sha1_file: change GIT_TRACE_PACK_ACCESS logging to use trace API

This changes GIT_TRACE_PACK_ACCESS functionality as follows:
 * supports the same options as GIT_TRACE (e.g. printing to stderr)
 * no longer supports relative paths
 * appends to the trace file rather than overwriting

Signed-off-by: Karsten Blees <blees@dcon.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git.txt

@@ -925,11 +925,11 @@ Unsetting the variable, or setting it to empty, "0" or
 "false" (case insensitive) disables trace messages.
 
 'GIT_TRACE_PACK_ACCESS'::
-	If this variable is set to a path, a file will be created at
-	the given path logging all accesses to any packs. For each
+	Enables trace messages for all accesses to any packs. For each
 	access, the pack file name and an offset in the pack is
 	recorded. This may be helpful for troubleshooting some
 	pack-related performance problems.
+	See 'GIT_TRACE' for available trace output options.
 
 'GIT_TRACE_PACKET'::
 	Enables trace messages for all packets coming in or out of a

sha1_file.c

@@ -36,9 +36,6 @@ static inline uintmax_t sz_fmt(size_t s) { return s; }
 
 const unsigned char null_sha1[20];
 
-static const char *no_log_pack_access = "no_log_pack_access";
-static const char *log_pack_access;
-
 /*
  * This is meant to hold a *small* number of objects that you would
  * want read_sha1_file() to be able to return, but yet you do not want
@@ -2085,27 +2082,9 @@ static void *read_object(const unsigned char *sha1, enum object_type *type,
 
 static void write_pack_access_log(struct packed_git *p, off_t obj_offset)
 {
-	static FILE *log_file;
-
-	if (!log_pack_access)
-		log_pack_access = getenv("GIT_TRACE_PACK_ACCESS");
-	if (!log_pack_access)
-		log_pack_access = no_log_pack_access;
-	if (log_pack_access == no_log_pack_access)
-		return;
-
-	if (!log_file) {
-		log_file = fopen(log_pack_access, "w");
-		if (!log_file) {
-			error("cannot open pack access log '%s' for writing: %s",
-			      log_pack_access, strerror(errno));
-			log_pack_access = no_log_pack_access;
-			return;
-		}
-	}
-	fprintf(log_file, "%s %"PRIuMAX"\n",
+	static struct trace_key pack_access = TRACE_KEY_INIT(PACK_ACCESS);
+	trace_printf_key(&pack_access, "%s %"PRIuMAX"\n",
 			 p->pack_name, (uintmax_t)obj_offset);
-	fflush(log_file);
 }
 
 int do_check_packed_object_crc;
@@ -2130,7 +2109,6 @@ void *unpack_entry(struct packed_git *p, off_t obj_offset,
 	int delta_stack_nr = 0, delta_stack_alloc = UNPACK_ENTRY_STACK_PREALLOC;
 	int base_from_cache = 0;
 
-	if (log_pack_access != no_log_pack_access)
 	write_pack_access_log(p, obj_offset);
 
 	/* PHASE 1: drill down to the innermost base object */