From 0412dea5ca9e56e06e35b09e1886f36c971e919d Mon Sep 17 00:00:00 2001
From: Peter Solnica <peter@solnica.online>
Date: Mon, 14 Sep 2026 08:28:22 +0000
Subject: [PATCH] fix(telemetry_processor): validate buffer config

---
 lib/sentry/config.ex                  | 41 ++++++++++++++++++++++++++-
 lib/sentry/telemetry/buffer.ex        |  6 ++++
 test/sentry/config_test.exs           |  8 ++++++
 test/sentry/telemetry/buffer_test.exs | 31 ++++++++++++++++++--
 4 files changed, 83 insertions(+), 3 deletions(-)

diff --git a/lib/sentry/config.ex b/lib/sentry/config.ex
index 475ec4eb1..2a1439fc0 100644
--- a/lib/sentry/config.ex
+++ b/lib/sentry/config.ex
@@ -742,7 +742,7 @@ defmodule Sentry.Config do
       """
     ],
     telemetry_buffer_capacities: [
-      type: {:map, {:in, [:error, :check_in, :transaction, :log, :metric]}, :pos_integer},
+      type: {:custom, __MODULE__, :__validate_telemetry_buffer_capacities__, []},
       default: %{},
       type_doc: "`%{category => pos_integer()}`",
       doc: """
@@ -1493,6 +1493,45 @@ defmodule Sentry.Config do
     {:error, "expected a Regex or a string pattern, got: #{inspect(term)}"}
   end
 
+  def __validate_telemetry_buffer_capacities__(capacities) when is_map(capacities) do
+    Enum.reduce_while(capacities, {:ok, capacities}, fn {category, capacity}, acc ->
+      case validate_buffer_capacity(category, capacity) do
+        :ok -> {:cont, acc}
+        {:error, _reason} = error -> {:halt, error}
+      end
+    end)
+  end
+
+  def __validate_telemetry_buffer_capacities__(other) do
+    {:error, "expected a map of category to capacity, got: #{inspect(other)}"}
+  end
+
+  defp validate_buffer_capacity(category, _capacity)
+       when category not in [:error, :check_in, :transaction, :log, :metric] do
+    {:error,
+     "expected category to be one of [:error, :check_in, :transaction, :log, :metric], " <>
+       "got: #{inspect(category)}"}
+  end
+
+  defp validate_buffer_capacity(category, capacity)
+       when not is_integer(capacity) or capacity < 1 do
+    {:error,
+     "expected capacity for #{inspect(category)} to be a positive integer, " <>
+       "got: #{inspect(capacity)}"}
+  end
+
+  defp validate_buffer_capacity(category, capacity) do
+    batch_size = Sentry.Telemetry.Category.default_config(category).batch_size
+
+    if capacity < batch_size do
+      {:error,
+       "capacity for #{inspect(category)} must be at least its batch size of #{batch_size}, " <>
+         "got: #{inspect(capacity)}"}
+    else
+      :ok
+    end
+  end
+
   def __validate_oban_tags_to_sentry_tags__(nil), do: {:ok, nil}
 
   def __validate_oban_tags_to_sentry_tags__(fun) when is_function(fun, 1) do
diff --git a/lib/sentry/telemetry/buffer.ex b/lib/sentry/telemetry/buffer.ex
index b281de720..bc133309e 100644
--- a/lib/sentry/telemetry/buffer.ex
+++ b/lib/sentry/telemetry/buffer.ex
@@ -136,6 +136,12 @@ defmodule Sentry.Telemetry.Buffer do
       last_flush_time: System.monotonic_time(:millisecond)
     }
 
+    if state.batch_size > state.capacity do
+      raise ArgumentError,
+            ":batch_size (#{state.batch_size}) cannot be greater than :capacity " <>
+              "(#{state.capacity}) for the #{inspect(category)} buffer"
+    end
+
     {:ok, state}
   end
 
diff --git a/test/sentry/config_test.exs b/test/sentry/config_test.exs
index 3a96f9f68..7d2fa07f7 100644
--- a/test/sentry/config_test.exs
+++ b/test/sentry/config_test.exs
@@ -367,6 +367,14 @@ defmodule Sentry.ConfigTest do
       assert_raise ArgumentError, ~r/telemetry_buffer_capacities/, fn ->
         Config.validate!(telemetry_buffer_capacities: %{log: 0})
       end
+
+      assert_raise ArgumentError, ~r/telemetry_buffer_capacities/, fn ->
+        Config.validate!(telemetry_buffer_capacities: %{log: 50})
+      end
+
+      assert Config.validate!(telemetry_buffer_capacities: %{log: 100})[
+               :telemetry_buffer_capacities
+             ] == %{log: 100}
     end
 
     test ":namespace with valid resolver" do
diff --git a/test/sentry/telemetry/buffer_test.exs b/test/sentry/telemetry/buffer_test.exs
index 107cc1213..a9f25158f 100644
--- a/test/sentry/telemetry/buffer_test.exs
+++ b/test/sentry/telemetry/buffer_test.exs
@@ -27,6 +27,21 @@ defmodule Sentry.Telemetry.BufferTest do
       GenServer.stop(pid)
     end
 
+    test "refuses a capacity smaller than the batch size" do
+      Process.flag(:trap_exit, true)
+
+      assert {:error, {%ArgumentError{message: message}, _stacktrace}} =
+               Buffer.start_link(
+                 category: :log,
+                 name: :test_buffer_capacity_below_batch,
+                 capacity: 2,
+                 batch_size: 5
+               )
+
+      assert message =~ ":batch_size"
+      assert message =~ ":capacity"
+    end
+
     test "allows overriding capacity, batch_size, and timeout" do
       assert {:ok, pid} =
                Buffer.start_link(
@@ -135,7 +150,14 @@ defmodule Sentry.Telemetry.BufferTest do
 
   describe "overflow behavior" do
     test "drops oldest item when buffer is full" do
-      {:ok, pid} = Buffer.start_link(category: :log, name: :test_buffer_overflow, capacity: 2)
+      {:ok, pid} =
+        Buffer.start_link(
+          category: :log,
+          name: :test_buffer_overflow,
+          capacity: 2,
+          batch_size: 1
+        )
+
       Buffer.add(pid, make_item("e1"))
       Buffer.add(pid, make_item("e2"))
       Buffer.add(pid, make_item("e3"))
@@ -172,7 +194,12 @@ defmodule Sentry.Telemetry.BufferTest do
 
     test "FIFO ordering is preserved after overflow" do
       {:ok, pid} =
-        Buffer.start_link(category: :log, name: :test_buffer_fifo_overflow, capacity: 3)
+        Buffer.start_link(
+          category: :log,
+          name: :test_buffer_fifo_overflow,
+          capacity: 3,
+          batch_size: 1
+        )
 
       for i <- 1..5, do: Buffer.add(pid, make_item("e#{i}"))