feat: Add C bindings for FDv2 data system

libs/server-sdk/include/launchdarkly/server_side/bindings/c/config/builder.h

@@ -4,6 +4,7 @@
 #pragma once
 
 #include <launchdarkly/server_side/bindings/c/config/config.h>
+#include <launchdarkly/server_side/bindings/c/config/fdv2_builder/fdv2_builder.h>
 #include <launchdarkly/server_side/bindings/c/config/lazy_load_builder/lazy_load_builder.h>
 #include <launchdarkly/server_side/bindings/c/hook.h>
 
@@ -256,6 +257,27 @@ LDServerConfigBuilder_DataSystem_LazyLoad(
     LDServerConfigBuilder b,
     LDServerLazyLoadBuilder lazy_load_builder);
 
+/**
+ * Configures the FDv2 data system. The builder is automatically consumed.
+ *
+ * This method is mutually exclusive with the BackgroundSync_Streaming,
+ * BackgroundSync_Polling, and LazyLoad builders.
+ *
+ * WARNING: Do not call any other LDServerFDv2Builder function on the provided
+ * builder after calling this function. It is undefined behavior.
+ *
+ * FDv2 receives flag delivery updates over the changeset-based protocol with
+ * built-in fallback and recovery semantics. An optional FDv1 fallback may be
+ * configured on the FDv2 builder for service-directed protocol fallback.
+ *
+ * @param b Server config builder. Must not be NULL.
+ * @param fdv2_builder The FDv2 builder. The builder is consumed; do not free
+ * it. Must not be NULL.
+ */
+LD_EXPORT(void)
+LDServerConfigBuilder_DataSystem_FDv2(LDServerConfigBuilder b,
+                                      LDServerFDv2Builder fdv2_builder);
+
 /**
  * Specify if the SDK's data system should be enabled or not.
  *

libs/server-sdk/include/launchdarkly/server_side/bindings/c/config/fdv2_builder/fdv2_builder.h

