fix(write): enforce JSON size limit after trailing whitespace

docs/releases/unreleased.md

@@ -5,3 +5,7 @@
 - All successfully read license states exit 0. Configuration, request, decoding, and output failures exit nonzero. No token or database header is sent to `/health`, and no authentication verification is performed. Normal connection selection, timeout, TLS verification, and identity opt-out settings apply.
 
 License installation and key/file configuration remain server responsibilities. Existing `ping -o json` license pass-through is unchanged. Closes [arcli#34](https://github.com/Basekick-Labs/arcli/issues/34).
+
+### Fixed
+
+- Enforce the 64 MiB JSON write input limit when a document ends with whitespace. Oversize input is rejected before upload, including when the bounded read would hide further trailing content.

internal/client/msgpackenc.go

@@ -172,6 +172,11 @@ func JSONToMsgPack(r io.Reader) ([]byte, MsgPackDocSummary, error) {
 	if _, err := dec.Token(); err != io.EOF {
 		return nil, sum, fmt.Errorf("invalid JSON: trailing content after the document")
 	}
+	// LimitReader also reports EOF after consuming its bound. Count
+	// trailing whitespace so that EOF cannot hide oversize input.
+	if cr.n > JSONDocMaxBytes {
+		return nil, sum, tooBig()
+	}
 	converted, err := convertTop(root, &sum)
 	if err != nil {
 		return nil, sum, err

internal/client/msgpackenc_test.go

@@ -3,6 +3,7 @@ package client
 import (
 	"bytes"
 	"encoding/hex"
+	"io"
 	"strings"
 	"testing"
 )
@@ -91,6 +92,36 @@ func TestJSONToMsgPack_RowBatchAndTags(t *testing.T) {
 	}
 }
 
+func TestJSONToMsgPack_TrailingWhitespaceSizeLimit(t *testing.T) {
+	const document = `{"m":"cpu","fields":{"x":1}}`
+	padding := strings.Repeat(" ", JSONDocMaxBytes-len(document))
+	for _, tc := range []struct {
+		name, suffix string
+		wantError    bool
+	}{
+		{"at limit", "", false},
+		{"one byte over", " ", true},
+		{"trailing document beyond limit", " {}", true},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			in := io.MultiReader(strings.NewReader(document), strings.NewReader(padding), strings.NewReader(tc.suffix))
+			out, sum, err := JSONToMsgPack(in)
+			if tc.wantError {
+				if err == nil || !strings.Contains(err.Error(), "JSON document exceeds") {
+					t.Fatalf("error = %v, want size-limit error", err)
+				}
+				if out != nil || sum != (MsgPackDocSummary{}) {
+					t.Errorf("oversize input produced output: %x, summary: %+v", out, sum)
+				}
+				return
+			}
+			if err != nil || len(out) == 0 || sum.Items != 1 || sum.Rows != 1 {
+				t.Errorf("input at limit: output=%x summary=%+v error=%v", out, sum, err)
+			}
+		})
+	}
+}
+
 func TestJSONToMsgPack_Rejections(t *testing.T) {
 	cases := map[string]string{
 		`{"columns":{"x":[1]}}`:                                        `"m" (measurement) must be a non-empty string`,

internal/commands/write_test.go

@@ -0,0 +1,40 @@
+package commands
+
+import (
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"sync/atomic"
+	"testing"
+
+	"github.com/basekick-labs/arcli/internal/client"
+)
+
+func TestWriteJSON_RejectsOversizeTrailingWhitespaceBeforeUpload(t *testing.T) {
+	var requests atomic.Int32
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		requests.Add(1)
+		_, _ = io.Copy(io.Discard, r.Body)
+		w.WriteHeader(http.StatusNoContent)
+	}))
+	defer srv.Close()
+	writeTestConfig(t, srv.URL, "tok")
+
+	const document = `{"m":"cpu","fields":{"x":1}}`
+	cmd := newWriteCmd()
+	cmd.SetIn(io.MultiReader(
+		strings.NewReader(document),
+		strings.NewReader(strings.Repeat(" ", client.JSONDocMaxBytes-len(document)+1)),
+	))
+	out, _, err := execCmd(t, cmd, "--format", "json")
+	if err == nil || !strings.Contains(err.Error(), "JSON document exceeds") {
+		t.Errorf("error = %v, want size-limit error", err)
+	}
+	if out != "" {
+		t.Errorf("stdout = %q, want no success output", out)
+	}
+	if got := requests.Load(); got != 0 {
+		t.Errorf("sent %d HTTP requests for rejected input", got)
+	}
+}