifier/gvCqzexZVqXjF8B5lKMcP5onmq3jJ6AKqzOW_WN0Fl2yTr9NKhPt9l_ClD2EOSlS": {Mode: fs.ModeSymlink | 0777, Data: []byte("../checksum/fHkl_RuHOoc4rso__nV-qreikovd6Yhrq5mpBlkf5hmPGaxDlik2bYOQ4dhUQjtl")},
+
+			"status": {Mode: fs.ModeDir | 0700},
+			"status/gvCqzexZVqXjF8B5lKMcP5onmq3jJ6AKqzOW_WN0Fl2yTr9NKhPt9l_ClD2EOSlS": {Mode: 0400, Data: []byte("\x00")},
+
+			"substitute": {Mode: fs.ModeDir | 0700},
+			"substitute/qOYrxy9ztKeOA96Os811_0Ox5sd8FBOxis6psJAnRJL5MLazFMaqmd4g7t7k1OHk": {Mode: fs.ModeSymlink | 0777, Data: []byte("../checksum/MGWmEfjut2QE2xPJwTsmUzpff4BN_FEnQ7T0j7gvUCCiugJQNwqt9m151fm9D1yU")},
+
+			"work": {Mode: fs.ModeDir | 0700},
+		}},
 	}
 	checkWithCache(t, testCases)
 }