[PATCH] fixincludes: fix portability issues about getcwd() [PR21283, PR80047]

[Xi Ruoyao <xry111@mengyan1223.wang>]

[Revised to handle PR 21283.]

POSIX says:

    On some implementations, if buf is a null pointer, getcwd() may obtain
    size bytes of memory using malloc(). In this case, the pointer returned
    by getcwd() may be used as the argument in a subsequent call to free().
    Invoking getcwd() with buf as a null pointer is not recommended in
    conforming applications.

This produces an error building GCC with --enable-werror-always:

    ../../../fixincludes/fixincl.c: In function ‘process’:
    ../../../fixincludes/fixincl.c:1356:7: error: argument 1 is null but
    the corresponding size argument 2 value is 4096 [-Werror=nonnull]

And, at least we've been leaking memory even if getcwd() supports this
non-standard extension.

And, MAXPATHLEN may be not unavailable on certain platform.  PATH_MAX is
POSIX, but getcwd() may produce a path with length larger than it.  So it's
suggested by POSIX [1] to call getcwd() with progressively larger buffers
until it does not give an [ERANGE] error.

[1]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/getcwd.html

fixincludes/ChangeLog:

	PR other/21823
	PR bootstrap/80047
	* fixincl.c (process): Allocate and deallocate the buffer for
	  getcwd() progressively.
---
 fixincludes/fixincl.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/fixincludes/fixincl.c b/fixincludes/fixincl.c
index 6dba2f6e830..1580c67efec 100644
--- a/fixincludes/fixincl.c
+++ b/fixincludes/fixincl.c
@@ -1353,9 +1353,18 @@ process (void)
   if (access (pz_curr_file, R_OK) != 0)
     {
       int erno = errno;
+      char *buf = NULL;
+      const char *cwd = NULL;
+      for (size_t size = 256; !cwd; size += size)
+	{
+	  buf = xrealloc (buf, size);
+	  cwd = getcwd (buf, size);
+	  if (!cwd && errno != ERANGE)
+	    cwd = "the working directory";
+	}
       fprintf (stderr, "Cannot access %s from %s\n\terror %d (%s)\n",
-               pz_curr_file, getcwd ((char *) NULL, MAXPATHLEN),
-               erno, xstrerror (erno));
+	       pz_curr_file, cwd, erno, xstrerror (erno));
+      free (buf);
       return;
     }
 
-- 
2.33.1