libc: use Id_space for FD allocator

This patch replaces the former use of an Allocator_avl with the Id_space
utility, which is safer to use and allows for the iteration of all
elements. The iteration over open file descriptors is needed for
implementing 'fork'.

Issue #3478

repos/libports/include/libc-plugin/fd_alloc.h

@@ -15,10 +15,11 @@
 #ifndef _LIBC_PLUGIN__FD_ALLOC_H_
 #define _LIBC_PLUGIN__FD_ALLOC_H_
 
-#include <base/allocator_avl.h>
 #include <base/lock.h>
 #include <base/log.h>
 #include <os/path.h>
+#include <base/allocator.h>
+#include <base/id_space.h>
 
 /* libc includes */
 #include <stdlib.h>
@@ -39,13 +40,27 @@ namespace Libc {
 
 	struct File_descriptor
 	{
-		int             libc_fd = -1;
-		char     const *fd_path = 0;  /* for 'fchdir', 'fstat' */
-		Plugin         *plugin  = 0;
-		Plugin_context *context = 0;
-		int             flags   = 0;  /* for 'fcntl' */
-		bool            cloexec = 0;  /* for 'fcntl' */
-		Genode::Lock    lock;
+		Genode::Lock lock { };
+
+		typedef Genode::Id_space<File_descriptor> Id_space;
+		Id_space::Element _elem;
+
+		int const libc_fd = _elem.id().value;
+
+		char const *fd_path = nullptr;  /* for 'fchdir', 'fstat' */
+
+		Plugin         *plugin;
+		Plugin_context *context;
+
+		int  flags   = 0;  /* for 'fcntl' */
+		bool cloexec = 0;  /* for 'fcntl' */
+
+		File_descriptor(Id_space &id_space, Plugin &plugin, Plugin_context &context)
+		: _elem(*this, id_space), plugin(&plugin), context(&context) { }
+
+		File_descriptor(Id_space &id_space, Plugin &plugin, Plugin_context &context,
+		                Id_space::Id id)
+		: _elem(*this, id_space, id), plugin(&plugin), context(&context) { }
 
 		void path(char const *newpath)
 		{
@@ -54,8 +69,7 @@ namespace Libc {
 				Genode::size_t const path_size = ::strlen(newpath) + 1;
 				char *buf = (char*)malloc(path_size);
 				if (!buf) {
-					Genode::error("could not allocate path buffer for libc_fd ",
-					              libc_fd, libc_fd == ANY_FD ? " (any)" : "");
+					Genode::error("could not allocate path buffer for libc_fd ", libc_fd);
 					return;
 				}
 				::memcpy(buf, newpath, path_size);
@@ -66,18 +80,24 @@ namespace Libc {
 	};
 
 
-	class File_descriptor_allocator : Allocator_avl_tpl<File_descriptor>
+	class File_descriptor_allocator
 	{
 		private:
 
 			Genode::Lock _lock;
 
+			Genode::Allocator &_alloc;
+
+			typedef File_descriptor::Id_space Id_space;
+
+			Id_space _id_space;
+
 		public:
 
 			/**
 			 * Constructor
 			 */
-			File_descriptor_allocator(Genode::Allocator &md_alloc);
+			File_descriptor_allocator(Genode::Allocator &_alloc);
 
 			/**
 			 * Allocate file descriptor

repos/libports/src/lib/libc/fd_alloc.cc

@@ -40,11 +40,9 @@ File_descriptor_allocator *Libc::file_descriptor_allocator()
 }
 
 
-File_descriptor_allocator::File_descriptor_allocator(Genode::Allocator &md_alloc)
-: Allocator_avl_tpl<File_descriptor>(&md_alloc)
-{
-	add_range(0, MAX_NUM_FDS);
-}
+File_descriptor_allocator::File_descriptor_allocator(Genode::Allocator &alloc)
+: _alloc(alloc)
+{ }
 
 
 File_descriptor *File_descriptor_allocator::alloc(Plugin *plugin,
@@ -53,44 +51,41 @@ File_descriptor *File_descriptor_allocator::alloc(Plugin *plugin,
 {
 	Lock::Guard guard(_lock);
 
-	/* we use addresses returned by the allocator as file descriptors */
-	addr_t addr = (libc_fd <= ANY_FD ? ANY_FD : libc_fd);
+	bool const any_fd = (libc_fd < 0);
+	if (any_fd)
+		return new (_alloc) File_descriptor(_id_space, *plugin, *context);
 
-	/* allocate fresh fd if the default value for 'libc_fd' was specified */
-	bool alloc_ok = false;
-	if (libc_fd <= ANY_FD)
-		alloc_ok = Allocator_avl_base::alloc_aligned(1, reinterpret_cast<void**>(&addr), 0).ok();
-	else
-		alloc_ok = (Allocator_avl_base::alloc_addr(1, addr).ok());
-
-	if (!alloc_ok) {
-		error("could not allocate libc_fd ", libc_fd,
-		      libc_fd <= ANY_FD ? " (any)" : "");
-		return 0;
-	}
-
-	File_descriptor *fdo = metadata((void*)addr);
-	fdo->libc_fd = (int)addr;
-	fdo->fd_path = 0;
-	fdo->plugin  = plugin;
-	fdo->context = context;
-	fdo->lock    = Lock(Lock::UNLOCKED);
-	return fdo;
+	Id_space::Id const id {(unsigned)libc_fd};
+	return new (_alloc) File_descriptor(_id_space, *plugin, *context, id);
 }
 
 
 void File_descriptor_allocator::free(File_descriptor *fdo)
 {
 	Lock::Guard guard(_lock);
+
 	::free((void *)fdo->fd_path);
-	Allocator_avl_base::free(reinterpret_cast<void*>(fdo->libc_fd));
+
+	Genode::destroy(_alloc, fdo);
 }
 
 
 File_descriptor *File_descriptor_allocator::find_by_libc_fd(int libc_fd)
 {
 	Lock::Guard guard(_lock);
-	return metadata(reinterpret_cast<void*>(libc_fd));
+
+	if (libc_fd < 0)
+		return nullptr;
+
+	File_descriptor *result = nullptr;
+
+	try {
+		Id_space::Id const id {(unsigned)libc_fd};
+		_id_space.apply<File_descriptor>(id, [&] (File_descriptor &fd) {
+			result = &fd; });
+	} catch (Id_space::Unknown_id) { }
+
+	return result;
 }