pw_sys_io: Add TryReadByte(byte* dest) function

This adds a non-blocking version of ReadByte() that enables writing, for
example, a superloop that combines custom processing simultaneously with
byte input handling for the RPC subsystem.

Change-Id: I9cb1e6ae8c944ca7cdff6d4bff101ac52e80b53a
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/23980
Reviewed-by: Armando Montanez <amontanez@google.com>
Commit-Queue: Keir Mierle <keir@google.com>

1 files changed, 10 insertions, 1 deletions

diff --git a/pw_sys_io/public/pw_sys_io/sys_io.h b/pw_sys_io/public/pw_sys_io/sys_io.h
index 5973579fc..477c71967 100644
--- a/pw_sys_io/public/pw_sys_io/sys_io.h
+++ b/pw_sys_io/public/pw_sys_io/sys_io.h
@@ -51,9 +51,18 @@ namespace pw::sys_io {
 // This function will block until it either succeeds or fails to read a byte
 // from the pw_sys_io backend.
 //
-// Returns Status::Ok() if a byte was successfully read.
+// Returns Status::Ok() - A byte was successfully read.
+//         Status::ResourceExhausted() - if the underlying source vanished.
 Status ReadByte(std::byte* dest);
 
+// Read a single byte from the sys io backend, if available.
+// Implemented by: Backend
+//
+// Returns Status::Ok() - A byte was successfully read, and is in dest.
+//         Status::Unavailable() - No byte is available to read; try later.
+//         Status::Unimplemented() - Not supported on this target.
+Status TryReadByte(std::byte* dest);
+
 // Write a single byte out the sys io backend.
 // Implemented by: Backend
 //