@@ -0,0 +1,285 @@
+/** @file fdv2_builder.h */
+// NOLINTBEGIN modernize-use-using
+
+#pragma once
+
+#include <launchdarkly/bindings/c/export.h>
+
+#include <stdbool.h>
+
+#ifdef __cplusplus
+extern "C" {  // only need to export C interface if used by C++ source code
+#endif
+
+typedef struct _LDServerFDv2Builder* LDServerFDv2Builder;
+typedef struct _LDServerFDv2StreamingBuilder* LDServerFDv2StreamingBuilder;
+typedef struct _LDServerFDv2PollingBuilder* LDServerFDv2PollingBuilder;
+
+/* Reused as the input handles for LDServerFDv2Builder_FDv1Fallback_*. Fully
+ * declared in <launchdarkly/server_side/bindings/c/config/builder.h>. */
+typedef struct _LDServerDataSourceStreamBuilder* LDServerDataSourceStreamBuilder;
+typedef struct _LDServerDataSourcePollBuilder* LDServerDataSourcePollBuilder;
+
+/**
+ * Creates an FDv2 builder pre-populated with the spec-recommended initializers,
+ * synchronizers, and FDv1 fallback. Suitable for most applications without
+ * further configuration.
+ *
+ * If not passed into the config builder, must be manually freed with
+ * LDServerFDv2Builder_Free.
+ *
+ * @return A new FDv2 builder with default sources.
+ */
+LD_EXPORT(LDServerFDv2Builder)
+LDServerFDv2Builder_Default(void);
+
+/**
+ * Creates an FDv2 builder with no initializers, no synchronizers, and no
+ * FDv1 fallback. Sources must be added explicitly via Initializer_*,
+ * Synchronizer_*, and FDv1Fallback_* methods.
+ *
+ * If not passed into the config builder, must be manually freed with
+ * LDServerFDv2Builder_Free.
+ *
+ * @return A new empty FDv2 builder.
+ */
+LD_EXPORT(LDServerFDv2Builder)
+LDServerFDv2Builder_Custom(void);
+
+/**
+ * Frees an FDv2 builder. Do not call if the builder was consumed by the
+ * config builder.
+ *
+ * @param b Builder to free.
+ */
+LD_EXPORT(void)
+LDServerFDv2Builder_Free(LDServerFDv2Builder b);
+
+/**
+ * Creates a new FDv2 Streaming source builder.
+ *
+ * If not passed into an FDv2 builder, must be manually freed with
+ * LDServerFDv2StreamingBuilder_Free.
+ *
+ * @return A new FDv2 Streaming source builder.
+ */
+LD_EXPORT(LDServerFDv2StreamingBuilder)
+LDServerFDv2StreamingBuilder_New(void);
+
+/**
+ * Sets the initial reconnect delay for the FDv2 streaming connection.
+ *
+ * The streaming service uses a backoff algorithm (with jitter) every time
+ * the connection needs to be reestablished. The delay for the first
+ * reconnection will start near this value, and then increase exponentially
+ * for any subsequent connection failures.
+ *
+ * @param b FDv2 Streaming source builder. Must not be NULL.
+ * @param milliseconds Initial delay for a reconnection attempt.
+ */
+LD_EXPORT(void)
+LDServerFDv2StreamingBuilder_InitialReconnectDelayMs(
+    LDServerFDv2StreamingBuilder b,
+    unsigned int milliseconds);
+
+/**
+ * Overrides the base URL used by this FDv2 streaming source. By default the
+ * streaming source reads its endpoint from the top-level ServiceEndpoints
+ * configuration; this override takes precedence for this source only.
+ *
+ * @param b FDv2 Streaming source builder. Must not be NULL.
+ * @param base_url Target URL. Must not be NULL.
+ */
+LD_EXPORT(void)
+LDServerFDv2StreamingBuilder_BaseURL(LDServerFDv2StreamingBuilder b,
+                                     char const* base_url);
+
+/**
+ * Frees an FDv2 Streaming source builder. Do not call if the builder was
+ * consumed by an FDv2 builder.
+ *
+ * @param b Builder to free.
+ */
+LD_EXPORT(void)
+LDServerFDv2StreamingBuilder_Free(LDServerFDv2StreamingBuilder b);
+
+/**
+ * Creates a new FDv2 Polling source builder.
+ *
+ * If not passed into an FDv2 builder, must be manually freed with
+ * LDServerFDv2PollingBuilder_Free.
+ *
+ * @return A new FDv2 Polling source builder.
+ */
+LD_EXPORT(LDServerFDv2PollingBuilder)
+LDServerFDv2PollingBuilder_New(void);
+
+/**
+ * Sets the interval at which this FDv2 polling source will poll for flag
+ * updates.
+ *
+ * @param b FDv2 Polling source builder. Must not be NULL.
+ * @param seconds Polling interval in seconds.
+ */
+LD_EXPORT(void)
+LDServerFDv2PollingBuilder_IntervalS(LDServerFDv2PollingBuilder b,
+                                         unsigned int seconds);
+
+/**
+ * Overrides the base URL used by this FDv2 polling source. By default the
+ * polling source reads its endpoint from the top-level ServiceEndpoints
+ * configuration; this override takes precedence for this source only.
+ *
+ * @param b FDv2 Polling source builder. Must not be NULL.
+ * @param base_url Target URL. Must not be NULL.
+ */
+LD_EXPORT(void)
+LDServerFDv2PollingBuilder_BaseURL(LDServerFDv2PollingBuilder b,
+                                   char const* base_url);
+
+/**
+ * Frees an FDv2 Polling source builder. Do not call if the builder was
+ * consumed by an FDv2 builder.
+ *
+ * @param b Builder to free.
+ */
+LD_EXPORT(void)
+LDServerFDv2PollingBuilder_Free(LDServerFDv2PollingBuilder b);
+
+/**
+ * Appends an FDv2 polling initializer to the FDv2 builder's initializers
+ * list. The source builder is automatically freed.
+ *
+ * WARNING: Do not call any other LDServerFDv2PollingBuilder function on the
+ * provided builder after calling this function. It is undefined behavior.
+ *
+ * @param b FDv2 builder. Must not be NULL.
+ * @param polling The FDv2 Polling source builder. The builder is consumed;
+ * do not free it. Must not be NULL.
+ */
+LD_EXPORT(void)
+LDServerFDv2Builder_Initializer_Polling(LDServerFDv2Builder b,
+                                        LDServerFDv2PollingBuilder polling);
+
+/**
+ * Appends an FDv2 streaming synchronizer to the FDv2 builder's synchronizers
+ * list. The source builder is automatically freed.
+ *
+ * Order in the list determines preference: the first entry is the primary
+ * synchronizer, subsequent entries are fallbacks.
+ *
+ * WARNING: Do not call any other LDServerFDv2StreamingBuilder function on
+ * the provided builder after calling this function. It is undefined behavior.
+ *
+ * @param b FDv2 builder. Must not be NULL.
+ * @param streaming The FDv2 Streaming source builder. The builder is
+ * consumed; do not free it. Must not be NULL.
+ */
+LD_EXPORT(void)
+LDServerFDv2Builder_Synchronizer_Streaming(
+    LDServerFDv2Builder b,
+    LDServerFDv2StreamingBuilder streaming);
+
+/**
+ * Appends an FDv2 polling synchronizer to the FDv2 builder's synchronizers
+ * list. The source builder is automatically freed.
+ *
+ * See LDServerFDv2Builder_Synchronizer_Streaming for ordering semantics.
+ *
+ * WARNING: Do not call any other LDServerFDv2PollingBuilder function on the
+ * provided builder after calling this function. It is undefined behavior.
+ *
+ * @param b FDv2 builder. Must not be NULL.
+ * @param polling The FDv2 Polling source builder. The builder is consumed;
+ * do not free it. Must not be NULL.
+ */
+LD_EXPORT(void)
+LDServerFDv2Builder_Synchronizer_Polling(LDServerFDv2Builder b,
+                                         LDServerFDv2PollingBuilder polling);
+
+/**
+ * Configures the FDv1 streaming source used as a last-resort fallback when
+ * the LaunchDarkly service signals (via the X-LD-FD-Fallback header) that
+ * the SDK should switch to FDv1. The fallback reads its endpoint from the
+ * top-level ServiceEndpoints configuration. The source builder is
+ * automatically freed.
+ *
+ * The fdv1_streaming parameter uses the same handle type as the
+ * BackgroundSync streaming synchronizer
+ * (LDServerDataSourceStreamBuilder_New); the underlying C++ type is shared.
+ *
+ * WARNING: Do not call any other LDServerDataSourceStreamBuilder function
+ * on the provided builder after calling this function. It is undefined
+ * behavior.
+ *
+ * @param b FDv2 builder. Must not be NULL.
+ * @param fdv1_streaming The FDv1 Streaming source builder. The builder is
+ * consumed; do not free it. Must not be NULL.
+ */
+LD_EXPORT(void)
+LDServerFDv2Builder_FDv1Fallback_Streaming(
+    LDServerFDv2Builder b,
+    LDServerDataSourceStreamBuilder fdv1_streaming);
+
+/**
+ * Configures the FDv1 polling source used as a last-resort fallback when
+ * the LaunchDarkly service signals (via the X-LD-FD-Fallback header) that
+ * the SDK should switch to FDv1. The fallback reads its endpoint from the
+ * top-level ServiceEndpoints configuration. The source builder is
+ * automatically freed.
+ *
+ * The fdv1_polling parameter uses the same handle type as the BackgroundSync
+ * polling synchronizer (LDServerDataSourcePollBuilder_New); the underlying
+ * C++ type is shared.
+ *
+ * WARNING: Do not call any other LDServerDataSourcePollBuilder function on
+ * the provided builder after calling this function. It is undefined behavior.
+ *
+ * @param b FDv2 builder. Must not be NULL.
+ * @param fdv1_polling The FDv1 Polling source builder. The builder is
+ * consumed; do not free it. Must not be NULL.
+ */
+LD_EXPORT(void)
+LDServerFDv2Builder_FDv1Fallback_Polling(
+    LDServerFDv2Builder b,
+    LDServerDataSourcePollBuilder fdv1_polling);
+
+/**
+ * Disables the FDv1 fallback. After this call, an FDv1 fallback directive
+ * from the service leaves the data source in the interrupted state and
+ * schedules an FDv2 retry on the directive's TTL.
+ *
+ * @param b FDv2 builder. Must not be NULL.
+ */
+LD_EXPORT(void)
+LDServerFDv2Builder_DisableFDv1Fallback(LDServerFDv2Builder b);
+
+/**
+ * Sets how long the active synchronizer may remain interrupted before the
+ * orchestrator falls back to the next-preferred synchronizer.
+ *
+ * @param b FDv2 builder. Must not be NULL.
+ * @param milliseconds Duration the synchronizer must be continuously
+ * interrupted for before fallback fires.
+ */
+LD_EXPORT(void)
+LDServerFDv2Builder_FallbackTimeoutMs(LDServerFDv2Builder b,
+                                      unsigned int milliseconds);
+
+/**
+ * Sets how long a fallback synchronizer must run successfully before the
+ * orchestrator attempts to recover to the primary synchronizer.
+ *
+ * @param b FDv2 builder. Must not be NULL.
+ * @param milliseconds Duration the fallback synchronizer must run before a
+ * recovery attempt is made.
+ */
+LD_EXPORT(void)
+LDServerFDv2Builder_RecoveryTimeoutMs(LDServerFDv2Builder b,
+                                      unsigned int milliseconds);
+
+#ifdef __cplusplus
+}
+#endif
+
+// NOLINTEND modernize-use-using

