add some notes about libev

Author: stevegrubb

src/libev/libev-select-notes.md

@@ -0,0 +1,21 @@
+# libev select backend notes
+
+## Build composition
+- `libev_la_SOURCES` only lists `ev.c` and `event.c` because `ev.c` directly
+  `#include`s the backend implementations such as `ev_select.c`. This means the
+  select backend is compiled whenever `ev.c` is built, without needing to list
+  the backend sources separately.
+
+## Signal handling and readiness notification
+- When a signal arrives, `ev_sighandler()` invokes `ev_feed_signal()`, which
+  records the pending signal and calls `evpipe_write()` to wake the loop via the
+  self-pipe using the async-signal-safe `write()` system call.
+- The select backend copies the tracked fd sets before calling `select()`.
+  If `select()` returns with `EINTR`, the backend simply returns to the main
+  loop. Because the self-pipe write occurred before the interruption completes,
+  the pipe remains readable and will be observed on the next poll iteration,
+  ensuring the readiness notification is delivered.
+- `select_poll()` also detects `EBADF` and `ENOMEM`, reporting them via
+  libev’s error helpers, and it retries interrupted `read()`/`write()` calls to
+  drain or signal the pipe, so the loop cannot miss the wakeup due to transient
+  EINTR interruptions.