shell: add new APIs to get the backend instances

Added some new APIs to get the backend instances more easily,
so that dev does not need to rely on `shell_backend_*_get_ptr`,
which looks more like a hack to access a local variable.

These APIs are basically copied from the log backend
implementation.

Added testcase for the APIs.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>

include/zephyr/shell/shell_backend.h

@@ -0,0 +1,61 @@
+/*
+ * Copyright (c) 2023 Meta
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#ifndef SHELL_BACKEND_H__
+#define SHELL_BACKEND_H__
+
+#include <zephyr/types.h>
+#include <zephyr/shell/shell.h>
+#include <zephyr/sys/iterable_sections.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @brief Get backend.
+ *
+ * @param[in] idx  Pointer to the backend instance.
+ *
+ * @return Pointer to the backend instance.
+ */
+static inline const struct shell *shell_backend_get(uint32_t idx)
+{
+	const struct shell *backend;
+
+	STRUCT_SECTION_GET(shell, idx, &backend);
+
+	return backend;
+}
+
+/**
+ * @brief Get number of backends.
+ *
+ * @return Number of backends.
+ */
+static inline int shell_backend_count_get(void)
+{
+	int cnt;
+
+	STRUCT_SECTION_COUNT(shell, &cnt);
+
+	return cnt;
+}
+
+/**
+ * @brief Get backend by name.
+ *
+ * @param[in] backend_name Name of the backend as defined by the SHELL_DEFINE.
+ *
+ * @retval Pointer to the backend instance if found, NULL if backend is not found.
+ */
+const struct shell *shell_backend_get_by_name(const char *backend_name);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* SHELL_BACKEND_H__ */

subsys/shell/shell.c

@@ -1488,6 +1488,17 @@ void shell_process(const struct shell *sh)
 	z_flag_processing_set(sh, false);
 }
 
+const struct shell *shell_backend_get_by_name(const char *backend_name)
+{
+	STRUCT_SECTION_FOREACH(shell, backend) {
+		if (strcmp(backend_name, backend->name) == 0) {
+			return backend;
+		}
+	}
+
+	return NULL;
+}
+
 /* This function mustn't be used from shell context to avoid deadlock.
  * However it can be used in shell command handlers.
  */

tests/subsys/shell/shell_backend/CMakeLists.txt

@@ -0,0 +1,8 @@
+# SPDX-License-Identifier: Apache-2.0
+
+cmake_minimum_required(VERSION 3.20.0)
+find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
+project(shell)
+
+FILE(GLOB app_sources src/*.c)
+target_sources(app PRIVATE ${app_sources})

tests/subsys/shell/shell_backend/prj.conf

@@ -0,0 +1,8 @@
+CONFIG_SHELL=y
+CONFIG_SHELL_BACKEND_DUMMY=y
+CONFIG_SHELL_BACKEND_SERIAL=y
+CONFIG_SHELL_CMD_BUFF_SIZE=90
+CONFIG_SHELL_PRINTF_BUFF_SIZE=15
+CONFIG_SHELL_METAKEYS=n
+CONFIG_LOG=n
+CONFIG_ZTEST=y

tests/subsys/shell/shell_backend/src/shell_backend_test.c

@@ -0,0 +1,41 @@
+/*
+ * Copyright 2023 Meta
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <zephyr/kernel.h>
+#include <zephyr/ztest.h>
+
+#include <zephyr/shell/shell.h>
+#include <zephyr/shell/shell_backend.h>
+#include <zephyr/shell/shell_dummy.h>
+#include <zephyr/shell/shell_uart.h>
+
+ZTEST(shell_backend, test_backend_apis)
+{
+	const struct shell *sh_dummy = shell_backend_get_by_name("shell_dummy");
+	const struct shell *sh_uart = shell_backend_get_by_name("shell_uart");
+
+	zassert_equal(shell_backend_count_get(), 2, "Expecting 2, got %d",
+		      shell_backend_count_get());
+
+	zassert_equal_ptr(sh_dummy, shell_backend_dummy_get_ptr(),
+			  "Unexpected shell_dummy backend");
+	zassert_equal_ptr(sh_uart, shell_backend_uart_get_ptr(), "Unexpected shell_uart backend");
+	zassert_equal_ptr(shell_backend_get_by_name("blah"), NULL, "Should be NULL if not found");
+
+	zassert_equal_ptr(shell_backend_get(0), sh_dummy < sh_uart ? sh_dummy : sh_uart,
+			  "Unexpected backend at index 0");
+	zassert_equal_ptr(shell_backend_get(1), sh_dummy < sh_uart ? sh_uart : sh_dummy,
+			  "Unexpected backend at index 1");
+}
+
+static void *shell_setup(void)
+{
+	/* Let the shell backend initialize. */
+	k_usleep(10);
+	return NULL;
+}
+
+ZTEST_SUITE(shell_backend, NULL, shell_setup, NULL, NULL, NULL);

tests/subsys/shell/shell_backend/testcase.yaml

@@ -0,0 +1,10 @@
+tests:
+  shell.backend:
+    min_flash: 64
+    min_ram: 32
+    tags:
+      - shell
+    filter: ( CONFIG_SHELL )
+    platform_allow:
+      - qemu_x86
+      - qemu_riscv32