Unusable DATA_DIR error names DATA_DIR; report file number is atomic.Int64 (closes #148, closes #161)

When the backend runs on its own, a DATA_DIR that could not be created
or read stopped the start with an error that named the path but not
DATA_DIR, and one where no file could be created let it start, to fail
at the first write of a report file. The start now creates a file in
DATA_DIR and removes it, and all three cases fail with
DATA_DIR "<value>": <reason>, the form config.New uses for the other
settings. Tests start the buffer with DATA_DIR below a regular file and
with DATA_DIR set to /proc.

The report file sequence counter is atomic.Int64 instead of
atomic.Uint64, as the Go style guide asks for int types. File names are
unchanged.

Model: opus-5-5

internal/reportbuf/reportbuf_test.go

@@ -108,6 +108,68 @@ func TestFailedFinalFlushFailsStop(t *testing.T) {
 	}
 }
 
+// TestUncreatableDataDirStopsStart sets DATA_DIR below a regular file,
+// where no directory can be created, even by root, as the tests run in
+// the backend image. The start must fail with an error naming DATA_DIR
+// and its value, as the errors for the other settings do.
+func TestUncreatableDataDirStopsStart(t *testing.T) {
+	file := filepath.Join(t.TempDir(), "file")
+	writeBytes(t, file, 1)
+
+	dir := filepath.Join(file, "reports")
+	t.Setenv("DATA_DIR", dir)
+
+	app := fxtest.New(t,
+		fx.Provide(
+			globals.New,
+			logger.New,
+			config.New,
+			reportbuf.New,
+		),
+		fx.Invoke(func(*reportbuf.Buffer) {}),
+	)
+
+	err := app.Start(t.Context())
+	if err == nil {
+		t.Fatal("start succeeded with DATA_DIR below a regular file")
+	}
+
+	want := fmt.Sprintf("DATA_DIR %q: ", dir)
+	if !strings.Contains(err.Error(), want) {
+		t.Fatalf("start error = %q, want it to contain %q", err, want)
+	}
+}
+
+// TestUnwritableDataDirStopsStart sets DATA_DIR to /proc, which exists
+// and can be read, but where no file can be created, even by root, as
+// the tests run in the backend image: root ignores a directory the test
+// makes read-only. The start must fail with an error naming DATA_DIR and
+// its value, rather than the first write of a report file.
+func TestUnwritableDataDirStopsStart(t *testing.T) {
+	dir := "/proc"
+	t.Setenv("DATA_DIR", dir)
+
+	app := fxtest.New(t,
+		fx.Provide(
+			globals.New,
+			logger.New,
+			config.New,
+			reportbuf.New,
+		),
+		fx.Invoke(func(*reportbuf.Buffer) {}),
+	)
+
+	err := app.Start(t.Context())
+	if err == nil {
+		t.Fatal("start succeeded with DATA_DIR /proc")
+	}
+
+	want := fmt.Sprintf("DATA_DIR %q: ", dir)
+	if !strings.Contains(err.Error(), want) {
+		t.Fatalf("start error = %q, want it to contain %q", err, want)
+	}
+}
+
 // TestStopWaitsForPeriodicWrite holds open the write the periodic flush
 // starts, then stops the app. The stop must wait until the write is
 // released, and the report must then be in its file. The test runs in