[PATCH v2] core: use env variable instead of config var to turn on logging pack access

["Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>]

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>
---
 >> +     if (!*log_pack_access) {
 >> +             log_pack_access = getenv("GIT_TRACE_PACK_ACCESS");
 >> +             if (!*log_pack_access)
 >> +                     log_pack_access = NULL;
 >> +             if (!log_pack_access)
 >> +                     return;
 >> +     }
 >
 > Have you ever tested this?
 >
 > Once log_pack_access goes to NULL (e.g. when it sees the empty
 > string it was initialized to), this new test will happily
 > dereference NULL.

 My bad. I did test when GIT_TRACE_PACK_ACCESS was set, not when it
 was set to an empty string. All cases tested now.

 Documentation/git.txt |  7 +++++++
 cache.h               |  3 ---
 config.c              |  3 ---
 sha1_file.c           | 10 ++++++++++
 4 files changed, 17 insertions(+), 6 deletions(-)

diff --git a/Documentation/git.txt b/Documentation/git.txt
index 68f1ee6..c760918 100644
--- a/Documentation/git.txt
+++ b/Documentation/git.txt
@@ -838,6 +838,13 @@ 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,
diff --git a/cache.h b/cache.h
index df532f8..4f41606 100644
--- a/cache.h
+++ b/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);
diff --git a/config.c b/config.c
index 830ee14..e68184f 100644
--- a/config.c
+++ b/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)
diff --git a/sha1_file.c b/sha1_file.c
index 67e815b..91c3627 100644
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -36,6 +36,8 @@ static inline uintmax_t sz_fmt(size_t s) { return s; }
 
 const unsigned char null_sha1[20];
 
+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
@@ -1956,6 +1958,14 @@ 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)
+			log_pack_access = NULL;
+		if (!log_pack_access)
+			return;
+	}
+
 	if (!log_file) {
 		log_file = fopen(log_pack_access, "w");
 		if (!log_file) {
-- 
1.8.2.83.gc99314b