please make PDF ID field more deterministic

Hello.

It would be convenient that, by default or with appropriate options,
PDFTeX produces reproducible results from given input files.  You may
find some motivations at
https://wiki.debian.org/ReproducibleBuilds/About#Why_do_we_want_reproducible_builds.3F
Despite the wording, few problems are specific to Debian.

By default, the CreationDate and ModDate fields reflect the build
date, but this can be overriden with
  \ifpdf\pdfinfo{/CreationDate($DATE)/ModDate($DATE)}\fi
or
  pdftex '\pdfinfo{/CreationDate($DATE)/ModDate($DATE)}\input{source.tex}'

Unfortunately, the implementation of the ID field is an MD5 hash of
the build date, the output directory path and the output file name.

A first suggestion is to avoid generating the ID field, which is
optional and not widely used.

A less intrusive option would provide a new primitive like
\pdfsetoutputfileid{}.

My favorite suggestion would let the default ID depend on
- the configurable CreationDate instead of a random gmtime().
- the output file name only, ignoring its directory path.
Even if everybody starts to use the ID field, I hardly imagine how
these changes could create a collision.

The attached patch demonstrates the idea on
  https://foundry.supelec.fr/scm/viewvc.php/*checkout*/trunk/source/src/texk/web2c/pdftexdir/utils.c?revision=463&root=pdftex
It requires a trivial, but less readable change, in order to actually
compile: swap the declarations of the start_time_str global variable
and the printID() procedure.

--
Please CC me, I do not read this list permanently.

--- utils.c.old	2015-05-17 19:42:26.237342143 +0200
+++ utils.c	2015-05-17 19:56:05.821406233 +0200
@@ -747,26 +747,17 @@
  */
 void printID(strnumber filename)
 {
-    time_t t;
-    size_t size;
-    char time_str[32];
     md5_state_t state;
     md5_byte_t digest[16];
     char id[64];
     char *file_name;
-    char pwd[4096];
     /* start md5 */
     md5_init(&state);
     /* get the time */
-    t = time(NULL);
-    size = strftime(time_str, sizeof(time_str), "%Y%m%dT%H%M%SZ", gmtime(&t));
-    md5_append(&state, (const md5_byte_t *) time_str, size);
+    initstarttime();
+    md5_append(&state, (const md5_byte_t *) start_time_str, strlen(start_time_str));
     /* get the file name */
-    if (getcwd(pwd, sizeof(pwd)) == NULL)
-        pdftex_fail("getcwd() failed (%s), path too long?", strerror(errno));
     file_name = makecstring(filename);
-    md5_append(&state, (const md5_byte_t *) pwd, strlen(pwd));
-    md5_append(&state, (const md5_byte_t *) "/", 1);
     md5_append(&state, (const md5_byte_t *) file_name, strlen(file_name));
     /* finish md5 */
     md5_finish(&state, digest);