io: use posix_fadvise() instead of readahead()

readahead() is a Linux-specific system call, and many C libraries (e.g.
Android's Bionic) do not declare it by default. A more-portable alternative is
to use posix_fadvise() with `POSIX_FADV_WILLNEED`. This is functionally
equivalent to readahead() on Linux.

Additionally, on FreeBSD, Dispatch is currently using fcntl() with `F_RDAHEAD`.
This is not exactly equivalent to Darwin's `F_RDADVISE`. Using posix_fadvise()
should work better here too.

Author: adierking

CMakeLists.txt

@@ -200,6 +200,7 @@ check_function_exists(mach_absolute_time HAVE_MACH_ABSOLUTE_TIME)
 check_function_exists(mach_approximate_time HAVE_MACH_APPROXIMATE_TIME)
 check_function_exists(mach_port_construct HAVE_MACH_PORT_CONSTRUCT)
 check_function_exists(malloc_create_zone HAVE_MALLOC_CREATE_ZONE)
+check_function_exists(posix_fadvise HAVE_POSIX_FADVISE)
 check_function_exists(pthread_key_init_np HAVE_PTHREAD_KEY_INIT_NP)
 check_function_exists(pthread_main_np HAVE_PTHREAD_MAIN_NP)
 check_function_exists(pthread_workqueue_setdispatch_np HAVE_PTHREAD_WORKQUEUE_SETDISPATCH_NP)

cmake/config.h.in

@@ -132,6 +132,9 @@
 /* Define if you have the Objective-C runtime */
 #cmakedefine HAVE_OBJC
 
+/* Define to 1 if you have the `posix_fadvise' function. */
+#cmakedefine HAVE_POSIX_FADVISE
+
 /* Define to 1 if you have the `pthread_key_init_np' function. */
 #cmakedefine HAVE_PTHREAD_KEY_INIT_NP
 

src/io.c

@@ -20,11 +20,6 @@
 
 #include "internal.h"
 
-#if defined(__FreeBSD__)
-#include <fcntl.h>
-#define F_RDADVISE F_RDAHEAD
-#endif
-
 #ifndef DISPATCH_IO_DEBUG
 #define DISPATCH_IO_DEBUG DISPATCH_DEBUG
 #endif
@@ -2235,9 +2230,8 @@ _dispatch_operation_advise(dispatch_operation_t op, size_t chunk_size)
 	(void)chunk_size;
 #else
 	if (_dispatch_io_get_error(op, NULL, true)) return;
-#if defined(__linux__) || defined(__FreeBSD__)
-	// linux does not support fcntl (F_RDAVISE)
-	// define necessary datastructure and use readahead
+#if !defined(F_RDADVISE)
+	// Compatibility struct whose values may be passed to posix_fadvise()
 	struct radvisory {
 		off_t ra_offset;
 		int   ra_count;
@@ -2262,20 +2256,25 @@ _dispatch_operation_advise(dispatch_operation_t op, size_t chunk_size)
 	}
 	advise.ra_offset = op->advise_offset;
 	op->advise_offset += advise.ra_count;
-#if defined(__linux__)
-	_dispatch_io_syscall_switch(err,
-			readahead(op->fd_entry->fd, advise.ra_offset, (size_t)advise.ra_count),
-		case EINVAL: break; // fd does refer to a non-supported filetype
-		default: (void)dispatch_assume_zero(err); break;
-	);
-#else
+#if defined(F_RDADVISE)
 	_dispatch_io_syscall_switch(err,
 		fcntl(op->fd_entry->fd, F_RDADVISE, &advise),
 		case EFBIG: break; // advised past the end of the file rdar://10415691
 		case ENOTSUP: break; // not all FS support radvise rdar://13484629
 		// TODO: set disk status on error
 		default: (void)dispatch_assume_zero(err); break;
 	);
+#elif defined(HAVE_POSIX_FADVISE)
+	err = posix_fadvise(op->fd_entry->fd, advise.ra_offset,
+			(off_t)advise.ra_count, POSIX_FADV_WILLNEED);
+	switch (err) {
+		case 0: break;
+		case EINVAL: break; // unsupported advice or file type
+		case ESPIPE: break; // fd refers to a pipe or FIFO
+		default: (void)dispatch_assume_zero(err); break;
+	}
+#else
+#error "_dispatch_operation_advise not implemented on this platform"
 #endif
 #endif
 }