Hi,

Please see the following.
If someone sets CCACHE_DIR to an empty / null string,
ccache -s
results in a segmentation fault.
It seems create_parent_dirs() is called infinite times since it seems to create
a parent of "/" infinite times and stack overflows, or something.

This happens with git trunk (don't be misled by the directory name below in the
shell prompt.)

ishikawa@ip030:/home/ishikawa/repos/ccache-gsplit-dwarf-support$ export CCACHE_DIR=
ishikawa@ip030:/home/ishikawa/repos/ccache-gsplit-dwarf-support$ ./ccache -s
Segmentation fault

ishikawa@ip030:/home/ishikawa/repos/ccache-gsplit-dwarf-support$ gdb ./ccache
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Reading symbols from ./ccache...done.
(gdb) run -s
Starting program: /home/ishikawa/repos/ccache-gsplit-dwarf-support/ccache -s

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff7591ca5 in ?? () from /lib/x86_64-linux-gnu/libc.so.6
(gdb) where
#0  0x00007ffff7591ca5 in ?? () from /lib/x86_64-linux-gnu/libc.so.6
#1  0x00007ffff7593e20 in malloc () from /lib/x86_64-linux-gnu/libc.so.6
#2  0x00007ffff759978a in strdup () from /lib/x86_64-linux-gnu/libc.so.6
#3  0x0000000000409019 in x_strdup (s=s@entry=0x5a1d30 "/ccache.conf")
    at util.c:620
#4  0x0000000000409376 in dirname (path=path@entry=0x5a1d30 "/ccache.conf")
    at util.c:809
#5  0x00000000004093d0 in create_parent_dirs (
    path=path@entry=0x5a1d30 "/ccache.conf") at util.c:473
#6  0x000000000040941d in create_parent_dirs (
    path=path@entry=0x5a1d10 "/ccache.conf") at util.c:483
#7  0x000000000040941d in create_parent_dirs (
    path=path@entry=0x5a1cf0 "/ccache.conf") at util.c:483
#8  0x000000000040941d in create_parent_dirs (
    path=path@entry=0x5a1cd0 "/ccache.conf") at util.c:483
#9  0x000000000040941d in create_parent_dirs (
    path=path@entry=0x5a1cb0 "/ccache.conf") at util.c:483
#10 0x000000000040941d in create_parent_dirs (
    path=path@entry=0x5a1c90 "/ccache.conf") at util.c:483

   ... well, no end in sight: I had to suspend gdb and killed it. ...

I set a break point and tried again.

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(gdb) break create_parent_dirs
Breakpoint 1 at 0x409460: file util.c, line 470.
(gdb) set CCACHE_DIR
No symbol "CCACHE_DIR" in current context.
(gdb) set environment CCACHE_DIR
Undefined command: "setenvironment".  Try "help".
(gdb) set environment CCACHE_DIR
Setting environment variable "CCACHE_DIR" to null value.
(gdb) run -s
Starting program: /home/ishikawa/repos/ccache-gsplit-dwarf-support/ccache -s

Breakpoint 1, create_parent_dirs (path=path@entry=0x41f410 "/ccache.conf")
    at util.c:470
470	{
(gdb) list
465	}
466	
467	/* Create directories leading to path. Returns 0 on success, otherwise -1. */
468	int
469	create_parent_dirs(const char *path)
470	{
471		struct stat st;
472		int res;
473		char *parent = dirname(path);
474	
(gdb) where
#0  create_parent_dirs (path=path@entry=0x41f410 "/ccache.conf") at util.c:470
#1  0x0000000000404115 in create_initial_config_file (
    path=0x41f410 "/ccache.conf", conf=0x41f030) at ccache.c:2973
#2  initialize () at ccache.c:3058
#3  0x0000000000407625 in ccache_main_options (argv=<optimized out>, 
    argc=<optimized out>) at ccache.c:3448
#4  ccache_main (argc=2, argv=0x7fffffffd9b8) at ccache.c:3484
#5  0x00007ffff7539b45 in __libc_start_main ()
   from /lib/x86_64-linux-gnu/libc.so.6
#6  0x0000000000403eee in _start ()
(gdb) up
#1  0x0000000000404115 in create_initial_config_file (
    path=0x41f410 "/ccache.conf", conf=0x41f030) at ccache.c:2973
2973		if (create_parent_dirs(path) != 0) {
(gdb) list
2968		uint64_t max_size;
2969		char *stats_dir;
2970		FILE *f;
2971		struct stat st;
2972	
2973		if (create_parent_dirs(path) != 0) {
2974			return;
2975		}
2976	
2977		stats_dir = format("%s/0", conf->cache_dir);
(gdb) print path
$1 = 0x41f410 "/ccache.conf"
(gdb) quit
A debugging session is active.

I think special handling of cases

- where the directory  is "/", or
- where the directory is write protected

may be required to here.
(I hate to think what would have happened if I run ccache 
with null CCACHE_DIR in superuser account by mistake ...)

Or simply barfing out "CCACHE_DIR is null. Set it to a proper value." and
bailing out is a way to go.

TIA