fsmonitor-ipc: create client routines for git-fsmonitor--daemon

Create fsmonitor_ipc__*() client routines to spawn the built-in file
system monitor daemon and send it an IPC request using the `Simple
IPC` API.

Stub in empty fsmonitor_ipc__*() functions for unsupported platforms.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fsmonitor-ipc.h

@@ -0,0 +1,48 @@
+#ifndef FSMONITOR_IPC_H
+#define FSMONITOR_IPC_H
+
+#include "simple-ipc.h"
+
+/*
+ * Returns true if built-in file system monitor daemon is defined
+ * for this platform.
+ */
+int fsmonitor_ipc__is_supported(void);
+
+/*
+ * Returns the pathname to the IPC named pipe or Unix domain socket
+ * where a `git-fsmonitor--daemon` process will listen.  This is a
+ * per-worktree value.
+ *
+ * Returns NULL if the daemon is not supported on this platform.
+ */
+const char *fsmonitor_ipc__get_path(void);
+
+/*
+ * Try to determine whether there is a `git-fsmonitor--daemon` process
+ * listening on the IPC pipe/socket.
+ */
+enum ipc_active_state fsmonitor_ipc__get_state(void);
+
+/*
+ * Connect to a `git-fsmonitor--daemon` process via simple-ipc
+ * and ask for the set of changed files since the given token.
+ *
+ * Spawn a daemon process in the background if necessary.
+ *
+ * Returns -1 on error; 0 on success.
+ */
+int fsmonitor_ipc__send_query(const char *since_token,
+			      struct strbuf *answer);
+
+/*
+ * Connect to a `git-fsmonitor--daemon` process via simple-ipc and
+ * send a command verb.  If no daemon is available, we DO NOT try to
+ * start one.
+ *
+ * Returns -1 on error; 0 on success.
+ */
+int fsmonitor_ipc__send_command(const char *command,
+				struct strbuf *answer);
+
+#endif /* FSMONITOR_IPC_H */