chore: add C binding example for server-side Redis Source (#470)

We had a ready-to-go example for C++, but the only good example for C
was buried in a unit test. I've extracted that out into a buildable
hello-example.

examples/CMakeLists.txt

@@ -6,4 +6,5 @@ add_subdirectory(client-and-server-coexistence)
 
 if (LD_BUILD_REDIS_SUPPORT)
     add_subdirectory(hello-cpp-server-redis)
+    add_subdirectory(hello-c-server-redis)
 endif ()

examples/hello-c-server-redis/CMakeLists.txt

@@ -0,0 +1,15 @@
+# Required for Apple Silicon support.
+cmake_minimum_required(VERSION 3.19)
+
+project(
+        LaunchDarklyHelloCServerRedis
+        VERSION 0.1
+        DESCRIPTION "LaunchDarkly Hello C Server-side SDK with Redis source"
+        LANGUAGES C
+)
+
+set(THREADS_PREFER_PTHREAD_FLAG ON)
+find_package(Threads REQUIRED)
+
+add_executable(hello-c-server-redis-source main.c)
+target_link_libraries(hello-c-server-redis-source PRIVATE launchdarkly::server_redis_source Threads::Threads)

examples/hello-c-server-redis/main.c

@@ -0,0 +1,119 @@
+#include <launchdarkly/server_side/bindings/c/sdk.h>
+
+#include <launchdarkly/bindings/c/context_builder.h>
+
+#include <launchdarkly/server_side/bindings/c/integrations/redis/redis_source.h>
+
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+// Set SDK_KEY to your LaunchDarkly SKD key.
+#define SDK_KEY ""
+
+// Set FEATURE_FLAG_KEY to the feature flag key you want to evaluate.
+#define FEATURE_FLAG_KEY "my-boolean-flag"
+
+// Set INIT_TIMEOUT_MILLISECONDS to the amount of time you will wait for
+// the client to become initialized.
+#define INIT_TIMEOUT_MILLISECONDS 3000
+
+// Set REDIS_URI to your own Redis instance's URI.
+#define REDIS_URI "redis://localhost:6379"
+
+// Set REDIS_PREFIX to the prefix containing the launchdarkly
+// environment data in Redis.
+#define REDIS_PREFIX "launchdarkly"
+
+char const* get_with_env_fallback(char const* source_val,
+                                  char const* env_variable,
+                                  char const* error_msg);
+
+int main() {
+    char const* sdk_key = get_with_env_fallback(
+        SDK_KEY, "LD_SDK_KEY",
+        "Please edit main.c to set SDK_KEY to your LaunchDarkly server-side "
+        "SDK key "
+        "first.\n\nAlternatively, set the LD_SDK_KEY environment "
+        "variable.\n"
+        "The value of SDK_KEY in main.c takes priority over LD_SDK_KEY.");
+
+    struct LDServerLazyLoadRedisResult result;
+    if (!LDServerLazyLoadRedisSource_New(REDIS_URI, REDIS_PREFIX, &result)) {
+        printf("error: couldn't instantiate Redis source: %s\n",
+               result.error_message);
+        return 1;
+    }
+
+    LDServerConfigBuilder config_builder = LDServerConfigBuilder_New(sdk_key);
+
+    LDServerLazyLoadBuilder lazy_builder = LDServerLazyLoadBuilder_New();
+    LDServerLazyLoadBuilder_SourcePtr(lazy_builder,
+                                      (LDServerLazyLoadSourcePtr)result.source);
+    LDServerConfigBuilder_DataSystem_LazyLoad(config_builder, lazy_builder);
+
+    LDServerConfig config = NULL;
+    LDStatus config_status =
+        LDServerConfigBuilder_Build(config_builder, &config);
+    if (!LDStatus_Ok(config_status)) {
+        printf("error: config is invalid: %s\n", LDStatus_Error(config_status));
+        return 1;
+    }
+
+    LDServerSDK client = LDServerSDK_New(config);
+
+    bool initialized_successfully = false;
+    if (LDServerSDK_Start(client, INIT_TIMEOUT_MILLISECONDS,
+                          &initialized_successfully)) {
+        if (initialized_successfully) {
+            printf("*** SDK successfully initialized!\n\n");
+        } else {
+            printf("*** SDK failed to initialize\n");
+            return 1;
+        }
+    } else {
+        printf("*** SDK initialization didn't complete in %dms\n",
+               INIT_TIMEOUT_MILLISECONDS);
+        return 1;
+    }
+
+    LDContextBuilder context_builder = LDContextBuilder_New();
+    LDContextBuilder_AddKind(context_builder, "user", "example-user-key");
+    LDContextBuilder_Attributes_SetName(context_builder, "user", "Sandy");
+    LDContext context = LDContextBuilder_Build(context_builder);
+
+    bool flag_value =
+        LDServerSDK_BoolVariation(client, context, FEATURE_FLAG_KEY, false);
+
+    printf("*** Feature flag '%s' is %s for this user\n\n", FEATURE_FLAG_KEY,
+           flag_value ? "true" : "false");
+
+    // Here we ensure that the SDK shuts down cleanly and has a chance to
+    // deliver analytics events to LaunchDarkly before the program exits. If
+    // analytics events are not delivered, the user properties and flag usage
+    // statistics will not appear on your dashboard. In a normal long-running
+    // application, the SDK would continue running and events would be delivered
+    // automatically in the background.
+
+    LDContext_Free(context);
+    LDServerSDK_Free(client);
+
+    return 0;
+}
+
+char const* get_with_env_fallback(char const* source_val,
+                                  char const* env_variable,
+                                  char const* error_msg) {
+    if (strlen(source_val)) {
+        return source_val;
+    }
+
+    char const* from_env = getenv(env_variable);
+    if (from_env && strlen(from_env)) {
+        return from_env;
+    }
+
+    printf("*** %s\n\n", error_msg);
+    exit(1);
+}