Introduce mmap_file function

New in v2:

- As Tom pointed out, we don't need to keep the fd around after
  mmapping.  This simplifies things quite a bit, since we don't need a
  new class.  It's now just a function that returns a scoped_mmap.

We already have scoped_mmap, which is a thin RAII layer over mmap.  If
one simply wants to mmap an entire file for reading, it takes a bit of
boilerplate.  This patch introduces the mmap_file function to make this
easier.

gdb/ChangeLog:

	* Makefile.in (COMMON_SFILES): Add common/scoped_mmap.c.
	* common/scoped_mmap.c: New file.
	* common/scoped_mmap.h (destroy): New method.
	(~scoped_mmap, reset): Use destroy.
	(scoped_mmap): New move constructor.
	(mmap_file): New declaration.
	* unittests/scoped_mmap-selftests.c (test_normal,
	test_invalid_filename, run_tests): New functions.
	(_initialize_scoped_mmap_selftests): Register selftest.

2 files changed, 70 insertions, 4 deletions

diff --git a/gdb/common/scoped_mmap.c b/gdb/common/scoped_mmap.c
new file mode 100644
index 0000000..aa2f703
--- /dev/null
+++ b/gdb/common/scoped_mmap.c
@@ -0,0 +1,48 @@
+/* scoped_mmap, automatically unmap files
+
+   Copyright (C) 2018 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include "defs.h"
+#include "scoped_mmap.h"
+#include "scoped_fd.h"
+
+#ifdef HAVE_SYS_MMAN_H
+
+scoped_mmap
+mmap_file (const char *filename)
+{
+  scoped_fd fd (open (filename, O_RDONLY));
+  if (fd.get () < 0)
+    perror_with_name ("open");
+
+  off_t size = lseek (fd.get (), 0, SEEK_END);
+  if (size < 0)
+    perror_with_name ("lseek");
+
+  /* We can't map an empty file.  */
+  if (size == 0)
+    error (_("file to mmap is empty"));
+
+  scoped_mmap mmapped_file (nullptr, size, PROT_READ, MAP_PRIVATE, fd.get (), 0);
+  if (mmapped_file.get () == MAP_FAILED)
+    perror_with_name ("mmap");
+
+  return mmapped_file;
+}
+
+#endif /* HAVE_SYS_MMAN_H */
diff --git a/gdb/common/scoped_mmap.h b/gdb/common/scoped_mmap.h
index 6070c37..b446163 100644
--- a/gdb/common/scoped_mmap.h
+++ b/gdb/common/scoped_mmap.h
@@ -41,8 +41,18 @@ public:
 
   ~scoped_mmap ()
   {
-    if (m_mem != MAP_FAILED)
-      munmap (m_mem, m_length);
+    destroy ();
+  }
+
+  scoped_mmap (scoped_mmap &&rhs)
+  {
+    destroy ();
+
+    m_mem = rhs.m_mem;
+    m_length = rhs.m_length;
+
+    rhs.m_mem = MAP_FAILED;
+    rhs.m_length = 0;
   }
 
   DISABLE_COPY_AND_ASSIGN (scoped_mmap);
@@ -58,8 +68,7 @@ public:
   void reset (void *addr, size_t length, int prot, int flags, int fd,
 	      off_t offset) noexcept
   {
-    if (m_mem != MAP_FAILED)
-      munmap (m_mem, m_length);
+    destroy ();
 
     m_length = length;
     m_mem = mmap (addr, m_length, prot, flags, fd, offset);
@@ -69,9 +78,18 @@ public:
   void *get () const noexcept { return m_mem; }
 
 private:
+  void destroy ()
+  {
+    if (m_mem != MAP_FAILED)
+      munmap (m_mem, m_length);
+  }
+
   void *m_mem;
   size_t m_length;
 };
 
+/* Map FILENAME in memory.  Throw an error if anything goes wrong.  */
+scoped_mmap mmap_file (const char *filename);
+
 #endif /* HAVE_SYS_MMAN_H */
 #endif /* SCOPED_MMAP_H */