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

TODO.md

@@ -22,12 +22,14 @@ Run `make frontend-viewport-test` in CI as its own step; it is not part of
 # Completed Steps
 
 - 2026-10-18: when the backend runs on its own, a `DATA_DIR` that cannot be
-  created or read stops the start with an error naming `DATA_DIR` and its value,
-  as the other settings' errors do
-  ([#148](https://git.eeqj.de/sneak/netwatch/issues/148)); it named only the
-  path. A unit test starts the buffer with `DATA_DIR` below a regular file and
-  checks the error. The report file number is an `atomic.Int64`, as the Go style
-  guide asks for int types
+  created or read, or where no file can be created, stops the start with an
+  error naming `DATA_DIR` and its value, as the other settings' errors do
+  ([#148](https://git.eeqj.de/sneak/netwatch/issues/148)); the first two named
+  only the path, and the third was found only at the first write of a report
+  file. At start the server creates a file in `DATA_DIR` and removes it. Unit
+  tests start the buffer with `DATA_DIR` below a regular file and with
+  `DATA_DIR` set to `/proc`, and check the error. The report file number is an
+  `atomic.Int64`, as the Go style guide asks for int types
   ([#161](https://git.eeqj.de/sneak/netwatch/issues/161)); file names are
   unchanged
 - 2026-10-18: a browser that cannot format the header's clocks still probes

internal/reportbuf/reportbuf.go

@@ -134,6 +134,19 @@ func New(
 				return fmt.Errorf("DATA_DIR %q: %w", b.dataDir, err)
 			}
 
+			// Creating and removing a file now stops the start if
+			// report files cannot be written, instead of the first
+			// write failing after reports were answered 200.
+			f, err := os.CreateTemp(b.dataDir, "")
+			if err != nil {
+				return fmt.Errorf("DATA_DIR %q: %w", b.dataDir, err)
+			}
+
+			err = errors.Join(f.Close(), os.Remove(f.Name()))
+			if err != nil {
+				return fmt.Errorf("DATA_DIR %q: %w", b.dataDir, err)
+			}
+
 			// Report files left by earlier runs count too, and are
 			// the first to be deleted to make room.
 			files, err := reportFiles(b.dataDir)

internal/reportbuf/reportbuf_test.go

@@ -140,6 +140,36 @@ func TestUncreatableDataDirStopsStart(t *testing.T) {
 	}
 }
 
+// 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