# gdb --args /path/to/httpd -f /path/to/httpd.conf -X
[...]
Reading symbols from bin/httpd...done.

(gdb) break spool_reqbody_cl
Function "spool_reqbody_cl" not defined.
Make breakpoint pending on future shared library load? (y or [n]) y
Breakpoint 1 (spool_reqbody_cl) pending.

(gdb) run
Starting program: /path/to/httpd -f /path/to/httpd.conf -X
[...]
# here httpd is waiting for input, run the from the browser or curl terminal

Thread 5 "httpd" hit Breakpoint 1, spool_reqbody_cl (req=0x7fffe00044d0, bytes_spooled=0x7ffff59d9910) at mod_proxy_http.c:432
432	    apr_pool_t *p = req->p;
(gdb) break apr_file_mktemp
Breakpoint 2 at 0x7ffff7ea1a7f: file file_io/unix/mktemp.c, line 182.
(gdb) break file_cleanup
Breakpoint 3 at 0x7ffff7ea1b8a: file file_io/unix/open.c, line 31.
(gdb) continue 
Continuing.

# the two first breakpoints are for another file for me, so I simply continue through them
Thread 5 "httpd" hit Breakpoint 2, apr_file_mktemp (fp=0x7ffff59d9780, template=0x7fffe0006e38 "/tmp/apr-tmp.XXXXXX", flags=0, p=0x7fffe0002b98) at file_io/unix/mktemp.c:182
182	                       APR_FOPEN_DELONCLOSE : flags;
(gdb) continue 
Continuing.

Thread 5 "httpd" hit Breakpoint 3, file_cleanup (file=0x7fffe0006e50, is_child=0) at file_io/unix/open.c:31
31	    apr_status_t rv = APR_SUCCESS;
(gdb) print *file
$6 = {pool = 0x7fffe0002b98, filedes = 13, fname = 0x7fffe0006ec8 "/tmp/apr-tmp.zmSk2p", flags = 2375, eof_hit = 0, is_pipe = 0, timeout = -1, buffered = 0, blocking = BLK_UNKNOWN, ungetchar = -1, 
  buffer = 0x0, bufpos = 0, bufsize = 0, dataRead = 0, direction = 0, filePtr = 0, thlock = 0x0}
(gdb) continue 
Continuing.

# now the relevant file, let's continue until file cleanup
Thread 5 "httpd" hit Breakpoint 2, apr_file_mktemp (fp=0x7ffff59d9868, template=0x7fffe0006f08 "/tmp/modproxy.tmp.XXXXXX", flags=0, p=0x7fffe0002b98) at file_io/unix/mktemp.c:182
182	                       APR_FOPEN_DELONCLOSE : flags;
(gdb) continue 
Continuing.

# here we are, step into the function with "next"
Thread 5 "httpd" hit Breakpoint 3, file_cleanup (file=0x7fffe0006f28, is_child=0) at file_io/unix/open.c:31
31	    apr_status_t rv = APR_SUCCESS;
(gdb) print *file
$7 = {pool = 0x7fffe0002b98, filedes = 13, fname = 0x7fffe0006fa0 "/tmp/modproxy.tmp.DB9KbL", flags = 2375, eof_hit = 0, is_pipe = 0, timeout = -1, buffered = 0, blocking = BLK_UNKNOWN, ungetchar = -1, 
  buffer = 0x0, bufpos = 0, bufsize = 0, dataRead = 0, direction = 0, filePtr = 0, thlock = 0x0}
(gdb) next
32	    int fd = file->filedes;
(gdb) next
37	    file->filedes = -1;
(gdb) next
39	    if (close(fd) == 0) {
(gdb) next
41	        if (!is_child && (file->flags & APR_FOPEN_DELONCLOSE)) {
(gdb) next
42	            unlink(file->fname);
# the above unlink() should delete the temporary file
(gdb) next
45	        if (file->thlock) {

# so here "/tmp/modproxy.tmp.DB9KbL" shouldn't exist anymore
# can you reach this point or something went wrong/elsewhere before?

# don't care thereafter, so to leave...
(gdb) quit