Improve gdb_realpath for Windows hosts

On Windows hosts, gdb_realpath is just an xstrdup. This makes filename
comparisons on Windows very chancy. Normally, we would normalize both
paths, and then compare.  But since the normalization doesn't do
anything, two equivalent names on Windows might not match.  This can
happen when trying to insert a breakpoint using the fullpath of a file.
For instance, if the compiler generated debug info that says that
the compilation directory is: `c:\\some\\double\\slashes\\dir', then
trying to insert a breakpoint on `c:/some/double/slashes/dir/foo.c:4'
does not work:

    (gdb) b c:/some/double/slashes/dir/foo.c:4
    No source file named c:/some/double/slashes/dir/foo.c:4.
    (gdb) b c:\some\double\slashes\dir\foo.c:4
    No source file named c:\some\double\slashes\dir\foo.c:4.

This fixes the problem by enhancing gdb_realpath on Windows hosts.
The code is inspired from libiberty's lrealpath.

gdb/ChangeLog:

        * utils.c (gdb_realpath): Add better support for Windows hosts.

2 files changed, 23 insertions, 0 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 6065377..a58e7c6 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,7 @@
+2011-12-27  Joel Brobecker  <brobecker@adacore.com>
+
+	* utils.c (gdb_realpath): Add better support for Windows hosts.
+
 2011-12-23  Kevin Pouget  <kevin.pouget@st.com>
 
 	Introduce gdb.FinishBreakpoint in Python.
diff --git a/gdb/utils.c b/gdb/utils.c
index 16405d1..d55e6f1 100644
--- a/gdb/utils.c
+++ b/gdb/utils.c
@@ -3341,6 +3341,25 @@ gdb_realpath (const char *filename)
   }
 #endif
 
+  /* The MS Windows method.  If we don't have realpath, we assume we
+     don't have symlinks and just canonicalize to a Windows absolute
+     path.  GetFullPath converts ../ and ./ in relative paths to
+     absolute paths, filling in current drive if one is not given
+     or using the current directory of a specified drive (eg, "E:foo").
+     It also converts all forward slashes to back slashes.  */
+  /* The file system is case-insensitive but case-preserving.
+     So we do not lowercase the path.  Otherwise, we might not
+     be able to display the original casing in a given path.  */
+#if defined (_WIN32)
+  {
+    char buf[MAX_PATH];
+    DWORD len = GetFullPathName (filename, MAX_PATH, buf, NULL);
+
+    if (len > 0 && len < MAX_PATH)
+      return xstrdup (buf);
+  }
+#endif
+
   /* This system is a lost cause, just dup the buffer.  */
   return xstrdup (filename);
 }