libs/server-sdk/src/CMakeLists.txt

@@ -111,6 +111,7 @@ target_sources(${LIBNAME}
         hooks/hook_executor.cpp
         bindings/c/sdk.cpp
         bindings/c/builder.cpp
+        bindings/c/fdv2_builder.cpp
         bindings/c/config.cpp
         bindings/c/all_flags_state/all_flags_state.cpp
         bindings/c/hook.cpp

libs/server-sdk/src/bindings/c/builder.cpp

@@ -225,6 +225,17 @@ LDServerConfigBuilder_DataSystem_LazyLoad(
     LDServerLazyLoadBuilder_Free(lazy_load_builder);
 }
 
+LD_EXPORT(void)
+LDServerConfigBuilder_DataSystem_FDv2(LDServerConfigBuilder b,
+                                      LDServerFDv2Builder fdv2_builder) {
+    LD_ASSERT_NOT_NULL(b);
+    LD_ASSERT_NOT_NULL(fdv2_builder);
+
+    auto* fb = reinterpret_cast<DataSystemBuilder::FDv2*>(fdv2_builder);
+    TO_BUILDER(b)->DataSystem().Method(*fb);
+    LDServerFDv2Builder_Free(fdv2_builder);
+}
+
 LD_EXPORT(void)
 LDServerConfigBuilder_DataSystem_Enabled(LDServerConfigBuilder b,
                                          bool const enabled) {