core: use env variable instead of config var to turn on logging pack access

5f44324 (core: log offset pack data accesses happened - 2011-07-06)
provides a way to observe pack access patterns via a config
switch. Setting an environment variable looks more obvious than a
config var, especially when you just need to _observe_, and more
inline with other tracing knobs we have.

Document it as it may be useful for remote troubleshooting.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git.txt

@@ -832,6 +832,13 @@ for further details.
 	as a file path and will try to write the trace messages
 	into it.
 
+'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
+	access, the pack file name and an offset in the pack is
+	recorded. This may be helpful for troubleshooting some
+	pack-related performance problems.
+
 GIT_LITERAL_PATHSPECS::
 	Setting this variable to `1` will cause Git to treat all
 	pathspecs literally, rather than as glob patterns. For example,

cache.h

@@ -772,9 +772,6 @@ extern int parse_sha1_header(const char *hdr, unsigned long *sizep);
 /* global flag to enable extra checks when accessing packed objects */
 extern int do_check_packed_object_crc;
 
-/* for development: log offset of pack access */
-extern const char *log_pack_access;
-
 extern int check_sha1_signature(const unsigned char *sha1, void *buf, unsigned long size, const char *type);
 
 extern int move_temp_to_file(const char *tmpfile, const char *filename);

config.c

@@ -675,9 +675,6 @@ static int git_default_core_config(const char *var, const char *value)
 		return 0;
 	}
 
-	if (!strcmp(var, "core.logpackaccess"))
-		return git_config_string(&log_pack_access, var, value);
-
 	if (!strcmp(var, "core.autocrlf")) {
 		if (value && !strcasecmp(value, "input")) {
 			if (core_eol == EOL_CRLF)

environment.c

@@ -37,7 +37,6 @@ size_t packed_git_window_size = DEFAULT_PACKED_GIT_WINDOW_SIZE;
 size_t packed_git_limit = DEFAULT_PACKED_GIT_LIMIT;
 size_t delta_base_cache_limit = 16 * 1024 * 1024;
 unsigned long big_file_threshold = 512 * 1024 * 1024;
-const char *log_pack_access;
 const char *pager_program;
 int pager_use_color = 1;
 const char *editor_program;

sha1_file.c

@@ -36,6 +36,9 @@ 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
@@ -1956,12 +1959,19 @@ 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 = NULL;
+			log_pack_access = no_log_pack_access;
 			return;
 		}
 	}
@@ -1992,7 +2002,7 @@ 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)
+	if (log_pack_access != no_log_pack_access)
 		write_pack_access_log(p, obj_offset);
 
 	/* PHASE 1: drill down to the innermost base object */