source.c:openp: save/restore errno

openp's return is documented as:

~~~
   If a file is found, return the descriptor.
   Otherwise, return -1, with errno set for the last name we tried to open.  */
~~~

By inspection, I noticed that there are function calls after the ones
that first set errno, and those may clobber errno.  It's safer to save
errno when see an open fail, and restore it on exit.

Tested on x86_64 Fedora 20.

gdb/ChangeLog:
2015-10-27  Pedro Alves  <palves@redhat.com>

	* source.c (openp): New local 'last_errno'.  Use it to
	save/restore errno.

2 files changed, 12 insertions, 0 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 6c81a9f..0e40c7e 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,10 @@
 2015-10-27  Pedro Alves  <palves@redhat.com>
 
+	* source.c (openp): New local 'last_errno'.  Use it to
+	save/restore errno.
+
+2015-10-27  Pedro Alves  <palves@redhat.com>
+
 	* psymtab.c (dump_psymtab_addrmap_1): Add casts.
 
 2015-10-27  Simon Marchi  <simon.marchi@polymtl.ca>
diff --git a/gdb/source.c b/gdb/source.c
index 3e5e15c..194b044 100644
--- a/gdb/source.c
+++ b/gdb/source.c
@@ -746,6 +746,9 @@ openp (const char *path, int opts, const char *string,
   struct cleanup *back_to;
   int ix;
   char *dir;
+  /* The errno set for the last name we tried to open (and
+     failed).  */
+  int last_errno = 0;
 
   /* The open syscall MODE parameter is not specified.  */
   gdb_assert ((mode & O_CREAT) == 0);
@@ -786,6 +789,7 @@ openp (const char *path, int opts, const char *string,
 	  filename = NULL;
 	  fd = -1;
 	}
+      last_errno = errno;
 
       if (!(opts & OPF_SEARCH_IN_PATH))
 	for (i = 0; string[i]; i++)
@@ -808,6 +812,7 @@ openp (const char *path, int opts, const char *string,
   alloclen = strlen (path) + strlen (string) + 2;
   filename = (char *) alloca (alloclen);
   fd = -1;
+  last_errno = ENOENT;
 
   dir_vec = dirnames_to_char_ptr_vec (path);
   back_to = make_cleanup_free_char_ptr_vec (dir_vec);
@@ -878,6 +883,7 @@ openp (const char *path, int opts, const char *string,
 	  fd = gdb_open_cloexec (filename, mode, 0);
 	  if (fd >= 0)
 	    break;
+	  last_errno = errno;
 	}
     }
 
@@ -895,6 +901,7 @@ done:
 	*filename_opened = gdb_abspath (filename);
     }
 
+  errno = last_errno;
   return fd;
 }