Leave out of the build context what the app's .dockerignore names (closes #274)

Docker does not apply .dockerignore to a build context sent as a tar,
so upaas sent every file in the clone. The build now reads the ignore
file as docker build does, <Dockerfile>.dockerignore next to the
Dockerfile if there is one, otherwise .dockerignore at the root, with
the ignorefile reader of moby/patternmatcher, and passes its patterns
as exclude patterns. As the docker command line does, the Dockerfile
and .dockerignore are never left out. The ignore file is read through
os.Root, so it cannot be read from outside the clone. An ignore file
that cannot be read, or a malformed pattern, fails the build.

Model: opus-5-5

internal/docker/buildcontext_test.go

@@ -0,0 +1,237 @@
+package docker //nolint:testpackage // tests the unexported performBuild
+
+import (
+	"archive/tar"
+	"errors"
+	"io"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"slices"
+	"strings"
+	"testing"
+
+	"github.com/docker/docker/client"
+)
+
+// File names used in more than one test build context, as constants to
+// satisfy the goconst linter.
+const (
+	testMainGo           = "main.go"
+	testSecretFile       = "secret.txt"
+	testDeployDockerfile = "deploy/Dockerfile"
+)
+
+// TestPerformBuildFollowsDockerignore runs builds against a fake Docker API
+// and checks which files the build context sent to it holds.
+func TestPerformBuildFollowsDockerignore(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name       string
+		dockerfile string
+		files      map[string]string // path in the context: contents
+		want       []string          // files sent in the build context
+	}{
+		{
+			name:       "no ignore file",
+			dockerfile: defaultDockerfileName,
+			files:      map[string]string{defaultDockerfileName: "", testMainGo: ""},
+			want:       []string{defaultDockerfileName, testMainGo},
+		},
+		{
+			name:       "excludes and re-includes",
+			dockerfile: defaultDockerfileName,
+			files: map[string]string{
+				defaultDockerignoreName: "secret.txt\n*.md\n!README.md\n",
+				defaultDockerfileName:   "",
+				"NOTES.md":              "",
+				"README.md":             "",
+				testMainGo:              "",
+				testSecretFile:          "",
+			},
+			want: []string{
+				defaultDockerignoreName, defaultDockerfileName, "README.md", testMainGo,
+			},
+		},
+		{
+			name:       "keeps the Dockerfile and .dockerignore",
+			dockerfile: defaultDockerfileName,
+			files: map[string]string{
+				defaultDockerignoreName: "Dockerfile\n.dockerignore\nsecret.txt\n",
+				defaultDockerfileName:   "",
+				testMainGo:              "",
+				testSecretFile:          "",
+			},
+			want: []string{defaultDockerignoreName, defaultDockerfileName, testMainGo},
+		},
+		{
+			name:       "an ignore file next to the Dockerfile wins over .dockerignore",
+			dockerfile: testDeployDockerfile,
+			files: map[string]string{
+				defaultDockerignoreName:          "main.go\n",
+				testDeployDockerfile:             "",
+				"deploy/Dockerfile.dockerignore": "secret.txt\n",
+				testMainGo:                       "",
+				testSecretFile:                   "",
+			},
+			want: []string{
+				defaultDockerignoreName,
+				testDeployDockerfile,
+				"deploy/Dockerfile.dockerignore",
+				testMainGo,
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			contextDir := t.TempDir()
+			writeFiles(t, contextDir, tt.files)
+
+			got, err := buildContextFiles(t, contextDir, tt.dockerfile)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			if !slices.Equal(got, tt.want) {
+				t.Errorf("build context holds %q, want %q", got, tt.want)
+			}
+		})
+	}
+}
+
+// TestPerformBuildFailsOnMalformedDockerignore checks that a pattern the
+// docker command line would reject fails the build.
+func TestPerformBuildFailsOnMalformedDockerignore(t *testing.T) {
+	t.Parallel()
+
+	contextDir := t.TempDir()
+	writeFiles(t, contextDir, map[string]string{defaultDockerignoreName: "[\n"})
+
+	_, err := buildContextFiles(t, contextDir, defaultDockerfileName)
+
+	want := "failed to create build context: " +
+		"invalid pattern in .dockerignore: syntax error in pattern"
+	if err == nil || err.Error() != want {
+		t.Errorf("got error %v, want %q", err, want)
+	}
+}
+
+// TestPerformBuildFailsOnUnreadableDockerignore checks that an ignore file
+// that cannot be read, here because it is a directory, fails the build.
+func TestPerformBuildFailsOnUnreadableDockerignore(t *testing.T) {
+	t.Parallel()
+
+	contextDir := t.TempDir()
+
+	err := os.Mkdir(filepath.Join(contextDir, defaultDockerignoreName), 0o750)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	_, err = buildContextFiles(t, contextDir, defaultDockerfileName)
+
+	want := "failed to create build context: failed to read .dockerignore: "
+	if err == nil || !strings.HasPrefix(err.Error(), want) {
+		t.Errorf("got error %v, want one starting %q", err, want)
+	}
+}
+
+// writeFiles writes files, a map of paths inside dir to their contents,
+// creating the directories they are in.
+func writeFiles(t *testing.T, dir string, files map[string]string) {
+	t.Helper()
+
+	for name, contents := range files {
+		path := filepath.Join(dir, name)
+
+		err := os.MkdirAll(filepath.Dir(path), 0o750)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		err = os.WriteFile(path, []byte(contents), 0o600)
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+}
+
+// buildContextFiles runs a build of contextDir against a fake Docker API and
+// returns the names of the files in the build context sent to it, sorted.
+func buildContextFiles(t *testing.T, contextDir, dockerfile string) ([]string, error) {
+	t.Helper()
+
+	sent := make(chan []string, 1)
+
+	srv := httptest.NewServer(http.HandlerFunc(
+		func(w http.ResponseWriter, r *http.Request) {
+			switch {
+			case strings.HasSuffix(r.URL.Path, "/version"):
+				_, _ = w.Write([]byte(`{"Version":"27.3.1","ApiVersion":"1.47"}`))
+			case strings.HasSuffix(r.URL.Path, "/session"):
+				serveSession(t, w, r, make(chan string, 1))
+			case strings.HasSuffix(r.URL.Path, "/build"):
+				sent <- tarFileNames(t, r.Body)
+			default:
+				t.Errorf("unexpected request to %s", r.URL.Path)
+			}
+		},
+	))
+	t.Cleanup(srv.Close)
+
+	dockerAPI, err := client.NewClientWithOpts(
+		client.WithHost("tcp://" + srv.Listener.Addr().String()),
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	c := &Client{docker: dockerAPI, log: slog.Default()}
+
+	_, err = c.performBuild(t.Context(), BuildImageOptions{
+		ContextDir:     contextDir,
+		DockerfilePath: dockerfile,
+	})
+	if err != nil {
+		return nil, err
+	}
+
+	return <-sent, nil
+}
+
+// tarFileNames returns the names of the regular files in the tar read from r,
+// sorted.
+func tarFileNames(t *testing.T, r io.Reader) []string {
+	t.Helper()
+
+	var names []string
+
+	reader := tar.NewReader(r)
+
+	for {
+		header, err := reader.Next()
+		if errors.Is(err, io.EOF) {
+			break
+		}
+
+		if err != nil {
+			t.Errorf("reading the build context: %v", err)
+
+			return nil
+		}
+
+		if header.Typeflag == tar.TypeReg {
+			names = append(names, header.Name)
+		}
+	}
+
+	slices.Sort(names)
+
+	return names
+}